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Science and Technology R

eviewLaw

rence Livermore N

ational LaboratoryP.O

.Box 808, L-664

Livermore, C

alifornia 94551

August 1995

Lawrence

Livermore

National

Laboratory

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Inside this issue:

• Scanning Tunneling Microscopy

• Risk Assessment

Top view of STM in ultra-high-vacuum chamber

Digital Mammography

SCIENTIFIC EDITOR

William A. Bookless

PUBLICATION EDITOR

Sue Stull

WRITERS

Lauren de Vore, Harriet Kroopnick,Ann Parker, Dale Sprouse, and Katie Walter

ART DIRECTOR AND DESIGNER

George Kitrinos

GRAPHIC ARTISTS

Treva Carey and Paul Harding

CONCEPTUAL ILLUSTRATOR

John Maduell

COMPOSITOR

Louisa Cardoza

PROOFREADER

Catherine M. Williams

2 The Laboratory in the News

3 Commentary on Science and Technology

Features4 Scanning Tunneling Microscopy: Opening a New Era

of Materials Engineering“Nanoengineering” is advancing the development of new materials with better performance characteristics.

12 Risk Assessments: From Reactor Safety to Health CareOur experiences with probabilistic risk assessments have led to rational and systematic approaches that improve the safety and performanceof complex engineering and operational processes.

Research Highlights22 Crucial Steps Taken in Laser Guide Star System

24 Forensic Science Center Update

27 Patents and Awards

28 Abstracts

S&TR Staff August 1995

LawrenceLivermoreNationalLaboratory

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UCRL-52000-95-8Distribution Category UC-700August 1995

This publication is a continuation of Energy and Technology Review.

R E V I E W

About S&TR

About the Cover August 1995

Lawrence

Livermore

National

Laboratory

Inside this issue:


• Risk Assessment

The Lawrence Livermore National Laboratory, operated by the University of California for theUnited States Department of Energy, was established in 1952 to do research on nuclear weapons andmagnetic fusion energy. Science and Technology Review (formerly Energy and Technology Review) is published monthly to communicate, to a broad audience, the Laboratory’s scientific and technologicalaccomplishments, particularly in the Laboratory’s core mission areas—global security, energy and the environment, and bioscience and biotechnology. Rather than just informing people of theseaccomplishments, the publication’s goal is to help readers understand them and appreciate their value tothe individual citizen, the nation, and the world.

Please address any correspondence concerning S&TR (including name and address changes) to Mail Stop L-664, Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, CA 94551, ortelephone (510) 422-8961. S&TR is also available on the Internet at http://www.llnl.gov/str/str.html, andour electronic mail address is [email protected].

Prepared by LLNL under contractNo. W-7405-Eng-48

Electronic Access

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S&TR is available on the Internet athttp://www.llnl.gov/str/str.html. As referencesbecome available on the Internet, they will beinteractively linked to the footnote references at the end of each article. If you desire moredetailed information about an article, click onany reference that is in color at the end of thearticle, and you will connect automatically withthe reference.

page 4

page 12

page 22

The scanning tunneling microscope (STM)provides an image of the atomic arrangement ofa material’s surface. The rendered enlargementshows bonds formed between adjacent atomsFor further information, see the feature articlebeginning on page 4.

What do you think?We want to know what you think of our

publication. Use the survey form on the insideback cover to give us your feedback.

The Laboratory in the News Commentary on Science and Technology

Science & Technology Review August 1995

Many applications for advanced cooling systemLaboratory scientists and industrial partners have

designed a new low-power, portable, fieldable, helium-basedcooling system. Originally designed for a radiation detectionsystem, it promises to find applications in many areas, fromfood refrigeration to scientific instrumentation. A patentapplication has been filed, and the Laboratory is seeking to license the design to industrial partners.

Key to the system is the use of microprocessors tominimize vibration; they allow the compressor’s motor tobecome more efficient, use less energy, and last longer.Instead of environmentally harmful chlorofluorocarbons, thenew system uses small amounts of helium as a coolant. Thedesign also eliminates the use of liquefied nitrogen (LN2).

LN2 has been a potentially hazardous component of most cooling systems that are built to deliver very lowtemperatures—for example, those used in radiation detectionsystems such as medical PET scans. Cooling systems thatuse liquefied nitrogen also are bulky and expensive tomaintain and require extensive safety mechanisms. Thus,they have been considered impractical.

The small size, low power requirement, LN2-freeoperation, and low vibration features of the Livermorecooling system are qualities researchers sought in a coolingsystem for a portable, fieldable radiation detector. Labresearchers expect their design to result in a number of portable, fieldable systems, including those forenvironmental monitoring and locating underground oildeposits.Contact: Ken Neufeld (510) 423-8718([email protected]).

Dismantlement approach seeks to reduce wasteThis September, the Laboratory plans to begin a year-long

technology development effort aimed at finding ways for thenation to dismantle its nuclear weapons with greatly reducedplutonium-contaminated waste. The small-scale effort to beconducted in the Laboratory’s plutonium facility will usemechanical and thermal approaches for reducing waste. First,researchers plan to use a cutter that does not produce wastechips to open a plutonium pit that has been removed from anuclear weapon. Then they will use a process called HYDEC(hydride-dehydride-cast), which uses hydrogen gas and heatto remove the plutonium and then casts it into small ingotsfor storage.

Plans call for processing about 20 to 25 pits from theDOE’s Rocky Flats Plant during the prototype test period.Afterward, the resulting plutonium ingots will be returned

there. The project could establish a more cost-effective, lesstime-intensive dismantlement procedure.Contact: Derek Wapman (510) 422-0826([email protected]).

Lab leads manufacturing of B Factory rf cavitiesThe Laboratory is directing the manufacture of all radio-

frequency (rf) cavities needed for operating the B Factoryaccelerator and detector—sometimes known as PEP-II andBaBar. The $177-million accelerator and the $75-milliondetector are designed to advance understanding in the field ofparticle physics. Located at the Stanford Linear AcceleratorCenter (SLAC), the B Factory is a collaboration betweenSLAC and the Lawrence Berkeley and Lawrence LivermoreNational Laboratories.

Each cavity will be 2 ft in diameter and weigh about450 lb. The rf cavities will be powered in pairs by a l-megawatt microwave generator. The first cavity was shippedto SLAC on May 30 as part of a $4-million project spread overthe next two years. In all, 26 additional rf cavities will beconstructed. While conventional machining on the rf cavitiesis being contracted to U.S. industry, critical fabrication andassembly activities are centered in Livermore, where about 60 technicians and machinists are involved.Contacts: Curt Belser (510) 423-2472 ([email protected]);Mark Franks (510) 423-4434 ([email protected]).

Workshop addresses planetary defenseAn international Planetary Defense Workshop was held

in Livermore May 22–26, 1995. Titled “An InternationalTechnical Meeting on Active Defense of the TerrestrialBiosphere from Impact of Large Asteroids and Comets,” theevent attracted about 150 scientists. After reviewing presentunderstanding of the basic nature of the threat posed byasteroid and comet impacts, attendees gathered in workinggroups to examine specialized issues, including detection,tracking, and categorization of threat objects, and technologiesand systems for threat object deflection and dispersion.

The workshop was co-hosted by LLNL and DOE.Sponsoring organizations were LLNL, DOE, NationalAeronautics and Space Administration, Los Alamos NationalLaboratory, Air Force Space Command, Air Force PhillipsLaboratory, Naval Research Laboratory, Russian FederalNuclear Centers VNIITF Chelyabinsk-70 and VNIIEFArzamas-16, Makeev State Rocket Center, the Russianministries of Energy and Defense, China’s Center ofAdvanced Science and Technology-World Lab (Beijing), and the World Laboratory (Erice and Geneva).Contact: Shirley Petty (510) 422-1175 ([email protected]).

2


HE external forces that are reshaping the Laboratory andrefocusing its programs are having a significant impact

on the disciplines as well. Chemistry and Materials Science,together with Physics, Engineering, and Computation, providethe disciplinary foundation for all of the Laboratory’sprograms. In many respects, recent changes are beneficial,particularly because they help us to sharpen our mission andpurpose as a discipline.

Chemistry and Materials Science plays a vital role in the success of Laboratory programs. Lasers, weapons,nonproliferation, environmental remediation, energytechnologies, advanced manufacturing, structural biology—these programs and others all involve challenging aspects ofchemistry and materials science. We must provide the core-discipline expertise to meet the objectives of existingprograms, plus we must continue to advance the state of the artin order to create the scientific foundations for new andevolving programs.

Our role is to be a supporting partner to existing andevolving Laboratory programs. By assigning personnelthrough our matrix system, we provide the programs with therequired mix of scientific skills in chemistry and materialsscience, as well as with individuals to assist in technicalproject management. To this end, it is essential that we forecastaccurately the types of scientific talent needed and that weattract and retain the best scientists in those areas. Given

current uncertainties regarding Laboratory missions andprogram directions, scientific flexibility and breadth of interestare increasingly important.

Synergy between the programs and disciplines is responsiblefor much of the Laboratory’s success over the years.Fundamental scientific discoveries open the doors to newprograms, and programmatic requirements push the state of theart in the disciplines. The article in this issue on scanningtunneling microscopy is an excellent example of this process.Many programs require ever-more detailed imaging methods—electronics engineers need to be able to fabricate microcircuitpatterns a few tens of atoms thick, biologists want to studysingle molecules of protein or DNA, and materials scientistsneed to be able to examine atomic-scale flaws in crystals orcoatings. As scanning tunneling microscopy and atomic-forcemicroscopy have evolved, these tasks become possible.

Indeed, one of our most difficult challenges is decidingwhich exciting innovations to pursue because the creativity ofour scientists far exceeds our resources. Only through closepartnership with the programs can we understand and anticipatetheir chemistry and materials science needs. And as westructure our research to develop the new processes, materials,and characterization techniques that will be needed, we makethe scientific and technical discoveries that lead to newprograms. Examples include the development ofnanoengineered materials (aerogels and multilayers) and newprocesses for forming ceramic–metal composites. In theseways, the disciplines participate in and influence the evolutionof the Laboratory’s programs.

Guiding the development of new disciplinary capabilitiesand innovations is especially challenging at a time whennational expectations of our Laboratory are changing rapidly.Regardless of how the Laboratory and its programs evolve,chemistry and materials science will play a key role. We aredoing all that is possible to provide the right people and theresearch environment to foster the continual advancement ofour technical capabilities and to sow the intellectual seeds fornew program directions.

3

Jeffrey WadsworthAssociate Director for Chemistryand Materials Science

T

5


Scanning Tunneling Microscopy

optoelectronics, switching speed in thecase of transistors for microelectronics,and hardness in the case of high-strength coatings—therefore dependscritically on the precise control of thedetails of atomic ordering duringmanufacture. This is where LLNL’ssurface physics facility in the Chemistryand Material Science Directorate entersthe picture with its ultra-high-vacuumscanning tunneling microscopycapabilities.

Analyzing AtomicArrangement

To diagnose the effect of atomicarrangement on material performance,materials scientists use a battery oftechniques. Traditionally, diffraction-based probes have been the mainstay of structural analysis, and haveprovided most of our basic knowledgeabout the atomic arrangement ofmaterials. In diffraction, a beam oflight or particles (neutrons, electrons,etc.) is scattered from an object, and thethree-dimensional, geometricdistribution of the scattered rays isdetermined by the structure of theobject. For example, the pattern ofvisible light reflected from the surfaceof an audio compact disc held underbright light indicates the spacing of bitswritten onto the disc. Similarly, thediffraction of beams of a wavelengththat is comparable to the spacingbetween atoms in a crystal indicates thespacing between the atoms.

X-ray diffraction, the primary toolfor analyzing the long-range, atomicordering of solids, enabled thedevelopment of crystallography andprovided the experimental data fromwhich the structure of DNA wasdeduced. Transmission electronmicroscopy, another diffraction-basedtool, is often used to provide images ofimperfections in crystals. In both x-raydiffraction and transmission electronmicroscopy, however, diffractionmeasurements reveal the internal atomicarrangements of a material only whencrystalline order extends over at leastseveral hundred atomic spacings; in thiscase the material is said to exhibit“long-range” order. In contrast, whencrystalline order exists over shorterdistances, the material is said to exhibit“short-range” order, which may not bedetected by diffraction.

Analyzing Surface Structure

These two diffraction techniquespresent the “bulk,” or three-dimensional, atomic arrangement of amaterial. In nanoengineering, however,we must control how the individualatomic layers of material are deposited.Because the structural integrity of eachatomic layer depends critically on thedetailed atomic ordering of the surfaceupon which it is deposited, we must beable to “see” the atomic ordering, orstructure, of that surface. To do this, weneed a separate class of diagnostics thatpresents the two-dimensional atomic

arrangement of the outermost layer ofatoms in a material, rather than its three-dimensional bulk structure.

Low-Energy Electron DiffractionFor many years, the characterization

of surface structure has relied on thediffraction of electrons of low energy(fewer than 200 V). Because such low-energy electrons do not penetrate beyonda few atomic layers into a crystal, theirdiffraction from a crystal yields the long-range atomic order on a surface.Although low-energy electron diffractionis responsible for most of our currentknowledge of surface crystallography, itcannot reveal the short-range crystallineorder of nanometer-scale dimensions.Yet it is on this very scale that clusters ofdeposited atoms initially aggregate, ornucleate, and influence the atomicarrangement of subsequently grownmaterial. In this regime, true atomicresolution is necessary, and the scanningtunneling microscope is indispensable.

Scanning Tunneling MicroscopyThe scanning tunneling microscope3

(STM) provides a picture of the atomicarrangement of a surface by sensingcorrugations in the electron density ofthe surface that arise from the positionsof surface atoms (see Figure 1). A finelysharpened tungsten wire (or “tip”) is firstpositioned within 2 nanometers of thespecimen by a piezoelectric transducer, aceramic positioning device that expandsor contracts in response to a change inapplied voltage. This arrangement

4


N the last decade, the ability ofmaterials scientists to “nanoengineer”

artificial materials—to build materialsatom by atom with a predeterminedarrangement and goal—has enabled thedevelopment of new technologies withapplications that range from thespectacular to the mundane.1,2 Forexample, x-ray mirrors composed of alternating, thin (less than 20-nanometer) films of molybdenumand silicon constitute the optics that areused to produce high-resolution picturesof the sun. Optoelectronic componentscomposed of alternating atomic layersof different elements are the devicesthat enable us to extract informationfrom video compact disks and togenerate and detect transoceanictelephone signals by fiberoptic cables.The alternating, ultrathin layers ofcobalt and iron in new high-densitymagnetic storage heads, andincreasingly miniature microelectronics,are fundamental constituents ofpowerful desktop computers, portablelaptops, and pocket-size wirelesstelephones.

The smaller these devices become,the more their performance depends onthe atomic ordering of their constituentmaterials. Such details include thearrangement of atoms in crystalstructures and the presence, size, anddensity of grain boundaries, impurities,dislocations, or other imperfections.Enhanced performance of a device—increased reflectivity in the case of x-ray mirrors, efficiency in the case of

Spectacular advances inthe development ofartificial materials—nowengineered on thenanometer scale—havespurred the paralleldevelopment of new toolsto characterize surfaceand interfacial structureat the atomic scale. Oursurface physics facilityapplies ultra-high-vacuumscanning tunnelingmicroscopy to acceleratethe development of suchadvanced materials.

ScanningTunneling Microscopy: Opening a New Era ofMaterials Engineering

ScanningTunneling Microscopy: Opening a New Era ofMaterials Engineering

I

How the Molybdenum–Silicon Interface Forms

Recently, we used this combinationof surface diagnostics to study thestructural development of thin films(films from one atom to several tens of nanometers thick) resulting fromdepositing molybdenum atoms onatomically clean silicon substrates. Thedata from this study can be used todevelop new processes for synthesizingfilms that can achieve higherperformance for particular applications.

For example, multilayer x-raymirrors composed of alternating, 5- to20-nanometer-thick layers ofmolybdenum and silicon achieve thebest reflectivity when the interfacesbetween molybdenum and silicon aremost abrupt—that is, when puremolybdenum is separated frompure silicon by a perfectly flatplane. However, molybdenum and silicon tend to react to form crystallinecompounds, or interfacialsilicides, which may adopt avariety of distinct crystalstructures called phases.Because these silicidesdegrade this interfacialabruptness, we are trying todefine processing conditionsthat minimize the amount ofinterfacial silicide.

However, molybdenumsilicides also appear inother applications, such ashigh-temperature coatingsand diffusion barriers forinterconnects in very large-scale integrated circuits. Forthese applications, it may bedesirable not to minimize theamount of interfacial silicide butrather to maximize the amount of aparticular silicide phase, which

may lead to enhanced performance. Ouranalysis is therefore intended to providea broad correlation between theprocessing conditions (for example,substrate temperature and depositionrate) and the microstructural details ofthe resulting films, which ultimatelydetermine how well the device willperform for a specific application. Wehave found that film morphology—characteristics such as roughness,crystalline structure, and grain size andorientation—depends strongly on smallstructures called precursors. Thesestructures form during the initial stagesof film growth and can only be detectedwith scanning tunneling microscopy.5

Phases and Atomic CompositionsThis reaction between molybdenum

and silicon exhibits a particularly richvariety of phases and relativecompositions of molybdenum andsilicon, such as MoSi2 and Mo3Si. Thestructure of disilicide thin films, asopposed to bulk crystals, is furthercomplicated by interfaces—both thesilicide/substrate interface and thesilicide surface itself. For example, thereis a thin disilicide film phase thatexhibits hexagonal symmetry that doesnot even appear in the bulk phase.Furthermore, the precise temperature at which this phase transforms to theequilibrium phase of tetragonalsymmetry appears to be highly processdependent. The STM can help usunderstand these phases andtransformations.

For example, we used the surface ofcrystalline silicon—designated Si(100)—as the starting substrate for filmdeposition. When clean Si(100) isexposed, the atoms of the surface arerearranged—as is the case with mostsemiconductor materials. In fact, bondsare formed between adjacent atoms, eachpair of which is called a “dimer.” Thedimers then align themselves into rowson the surface, as shown in Figure 2.These surface structures are important to STM studies that seek to extractchemical information about a particularsurface (see box on page 8).

Regimes of Silicide Film GrowthWhen we use STM to examine these

films, we find several regimes of silicidefilm growth. When molybdenum is

7Scanning Tunneling Microscopy6 Scanning Tunneling Microsopy

enables us to control the motion of thetip with subnanometer precision. At this small separation, as explained bythe principles of quantum mechanics,electrons “tunnel” through the gap, theregion of vacuum between the tip andthe sample. If a small voltage (bias) isapplied between the tip and the sample,then a net current of electrons (the“tunneling current”) flows through thevacuum gap in the direction of the bias.For a suitably sharpened tip—one thatterminates ideally in a single atom—thetunneling current is confined laterally toa radius of a few tenths of a nanometer.The remarkable spatial resolution of the STM derives from this lateralconfinement of the current.

Next, additional piezoelectrictransducers are used to raster the tipacross a small region of the sample. Asthe tip scans the surface, corrugations inthe electron density at the surface of thesample cause corresponding variationsin the tunneling current. By detectingthe very fine changes in tunnelingcurrent as the tip is swept across the surface, we can derive a two-dimensional map of the corrugations in electron density at the surface.4Procedures for synthesizing variousnanoengineered materials often involvedepositing the atoms onto a surface insuch a way that the surfaces remain freeof contamination. The use of ultra-highvacuum enables the preparation andatomic-resolution imaging of atomicallyclean surfaces, which would otherwisebe contaminated immediately in air.

That is why we integrated a scanningtunneling microscope into an ultra-high-vacuum environment thatincludes facilities for the preparationand maintenance of atomically cleansurfaces, as well as sources of thematerial to be deposited. We alsointegrated complementary,conventional surface diagnosticsequipment, such as a low-energyelectron diffraction probe, into thisenvironment. The latter measures thelong-range order on a surface, andSTM presents the short-range orderthat otherwise might not be detected.In this environment, the STM offers anew opportunity for direct diagnosis ofhow the processing conditions affectthe atomic details of surfaces.


Figure 1. Artist’s renderings of a scanning tunnelingmicroscope (STM). (a) Plan view of the STM mountedin an ultra-high-vacuum chamber. (b) The probe tip asheld by a tripod, which consists of three piezoelectriccylinders that expand or contract in the directions(x,y,z) shown to displace the tip. (c) A close-up of thetip within tunneling distance of the surface of thespecimen being viewed, showing the ribbon-like paththat the tip follows above the surface atoms duringscanning.

V bias

Current flow

z

y

Piezoelectriccylinders

Piezoelectriccylinder

x

(a)

(b)

(c)


Figure 2. (Below) A 650-angstrom (Å)STM image of the surface Si(100), inwhich each stripe represents a row ofdimers. (Above) Illustration of the atomicstructure of the Si(100) surface. Theoutermost, dimerized atoms thatcontribute to the STM image below areshown in red in the illustration above.

deposited on Si(100) at 475°C, a novelordering of atoms occurs only withinthe outermost layer of the surface(Figures 3 and 4). Because this ordering

cannot be detected by conventional x-ray crystallography and is not readilydetectable by electron diffraction, theresulting surface previously wasthought to be amorphous. With STM, itis now possible to identify the presenceand locally ordered character of thisnew interfacial material.

At higher temperatures (between650 and 750°C) in this process, some of the material nucleates into thehexagonal phase of disilicide MoSi2(Figure 5). This nucleation acts as aprecursor for disilicide grains that growwhen additional molybdenum isdeposited on the surface.

The third regime of disilicidegrowth occurs above 750°C. Whenmolybdenum is deposited at 770°C,tetragonal MoSi2 is formed. Figure 6,an STM image of the resulting surface,displays plateaus with large, flatterraces. Despite the variety of atomicarrangements observed in this image,each superstructure suggests a simplerelationship to the periodicity (ordered,repeated atomic arrangement) of a

specific face of tetragonal MoSi2. Thecrystal face visible in the image thenspecifies the orientation of growth of the specific grain, which ispreferentially oriented with respect tothe silicon substrate.

For example, the sets of smallcircles in the three diagrams on theright-hand side of Figure 6 represent theatomic arrangement of the [001] planesof tetragonal MoSi2. The large, shadedcircles represent atomic sites in asuperstructure that would correspond tothe periodicity observed in the regionsof the STM image. In the diagram onthe left-hand side, the small, whitecircles represent silicon atoms in [100]planes, and the small, dark circlesrepresent molybdenum atoms in thoseplanes. The large shaded circles thenconstitute a superstructure that wouldcorrespond to the periodicity in theindicated region of the image rotatedapproximately 37 deg with respect tothe neighboring regions. Such rotationwould be required to achieve alignmentbetween a disilicide crystallite growing

9


Scanning Tunneling Microscopy8


Scanning Tunneling Microsopy

Imaging Surface Electronic Structure with STM

Because the trajectory executed by an STM tip isdetermined by the overlap of the electronic-state density ofthe tip with the local electronic-state density of the surface,the STM can be used to extract chemical informationassociated with a particular surface structure. On theSi(l00) surface, for example, dimerization results in aconcentration of valence electron density betweendimerized atoms (bonding orbitals) and a depletion ofvalence electron density in “anti-bonding” orbitals, ofwhich there is one for each surface atom. In the modelbelow, which presents the bonding geometry of a pair ofdimers on the surface of Si (100), the bonding orbitals arerepresented in red, and the antibonding orbitals in gold.

Figure 4. Filled- andempty-state STM imagesshowing a 120-Å detail ofthe region shown in Figure 3. Each spotcorresponds to a singlesurface atom. Variousatomic arrangements areindicated with arrows.

Figure 3. A 500-Å atomic-resolution STMimage of Si(100) following deposition of one-half monolayer of molybdenum at 475°C. Thesurface atoms form ordered arrangements overshort ranges.

If the STM is operated with a positive tip-to-samplejunction, so that electrons must tunnel from the surface tothe tip, the concentration of surface valence electrondensity within dimers causes the STM image to displayrows of dimers, as in image (a) below. The individualdimers, displayed in red in image (a) correspond to thedimer bonds shown in red in the model. If the polarity isreversed, and electrons tunnel from the tip to theantibonding orbitals of the surface, then the resulting image(b) will enhance individual atoms. Each gold spot in image (b) then corresponds to one antibonding orbital,which in turn is associated with a single surface atom, asshown in the model.

(a)

(a)

(b)

(b)

Molybdenum islands

Si dimers

2D surface phases

Filled states Empty statesFilled states Empty states

500 Å

120 Å 120 Å

along its <100> axis and one growingalong its <001> axis. Both orientationsof tetragonal MoSi2 relative to theSi(100) substrate are consistent withthose reported previously for thickerdisilicide films. Neither the existence ofthese superstructures nor their relativeprevalence was accessible frommeasurements used in previous analysesof the molybdenum/silicon system.

By associating specificmicrostructures with the temperatures at which they are processed, we are now equipped to determine the bestprocedure for synthesizing thin filmsthat have the microstructures necessaryfor particular applications. For example,the temperature stability of multilayersused in x-ray mirrors is known to berelated strongly to the microstructure ofthe interfaces between individualmolybdenum and silicon layers.6 Thedeliberate promotion during fabricationof one or another of the regimes ofMo/Si interfacial structure that we haveidentified above may therefore lead tomultilayers with internal structureengineered for enhanced thermalstability.

With the advanced capabilities ofSTM, the Laboratory can evaluate howprocessing parameters affect the atomicstructures of interfaces, identify surfacedefects that have a critical influence onfilm growth, and control theiroccurrence, which will lead to improvednew materials with better performancecharacteristics.

Key Words: atomic ordering, disilicide,interface, molybdenum/silicon,nanoengineer, scanning tunnelingmicroscopy, silicide, surface physics, thinfilm.

Notes and References1. “Materials by Computer Design,”

Energy & Technology Review,August–September 1994, LawrenceLivermore National Laboratory,Livermore, CA, UCRL-52000-94-8/9(1994).

2. T. W. Barbee, “High-Performance,Nanoengineered Multilayer Materials,”Chemistry and Materials Science AnnualReport, 1990–1991, UCRL-53943-91(1991).

3. G. Binnig and H. Rohrer, “ScanningTunneling Microscopy—from Birth toAdolescence,” Reviews of ModernPhysics 59, 615 (1987).

4. J. Golovchenko, “The TunnelingMicroscope: A New Look at the AtomicWorld,” Science 232, 48 (1986).

5. P. Bedrossian, “One-DimensionalOrdering at the Mo/Si Interface,”Surface Science 320, 247 (1994); also P. Bedrossian, “Nucleation andOrdering of MoSi2 on Si(100),” SurfaceScience 322, 73 (1995).

6. R. Rosen, D. Stearns, M. Viliardos, M. Kassner, and S. Vernon, “ThermalStability of Mo/Si Multilayers,”Chemistry and Materials ScienceAnnual Report 1990–1991, LawrenceLivermore National Laboratory,Livermore, CA, UCRL-53943-91(1991).

For further information contact Peter J. Bedrossian (510) 423-5938([email protected]).

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Scanning Tunneling Microsopy

Si Dimers

3.2 Å7.86 Å

Si <010>

MoSi2(001) (3¥3)

MoSi2(001) p(2¥2)

MoSi2(001) c(2¥2)

MoSi2(100)(2¥9)

C40 (hex)MoSi2(b)

(a)

Figure 5. (a) A 500-ÅSTM image of Si(100)following deposition ofone monolayer ofmolybdenum at roomtemperature andannealing at 640°C. (b) 125-Å detail showingindividual silicon dimersand silicide islands.

Figure 6. A 225-Å STM image ofthe surface resulting from thedeposition of four monolayers ofmolybdenum on Si(100) at 770°C.The spacings between the atomsare indicated in the diagram.

About the Scientist

PETER BEDROSSIAN came to Livermore in 1992, when hejoined the Chemistry and Materials Science Department as aStaff Physicist. He also works with the AdvancedMicrotechnology Program. In his short career here, he was acollaborator with a group at Sandia National Laboratories,Albuquerque, that received the 1994 Materials Science Awardfor Sustained Outstanding Research in Metallurgy and Ceramics

from the U.S. Department of Energy. He has published 20 papers in the field ofatomic-scale physics, including a number of articles in scanning tunnelingmicroscopy.

Bedrossian received his A.B., A.M., and Ph.D. in Physics from HarvardUniversity in 1985, 1987, and 1989, respectively. Prior to coming to Livermore, heworked at Sandia National Laboratories, Albuquerque, in the Surface ScienceDivision from 1991 to 1992 and was a Humboldt Research Fellow atForschungszentrum Jülich, Germany, in 1993.

500 Å

225 Å

4.5 Å

125 Å

product, process, or facility and enterthe environment. This type of riskassessment is often referred to as a health risk assessment and iscommonly undertaken by agencies ofthe federal government that deal withpublic health and safety, e.g., theEnvironmental Protection Agency, theFood and Drug Administration, theConsumer Product Safety Commission,and the Occupational Safety andHealth Administration.

Other times, a risk assessmentfocuses on the health effects that canoccur when an “engineered” systemfails, because of a natural or human-initiated event or when the protectivebarrier between the environment andthat system fails (Figure 1). Known asengineering risk assessments, they arecommonly carried out by agencies ofthe federal government that makesafety, health, or design decisionsabout risk-posing facilities orequipment. Examples of agencies thatuse engineering risk assessments are • Department of Energy, in evaluatingthe radiological and chemical risksfrom various types of nuclear and non-nuclear facilities.• Department of the Interior, inanalyzing dam safety, assessingdamage from ecological disasters, andhelping to predict natural hazards, such

as earthquakes, floods, or volcanoes.Much of this risk assessment work is directed toward improving theprobability distributions that describethe recurrence of these natural hazardsand their possible intensity.• Federal Aviation Administration, inanalyzing potential collision scenarios,such as the simultaneous approach oftwo aircraft on closely spaced, parallelrunways in inclement weather.• National Aeronautics and SpaceAdministration, in assessing thepossibility of shuttle accidents thatmight result in the release ofradioactive material from radioactivepower sources.• Nuclear Regulatory Commission, inanalyzing risks of low-level radioactivewaste disposal, evaluating performanceof high-level waste repositories, andevaluating risks associated with nuclearpower plant accidents.

The Assessment Process

Although the process used to assessrisks from engineered systems varieswith each user and application, itusually retains five common elements(Figure 2):• A description of the system’shardware components, operatingenvironment, and staff operators.

• A hazard identification analysis todetermine the events or conditions thatmight lead to accidents or failures.• An analysis to estimate the frequencyof events that must occur before healthimpacts could occur.• An analysis to determine the healtheffects, i.e., the consequences of theseevents to workers and the public.• A procedure to quantify assessedrisks, including the uncertaintiesinherent in any risk evaluation.

In 1983, the National Academy ofSciences published a document thatstandardized the process for health riskassessment. The book, RiskAssessment in the FederalGovernment: Managing the Process,1is also known informally as the “RedBook.”

The Red Book breaks the riskassessment process into four basicelements: • A hazards identification analysis todetermine whether a particularchemical is or is not causally linked toa particular health effect.• An exposure assessment to determinethe extent of human exposure before orafter the application of regulatorycontrols.• A dose-response assessment todetermine the relation between themagnitude of exposure to a chemical

13


Risk Assessment

UR expertise in risk assessment hasevolved over 20 years of experience.

Lawrence Livermore National Laboratory’sFission Energy and Systems SafetyProgram (FESSP) first helped the NuclearRegulatory Commission (NRC) to set upguidelines for safely siting and buildingnuclear power reactors. Today’s challengeis to meet increasing needs to evaluate thesafety risks of diverse, engineered systems.

Risk-analysis techniques have beenused by both government and industry tostudy and assess the safety, reliability, and effectiveness of various products,processes, and facilities. We performedoriginal probabilistic risk analyses in threeimportant areas: seismic safety in U.S.nuclear power plants, regulations intransporting spent nuclear reactor fuel,and, most recently, human-initiated risk inusing a nuclear medical device. Theseassessments have evolved into thedevelopment of new methods andtechniques, subsequently affectingregulatory developments and broadeningthe range of applications and usefulness forrisk analysis.

Health Versus EngineeringRisk Assessments

In many cases, a risk assessmentfocuses on the health effects that occurwhen toxic chemicals are released from a

12


From Reactor Safety to Health CareRisk Assessments:

In LLNL risk-assessment

experience, the most useful

aspects of risk assessment are

not exclusively the risk

numbers that are generated,

but also the insight gained

from a systematic and

methodical consideration of

what can go wrong with a

system.

O

Preventivebarriers

Preventivebarriers

AccidentInitiator

Mitigativecontrolss

ve Mitigativecontrols

Hazard ConsequencesHazard Consequences

Engineering Systems Risk Assessment

Health Risk AssessmentFigure 1. The interplayof the elements ofengineering and healthrisks that we evaluate.

Our Focus: EngineeringRisk Assessment

Depending on its application, anengineering risk-assessment study canfall into one of five classes: it can be aconceptual design evaluation, adetailed design study, a facilityoperations study, a managementsupport study, or a policy andstandards development study. Table 1offers examples of the applications oractivities appropriate to each class.Conceptual design evaluations anddetailed design studies tend to focuson equipment or one facility at a time;facility operations, managementsupport, and policy standards anddevelopment studies can focus on asingle facility or on multiple facilitiesand activities.

The FESSP specializes inintegrating advanced analytic methodswith an understanding of nucleartechnologies, economics, and policy-making. Over the last 20 years, wehave performed a number of originalrisk-assessment studies to supportregulatory developments at the NRC.We concentrate on safety issuesrelating to engineered systems thateither use or contain nuclear material,as shown in the following four cases:• An analysis to develop seismiccriteria for the siting and design ofnuclear power plants.• A risk analysis of reactor coolantpiping systems to establish new pipingdesign objectives and increase nuclearpower plant safety.• A study of risks involved in thetransport of spent reactor fuel todetermine the level of safety providedduring transport and the adequacy ofexisting transport regulations for suchmaterial.• The development of an approach toidentify human-initiated risks in theuse of nuclear medical devices such asthe Gamma Knife.2

Depending on the nature of theproblem, the detailed methods used ineach study vary in that they may includeany or all of the basic elements of theengineering risk-assessment process.However, each study is similar in that itconstitutes a rational and systematicapproach to obtaining information thatcan be used to increase safety,

formulate policy, develop standards,omit costly duplications, or implementregulatory guidelines.

Our evolving experience base thus provides the government withrecommendations of risk-basedregulations and prioritizations for resource allocations. It shows where regulatory reform can help the

15


Risk Assessment

and the probability of occurrence of thehealth effect in question.• A risk characterization procedure todescribe the nature and magnitude ofhuman risk, including any attendantuncertainty.

If we compare these health riskassessment elements to the five basic elements of engineering riskassessment, we find both similaritiesand differences between the twoprocesses. In an engineering riskassessment, the event consequence stepcontains the first three steps describedin the Red Book (Figure 2). In NRCstudies that analyze the impact from arelease of radioactive material, thisconsequence would be the dispersion ofmaterial in the environment; the uptakeof the material via inhalation, ingestion,or other exposure pathways; and theresponse of various body organs tosuch exposures. The results would leadto an estimate of the probability ofcancer incidence or fatality, given thatthe radioactive release had occurred.

Perhaps the most significantdifference between the two processes isthe treatment of event frequencies. Inan engineering risk assessment, theanalyst considers both the frequency ofan event (e.g., a large earthquakeoccurring near a nuclear power plant)and the probabilities of different

failures within the engineered system.Different combinations of failures canlead to health threats of differentseverity. For example, an earthquakecould produce a variety of damage in anuclear power plant, including nodamage at all. These damage statescould, in turn, lead to a variety ofpotential radioactive releases, or norelease at all. Thus, a single initialevent can lead to a variety of possiblehealth effects, each with its ownprobability.

On the other hand, in a health riskassessment, the analyst deals primarilywith situations involving chronicreleases to the environment with arelease probability of 1, that is, theassumption that such a release willabsolutely occur. This type ofassessment would propose to restrict oreliminate the material’s presence ratherthan mitigate with engineering controlsor boundaries.

The differences between engineered-system risk assessment and health riskassessment thus have a significantimpact on risk-management strategies.Although eliminating hazards is aneffective strategy, it is not alwayspractical in an industrialized economy.Engineering risk assessment supportsthe management of risk through design,maintenance, and administrative

controls. Reducing the possibility thataccident initiators and hazards cancause consequences—through effectiveand reliable engineered barriers andmitigative controls—provides a meansof managing risks in industrial activitywhile protecting the environment,safety, and health of the public.

Another important differencebetween the two processes has to do with consequence measures, orendpoints, of risk assessment. Healthrisk assessment is specific to exposuresfrom toxic chemicals and the associateddose response; hence, the ultimateendpoint can be cancer fatality. Inengineering risk assessment, theendpoint varies. Common endpointsinclude worker health and safety, loss of a facility or piece of equipment (forexample, the crash of an airplane andthe associated, implicit health effects),immediate loss of life (one of the resultsof a large earthquake), or long-term lossof life from cancer (one of the results of a nuclear power plant accident). In addition to these consequences,engineering risk assessment can haveother nonhealth-related endpoints. Forexample, the endpoint of a Departmentof the Interior risk assessment study ondam failure involved the economicimpact that failure would have on thesurrounding community.

14


Risk Assessment

System description

Hazard(s) identification

Event frequency

Event consequences

Risk quantification

Not applicable

Engineering risks Health risks

Not applicable

Not applicable

Hazard(s) identification

Dose response assessment

Exposure assessment

Risk characterization

Figure 2. Somecomponents ofengineering systemsrisk assessmentsoverlap those forhealth riskassessments. Table 1. Classification of engineering risk assessment by application or activity.

Type of engineering risk assessment Application or activity

Conceptual design evaluations • Determine the viability of a particular site for a particular facility.• Analyze and compare competing technologies or processes.• Evaluate the risks of emerging technologies.

Detailed design studies • Identify risk-dominant scenarios to provide guidance for refinements in the design of a system or facility.• Analyze and compare the reliability or availability of system/component options.• Provide specifications to design components, systems, or structures that will have high reliability and protection against severe natural phenomena.• Analyze and improve a facility’s training programs, operator–equipment interfaces, and operating procedures.• Determine optimum safety limits, equipment outage times, and testing frequencies to minimize risk.• Analyze acceptable risk to document the importance of risk-based design features and systems interactions data.

Facility operations studies • Carry out a risk-based analysis of operating events.• Design and implement risk-based trends and patterns.• Improve system availability.• Enhance component inspection, testing, monitoring, and maintenance based on component failure analysis.• Evaluate and prioritize safety issues.• Evaluate, select, and schedule modification.• Assess continued operations.• Enhance safety, emergency, and accident management information and training.

Management support studies • Provide risk-based perspectives for decision-making.• Provide information for allocating resources (staff, budgets) and identifying research needs.• Measure safety performance.• Perform risk-based quality assurance and audits.

Policy/standards development studies • Assess and develop rules, standards, and safety criteria.• Develop safety measures, goals, and criteria.• Assure coordination and consistency of safety goals and criteria.

Ann

ual p

roba

bilit

yof

exc

eeda

nce

Earthquakemagnitude

m

Dam

age

ratio

, %

0Earthquakemagnitude

Uncertainty band

Uncertainty band

Fault line

m

Ann

ual p

roba

bilit

y o

f exc

eeda

nce

Damage ratio, %

Case 2: Safety of ReactorCoolant Piping

This safety assessment was one ofthe first Laboratory studies in whichrisk-assessment techniques resulted inregulatory change. It is also a classicexample of a substitution risk, that is,substituting a change in risk for asavings in dollars.

The Code of Federal Regulations3

requires that structures, systems, andcomponents important to the safety ofnuclear power plants be designed towithstand the effects of naturallyoccurring hazards as well as the effectsof normal and accident conditions.Design criteria require that safety-related structures, systems, andcomponents of nuclear power plants bedesigned to withstand the effects of alarge loss-of-coolant accident. Toaccount for these effects, nuclear power plants have been designed toaccommodate postulated, double-

ended, “guillotine” breaks in their high-energy piping systems, particularly themassive ones about a meter in diameterthat circulate primary reactor coolant(see Figure 4a).

The difficulty—and cost—ofdesigning a nuclear power plant forpostulated pipe breaks was exacerbatedby a related requirement that thehydrodynamic loads be combined withthe vibratory loads that result from a“safe shutdown earthquake,” themaximum design-basis earthquake for a nuclear power plant. In effect, this requirement presumed that anearthquake could cause pipe breaks inall high-energy piping systems. Thisrequirement was also problematicbecause the design objectives for safepiping systems under normal conditionscontradicted those for safe pipingsystems under earthquake conditions.

During normal operation, pipingsystems must be flexible enough toexpand to relieve the thermal stressesthat can drive cracks through their wallsand cause leaks or breaks. However,during a large earthquake (which ismost likely a once-in-a-plant-lifetimeoccurrence), stiff piping is needed toassure that seismically induced breaks

17


Risk Assessment

government—and the country—workbetter and safer for less.

Case 1: Seismic Criteria forSiting Nuclear Power Plants

Since the early 1970s, the Laboratoryhas worked with the NRC to establishseismic criteria for regulating the sitingof nuclear power plants. Most of thesecriteria are deterministic in that they are based on the determined size and location of the most credibleseismic event, not on its frequency of occurrence or the possibleconsequences. In areas where very largeearthquakes have occurred (such asNew Madrid, Missouri, or Charleston,South Carolina) or cannot be excludedfrom occurring, even if the likelihood ofoccurrence is very small, the applicationof siting regulations based on thesecriteria could lead to very conservativedesign criteria and prohibitive costs.

To help the NRC evaluate the effectof such siting regulations, we proposedto assess the seismic hazard by using aprobabilistic methodology—that is, weweighted all the possible earthquakesthat could affect a site by theirlikelihood of occurrence. By couplingthis methodology with a newlydeveloped systems analysis concept,we systematically analyzed the seriesof causative events and the behavior of all structures, systems, andcomponents in the plant. We thenidentified the failure modes andquantified their consequences. Thetotal risk was obtained by consideringthe entire spectrum of earthquakes andall possible modes of failure andintegrating their calculatedconsequences (Figure 3).

Sponsored by the NRC, this firstU.S. seismic probabilistic riskassessment for nuclear power plantsfrom 1978 to 1985 cost $18 million.The same methodology was then usedby the nuclear industry to assess 35 nuclear power plant sites. The

majority of seismic probabilistic risk-assessment knowledge existing in thetechnical community today was gainedthrough this massive exercise.

Our methodology is now widelyused by the NRC and other publicutilities to evaluate and compare, on arelative scale, the risks associated withexisting nuclear power plants. In manycases, its use has led to retrofitting,reinforcement, and redesign ofcomponents or systems to achievecomparable levels of risk across theentire population of plants.

Currently, we are helping the NRCto overhaul the seismic siting criteria

16


Risk Assessment

Figure 3. Graphs show relationships among(a) probabilities, (b) consequences, and (c) total risks of earthquakes and other seismiccauses. In (a) and (b), the point labeled m,representing a particular earthquakemagnitude, corresponds to a mean probabilityin (a) and a mean consequence in (b). Graph(c), representing the combination of graphs (a)and (b), gives probability of damage to areactor from an earthquake in the vicinity.

(a) A reactor coolant piping system

(b) Failure probability

Pressurizer

Cold leg

Crossover leg

Applied stressmeasure (ß)

prob (ß) prob (S) prob (ß)ß

S

Sß

prob (Y)

Strengthmeasure (S)

Stress (ß),strength (p)

Safety margin(Y = S – ß)

0 0 0 0

Hot leg

Reactorcoolantpump

Reactor pressure

vessel

Steam generator

Figure 4. (a) LLNL developed standards andproposed regulations concerning the high-energy piping systems that circulate primaryreactor coolant. (b) Probabilistic approach for assessing component adequacy forpostulated load conditions in piping. In thisapproach, failure is possible only in the regionshaded red.

for new nuclear power plants. Ourexperience base has been used to helpdevelop proposed risk-basedregulations now under public review.Previous regulations were based onmethodologies that rely on singledeterministic models. Often suchmodels pit one group of experts againstanother group, creating time delays andthus protracting the plant licensingprocess. The proposed changes toregulations are based on a methodologythat provides a framework for assessingall information and makes maximumuse of existing data and factors from all possible modeling and scientificalternatives. As such, the changesshould help streamline the plantlicensing process.

(a)

(b)

(c)

has resulted in savings of tens ofmillions of dollars for each nuclearpower plant.

Case 3: Assessments forTransporting Spent NuclearFuel

Tens of thousands of spent nuclearfuel assemblies from U.S. nuclearpower plants are currently being storedat the plants. In the near future, thesespent fuel assemblies will be placed in a federal repository for permanentstorage.

From 1985 to 1987, we performed atransportation model study for the NRC to determine the level of safetyprovided when spent reactor fuel is transported to a nuclear wasterepository. During transport, theprotective casks carrying the fuel couldbe exposed to highway or railwayaccidents. Our task was to evaluate anddocument what might happen to thecasks under severe conditions and toassess how effectively the currentfederal transport regulations wouldprotect the public.

This assessment represented adeparture in risk-assessment techniquesfrom reactor safety studies. The nuclearpower plant probabilistic risk assess-ment addresses stationary facilities,with system functions and potentialfaults fairly well understood. In thisassessment, a first in transportationregulations, we studied scenarioshaving nuclear material moving

Rigid Piping Program,” “PipingReliability Program,” and “LoadCombination Program” carried outbetween 1981 and 1985 at a cost of $3.5 million.

The results of this analysis indicatedthat the probability of this kind of breakin a PWR’s coolant loop piping is lowenough under all plant conditions,including earthquakes, to justifyeliminating it as a basis for plant design.Our analysis also showed that theprobability of a pipe break being causedby an earthquake is significantly less,by a factor of 10 to 100, than theprobability of a pipe break being causedby thermal stress. The results of acompanion probabilistic analysis of stiffversus flexible piping supported theopinion that inadvertent stiffness(resulting, for example, from failed pipesnubbers) can indeed reduce nuclearpower plant safety.

On the basis of these technicalresults, we recommended that the NRCeliminate the double-ended guillotinebreak requirement in the reactor coolantloop of PWR designs. After anexhaustive peer review of the results by technical experts, the provisions of General Design Criterion 4 weremodified by excluding from the designbasis any dynamic effects associatedwith loss-of-coolant accidents. Ourtechnical analyses made it possible toapply the new exclusion rule to themain reactor coolant loop piping in allU.S. PWR plants.

The rule change also indicated theremoval of pipe snubbers—a decisionthat had two major effects. First, itreduced the amount of time thatmaintenance and inspection personnelhad to spend in high radiation areas,thus reducing their exposure toradiation. Second, the nuclearpower industry no longer had todesign, fabricate, install, and maintainthe costly snubber equipment. Industryspokespersons say that the rule change

through populations, with variouspotential highway and rail accidents.

Spent fuel shipments, now occurringat a very low rate, are regulated by boththe Department of Transportation(DOT) and the NRC. The NRCevaluates and certifies the design of theshipping casks used to transport spentfuel, and DOT regulates vehicles anddrivers. Current NRC regulationsrequire that shipping casks meet certainperformance standards. For example,under normal operating conditions andhypothetical accident conditions, a caskmust limit releases of radioactivematerial and minimize externalradiation levels, and it must assure thatthe spent fuel will remain subcritical(not undergo a self-sustaining nuclearchain reaction).

The study evaluated the possiblemechanical and/or thermal forcesgenerated by actual truck and railroadaccidents. The magnitudes of forcesfrom actual accidents were comparedwith forces attributed to the hypo-thetical accident conditions defined inthe NRC and DOT regulations (Figure 5). The frequency of accidents

that can produce defined levels ofthermal or mechanical force was alsodeveloped. With this information, thestudy results showed that for certainbroad classes of accidents, spent fuelcasks provide essentially completeprotection against radiological hazards.For extremely severe accidents imposingforces on the cask greater than thoseimplied by the hypothetical accidentconditions, we made calculations of thelikelihood and magnitude of anyradiological hazard.

The study also contained anevaluation of the radiological risk from accidents during transport. Riskrepresents the summation of the productsof the magnitude and likelihood of allaccident outcomes. The purpose formaking the risk calculations was tocompare the resulting values with thosepreviously used by the NRC in judging the adequacy of its regulations. Weconfirmed the adequacy of existingregulations. Our methods subsequentlyhave become the basis for othertransportation risk studies required by DOE/Defense Programs andDOE/Office of Civilian RadioactiveWaste Management.

Case 4: Identifying Risks ofUsing Nuclear MedicalDevices

Our experience with analyzing risksof radiation-emitting systems has led to performing other radiation-basedanalyses. In one of these cases,involving a medical application, wefound it necessary to develop newtechniques to evaluate the potentialrisks of a relatively new device forwhich operators have limited operatingexperience and processes havesubstantial human-factor considerations.

As part of its public health and safetycharge, the NRC is responsible forregulating radiation from nuclearbyproduct material. Current NRCregulations address procedures forconventional cobalt-60 teletherapydevices, but do not necessarily addressappropriate or comparable proceduresfor the Gamma Knife (Figure 6), acommercially available external-beamradiation device. It is used to locate andsurgically treat inaccessible lesions inthe brain while sparing healthy tissuealong some 200 radiation entry paths.

19


Risk Assessment

do not occur. Designers have met these cross purposes by using “pipesnubbers,” elaborate mechanical and/orhydraulic devices that allow pipes tomove during normal operation butanchor them rigidly when they aresubjected to rapid (i.e., seismic) loads.Pipe snubbers not only require periodictesting and maintenance—in areas ofhigh radiation and difficult access—buthave proved unreliable. Many havebeen found to lose their earthquake-resisting function; others have beenfound to restrict normal thermalexpansion and seriously increase pipestresses. (In the latter mode, then, thesesafety devices can actually increase thelikelihood of pipe failure.)

For years, nuclear plant designershave contended that the likelihood ofseismically induced breaks is lowenough to be considered negligible.They believed that protective measuressuch as pipe whip restraints and jetimpingement barriers may actuallydecrease the reliability of pipingsystems. In the early 1980s, the nuclearindustry sought to exempt itself fromthe NRC piping safety regulations by doing extensive research indeterministic fracture mechanics so thatit could argue the merits of a “leak-before-break” concept. That is, becauseof the very tough materials used innuclear piping, even large cracksthrough walls would remain stable andnot result in a double-ended guillotinebreak. The NRC sought additionaltechnical information to respond to theexemption request.

The FESSP engineers, in anindependent confirmatory researcheffort funded by the NRC Office ofNuclear Regulatory Research,developed and applied risk-assessmenttechniques (Figure 4b) to estimate thelikelihood of a double-ended guillotinebreak in the coolant loop piping of apressurized water reactor (PWR). Thiseffort consisted of the “Flexible vs

18


Risk Assessment

Figure 5. Comparisons weremade of forces from actualtransportation accidents andhypothetical conditions for a riskassessment of transporting spentnuclear fuel. Damage to the caskdepends on the velocity of thecask and its orientation when itimpacts a hard surface.

å

ı

å = Impact angleı = Cask orientation angleImpact velocity

Cask velocity

Object surface

physical processes resulting from severallow-frequency failures, or long-termhealth effects from potentially toxicmaterials. Furthermore, because a riskassessment often deals with low-frequency but high-consequenceaccident risks, there is considerablepotential for its results to bemisunderstood.

In our experience, the most usefulaspects of risk assessment are notexclusively the risk numbers that aregenerated; they are also the insightgained by a systematic and methodicalconsideration of what can go wrong witha system. A procedural analysis helps usto understand the likely vulnerabilitiesof the system, the threats they pose, andthe measures that could be applied tomitigate or prevent them.

Risk assessment is a particularlypowerful tool when there is only alimited set of alternatives for riskevaluation. “Real-world” managers, too,often have only limited resources toimprove safety. Ultimately, the “best”choice will depend on the context of themanager’s problem, as illustrated by ourpiping safety study.

We have found that it is important todo sensitivity, or “what-if,” analyses todetermine the relative importance ofinput to a risk assessment. Varied inputallows us to (1) distinguish risks fromvariations in assumptions, modeling, or data; (2) identify where a lack ofinformation is crucial; (3) determinewhich factors contribute the most to risk;and (4) investigate potential preventiveor mitigative solutions that combinevarious risk-reduction measures.Because evaluations of alternatives orsensitivity analyses do not requireabsolute risk values, we can use relativerisk estimates or risk rankings tocompare risks. Relative risk estimatesare adequate to compare alternativeapproaches to the same problem or toachieve comparable levels of risk across

a population of similar systems. Thus,meaningful insights can be obtained bya risk assessment without depending onthe accuracy of an “actual risk” value—such values are notoriously difficult to ascertain.

Uncertainty is a very important partof any risk assessment, particularlywhen there is an attempt to accuratelyquantify an actual risk. Uncertaintystudies should be performed to evaluatethe dependence of the assessmentresults on uncertainty values. Sources ofuncertainty occur in models, methods,and data. Given the uncertaintiesinherent in any risk assessment, expertanalysts may disagree over riskcharacterization values. Sometimesconsensus is obtained by defaulting to the most conservative estimates.Such practices tend to “ratchet-up”prescriptive risk standards.

Because a risk analysis receives somuch scrutiny, the risk assessment mustbe documented and understood. It isalso extremely important to have theassessment reviewed by independentagents both internal and external to theorganization performing the assessment.

Finally, the results of a riskassessment are only one of many inputsto a decision. Other factors—which may

have nothing to do with technical riskper se—include cost considerations,compliance with rules and regulations,mission objectives, businessoperations, and public perceptions.The relationships among these factorscan be complex, and the relative valueof each is context dependent.Integrating these factors into thedecision-making process is essential.

Key Words: engineering risk assessment;Fission Energy Systems Safety Program(FESSP), Nuclear Regulatory Commission(NRC); probabilistic, risk, risk assessment,

References1. National Research Council, Risk

Assessment in the Federal Government:Managing the Process, NationalAcademy Press, Washington, DC(1983). This guideline has recentlybeen augmented by a new NationalResearch Council study, Science andJudgment in Risk Assessment, NationalAcademy Press, Washington, DC(1994).

2. Registered trademark of ElektaInstruments, Inc.

3. Code of Federal Regulations, Title 10,Part 50.

For further information contact Edwin Jones (510) 422-8259([email protected]).

21


Risk Assessment

Reports received by the NRC pointed tosome cases of misadministration inconventional teletherapy that haveresulted from equipment malfunctions or human errors in treatment planning,dose calculations, and measurements. Itwas reasonable to project thatcomparable events may occur with theGamma Knife.

The NRC therefore asked us toperform a preliminary risk analysis ofthe use of the Gamma Knife. Our reviewof cases of misadministrations andabnormal occurrences for conventionalteletherapy indicated that the assessmentof the risks of such an external beamtherapy system should be balancedbetween equipment failures and humanmistakes, if not skewed toward thehuman errors.

The Gamma Knife is used to delivergamma radiation from cobalt-60 toprecisely defined, intracranial targets. Its relatively simple hardware systemrequires significant human control, butbecause the instrument is relatively new,very little operating failure data existsfor it. Most operational informationresides in the, as yet, limited and little-documented experience base of themanufacturer and operators. FESSP wasasked to identify the high-risk, human-initiated actions and failure modes that

are most likely to occur and to evaluatetheir relative importance.

To do that, we adopted an approachthat relied on empirical evidence,observations, and expert experience. Inthis approach, an analysis of the GammaKnife treatment tasks provided asystematic framework that couldadequately account for and describeactivities and equipment that could leadto undesirable events or consequences.We relied on experts’ estimates oflikelihood, consequence, and risk for theprimary tasks, and compared them bymeans of relative risk rankings and riskprofiles. These estimates aided theidentification of the highest-risk orcritical tasks, without requiring anabsolute quantification of risk for each task.

We believe the approach may be bestused to identify weaknesses in processesand to support the development ofpositive performance measures, ratherthan to predict the numerical riskassociated with poor performance.Perhaps most effective in nuclearmedical applications that are not highly structured, the approach couldserve to produce reliable processes and procedures to preventmisadministrations that result frommistakes. We have yet to apply theseprinciples and techniques elsewhere, butwe expect them to be applicable where

human-initiated actions are important.The lesson learned is that informativeassessments can be made from a relativerisk analysis; the approach is alsoinexpensive and practical.

When to Perform RiskAssessments

Risk assessment is an excellent risk-analysis tool in that it allows us to • Systematically examine a broad set ofdesign and operational features.• Integrate the influence of systeminteractions and human–systeminteractions.• Explicitly consider uncertainties inestimates.• Consider and analyze competingrisks—those of one system versusanother, or of one set of modificationsversus another.• Measure the relative importance ofsystems, components, and otherengineered elements to risk.• Quantify the overall level of risk for asystem.• Identify relative risks versus costtradeoffs in design and operationalmodifications.

However, risk assessment also has itslimitations. It may sometimes exclude ornot adequately quantify potentiallyimportant risk factors, such as very-low-frequency accident initiators, variousfailures derived from a common event,

20


Risk Assessment

Figure 6. Rendering of the Leksell GammaKnife, which is used for intracranial radiationtherapy. We identified risks and their relativeimportance for operating this system.

About the Engineer

ED JONES came to the Laboratory in 1991, when he joined theFission Energy and Systems Safety Program. Since 1993, he hasbeen the Deputy Associate Program Leader for Risk Assessment,System Engineering, and Human Performance. He has written ten papers on risk assessment since arriving in Livermore.

Jones received his B.S. degree in Engineering and Physics in1975 from the University of California, Berkeley. He did

graduate work in engineering and physics at Stanford University from 1975 to 1978and doctoral research in physics from 1979 to 1981 at the University of Oxford.Before coming to LLNL, Jones worked at Eyring Research Institute from 1983 to1987 and at BDM International, Inc., from 1987 to 1990; he was president of Jonesand Associates from 1990 to 1991.

23

the end of last year, they moved the adaptive optics to the 3-mtelescope and, again using natural guide stars, corrected to itsdiffraction limit in the near-infrared. The last step was toinstall the laser under the guidance of Herb Friedman, projectscientist for the laser subsystem.

According to Olivier, the adaptive optics on the 3-mtelescope allow astronomers to resolve objects more than 10times smaller than before, when viewing in the near-infrared.With the addition of the laser guide star, astronomers are nowable to perform these high-resolution observations over alarge fraction of the sky. “This combination,” notes Olivier,“makes this system arguably the world’s most powerful toolfor high-resolution, near-infrared astronomy.”

Now that the Lick system is up and running, the Livermoreteam and other UC astronomers are beginning high-resolution,near-infrared observations of star-forming regions, quasars,and other interesting astronomical objects. Preliminary resultsfrom this research will be available later this fall.

In addition, based largely upon experience gained inbuilding the Lick system, the Livermore team was recentlyawarded a contract to build the major components of a laserguide star adaptive optics system for the largest telescope inthe world, the 10-m Keck telescope in Hawaii, owned by UCand the California Institute of Technology. This system,scheduled for completion in 1997, will become the world’smost powerful tool for high-resolution near-infraredastronomy as we enter the 21st century.


Guide Star

Creating a Guide StarLaser guide star efforts have generally focused on two

methods of creating artificial stars. The first method usesvisible or ultraviolet light to reflect off air molecules in thelower atmosphere from fluctuations (Rayleigh scattering),creating a star at an altitude of about 10 km. The othermethod uses yellow laser light to excite sodium atoms atabout 90 km. The sodium-layer laser guide star turns out tobe crucial for astronomy, because astronomers need largetelescopes to see objects that are very far away and thereforevery dim. These large telescopes require the laser guide starto be as high as possible so that the light from the laser starand the observed object pass through the same part of theatmosphere. With a guide star at the lower elevation, thesystem senses and corrects for only about half of theatmosphere affecting the light from a distant object.

The Laboratory’s key contribution to this field has beenthe introduction of the sodium-layer laser guide star based onAVLIS dye laser technology. Claire Max, the project’sprincipal investigator and the current Director of UniversityRelations at LLNL, was a co-inventor of the idea of using alaser guide star in the sodium layer of the atmosphere forastonomical telescopes.

2.0 ¥ 104

1.5 ¥ 104

1.0 ¥ 104

5.0 ¥ 103

0

0.125 arcsecper pixel

AO off

AO on

22 Research Highlights

ARTH-BOUND astronomershave long sought to diminish

the effects of the atmosphere ontheir observations. Stars that appearas sharp pinpricks to the eye becomesmeared “blobs” by the time they areimaged by large ground-basedtelescopes.

At the University of California’sLick Observatory on MountHamilton near San Jose, California,Laboratory researchers and their UCcolleagues are installing a system onthe 3-m Shane telescope that willcorrect these troublesomedistortions. The system includes adye laser that will create a “guidestar” in the upper atmosphere andvery sensitive adaptive optics thatwill measure and correct foratmospheric distortions. Accordingto Scot Olivier, project scientist forthe adaptive optics subsystem, the Shane is the first majorastronomical telescope with such a laser system.

Other groups have been using adaptive optics systems withnatural guide stars. However, it turns out that not just any starwill do. It must be bright enough, that is, generate enoughlight to serve as a reference. When observing at visiblewavelengths, astronomers using adaptive optics require afifth-magnitude star, one that is just bright enough to be seenunaided. For near-infrared observations, only a tenth-magnitude star is needed, which is 100 times fainter.

The problem, Olivier noted, is that even though there maybe hundreds of thousands or even a million stars brightenough to be guide stars, they only cover a small fraction ofthe sky. “Many times, there just isn’t a natural guide star inthe area you want to observe,” he said. “This is the kind ofsituation where a telescope equipped with a laser guide starcomes out ahead.” (See box.)

The guide star is created by a dye laser system, which is asmall, closely related version of the system used by the

Laboratory’s Atomic Vapor LaserIsotope System (AVLIS) program. AtLick, green light from solid statelasers beneath the main floor of thetelescope travels through fiber opticsto a compact dye laser mounted on theside of the Shane telescope. A beamprojector then directs the yellow dyelaser light up through the atmosphere.At about 100 km, the laser beam hits alayer of sodium atoms created bymicrometeorites, which vaporize asthey enter the upper atmosphere. The yellow laser light, tuned to 0.589 micrometers, excites the sodiumatoms, which then emit this yellowlight in all directions, creating aglowing guide star in the upperatmosphere wherever the astronomerneeds it.

Some of the light from thisartificially created star travels back

through the atmosphere into the Shane telescope. There, anadaptive optics system measures and corrects the guide starimage for atmospheric distortions caused by air turbulence andtemperature changes. Small sensors continuously monitorchanges in the direction of light waves from the guide star.The sensors send this information to a computer, which in turncontrols the movements of hundreds of tiny actuators attachedto the back of a flexible mirror. Moving hundreds of times asecond, the actuators deform the surface of the mirror to“smooth out” the image of the guide star.

When the telescope is viewing a celestial object, light from the guide star and the object travel through the sameturbulence and receive the same corrections from thedeformable mirror. The result is a clearer image of the objectas well.

Last year, Livermore scientists, operating the adaptiveoptics system on Lick’s 1-m telescope, observed objects atvisible wavelengths. Using natural stars as guides, theycorrected images to the diffraction limit of the telescope. At


(Top left) The black box, highlighted inwhite, holds the laser package thatLivermore researchers developed forthe 3-m Shane telescope at LickObservatory. (Bottom left) Outdoors,the laser beam can be seen for miles.

(Right) The graph shows the data taken on the Shanetelescope in the near infrared (2.1 micrometers) forLambda Bootis, a nearby star in the same constellationas Arcturus. The vertical axis shows the intensity of lightin one pixel, which corresponds to the telescope’sdiffraction limit. The smaller bump shows the datagathered with the adaptive optics (AO) system turned off.The second image, taken with the adaptive optics systemturned on, shows most of the light concentrated in onepixel. If the image were taken in space, away from theatmosphere, that peak would be about four times higherthan it appears here.

Crucial Steps Takenin Laser Guide StarSystem

Crucial Steps Takenin Laser Guide StarSystem

E

For further information contact Scot Olivier (510) 423-6483([email protected]).

25


example, ingestion of or exposure to certain chemicals,including illegal drugs, can be identified in human hair. Sincehuman hair generally grows at about one-half inch per month,analysis of a person’s hair along its length can provide achronology of drug use over time (see photos above). Or thehair of a dog known to have been kept as a pet at a suspecteddrug manufacturing facility can be analyzed to determinechemicals associated with chemical spills and exposures at thedrug lab. Positive identification of chemicals in the dog’s hair,indicative of the lab’s operations, could serve as criminalevidence in a trial.

Although this technique is still in its infancy, its potential could beenormous. As lasers become easier touse, smaller and smaller particles andfibers will be sampled and characterizedin forensic investigations.

Miniaturizing the GC/MSThe Forensic Science Center is also at

the forefront in developing new, portablesystems capable of real-time analysis inthe field. These units have numerousapplications, from identifyingmaterials to support verification of theChemical Weapons Convention toinvestigating criminal activities.

Almost five years ago, the Center developed a suitcase-size gas chromatograph/mass spectrometer (GC/MS) for on-site identification of ultratrace (microgram or less) quantitiesof certain compounds in complex mixtures. The systemweighed 68 kg (150 lb), which made it portable, but onlybarely. Three years later, the system’s weight had been cut by more than half to 32 kg (70 lb), still a hefty load. Today, at 20 kg (44 lb), with an accompanying laptop computer, this system can realistically be considered portable. Thisrugged, all-metal vacuum vessel can be carried on board anairplane and put into the overhead compartment, while itsaccompanying generator and off-line vacuum reconditioningpumping unit travel in the baggage compartment.

Reduction in size does not mean a reduction inperformance. The latest complete GC/MS unit is able toachieve the almost-perfect vacuum required for accurateanalysis. It can run for 12 hours in the field, and, like a 500-lb bench-top model, can perform up to 200 operator-assisted analyses per day. While the operator sleeps, the

Forensic Science Center Update

The photo above shows the imaging laser-ablation massspectrometer system in action. From the left, the 2.5-gigawatt-per-cm2 Nd:YAG (neodymium yittrium-aluminum-garnet) laserbeam is passing through steering and alignment optics in theimaging system and into the mass spectrometer. A display ofthe target material is visible on the videoscreen. To the left is amagnified human hair after bombardment by the laser beam.

24


Research Highlights

OR most people, “forensic science” means cops andfingerprints and DNA analysis. All of that is still true,

but these days forensic science encompasses much more.Some “whodunits” are more complicated and can involve aninternational cast of characters. Forensic science now also isused to verify and monitor compliance with such internationalagreements as the Nuclear Non-Proliferation Treaty and theChemical Weapons Convention, and to learn whether acountry is developing a clandestine nuclear weapons program.

The Laboratory’s Forensic Science Center was establishedin 1991, and in its short life has become a leader in lawenforcement, national security, defense, and intelligenceapplications. Using sophisticated analytical equipment,experts in organic, inorganic, and biological chemistry candetermine the composition and often the source of the mostminute samples of material. Lasers are also being used to“interrogate,” or examine, a variety of materials.

The March 1994 issue of Energy & Technology Reviewdescribed in detail the workings of the Forensic ScienceCenter. It reported on the Center’s excellent performance in a“round-robin” series of exercises with analytical chemistryfacilities from around the world. The Center has done so well in these exercises over the years that it is no longer just a participant. Its staff also prepares samples for otherlaboratories to analyze. Following is an update on activities at the Forensic Science Center since early 1994.

What You See Is What You GetBy combining three technologies into a single system—an

ion trap mass spectrometer for analysis, a high-poweredmicroscope for viewing, and a laser for ionizing samples—theCenter has created something entirely new for forensicanalysis: imaging laser-ablation mass spectroscopy.Conceived in 1994 and still being refined, this new processallows considerably more accuracy in analyzing samples thanstandard mass spectroscopy.

Sampling material is placed on the tip of a probe that is inserted into the source region of an ion trap massspectrometer. With a microscope outside the vacuumchamber, the sampling material is viewed from above at 250 ¥magnification. A laser beam is then directed at precisely the10- to 50-micrometer spot on the probe tip from which thesample’s mass spectral data is desired. The intensity of thelaser beam can be adjusted to instantaneously vaporize moreor less sampling material, depending on the size of thesample. The laser ionizes the material, and the massspectrometer sorts these fragments according to theirmolecular weights. Once sorted, each chemical componentproduces a characteristic mass spectral fragmentation patternthat is used by the operator to identify the entire sample.

There are several benefits of this method. The new imagingcapability allows for a more accurate focus of the laser beam,which means more accuracy in sampling and more accuracyin analysis. By having the sampling material inside the massspectrometer vacuum chamber before it is hit with the laserbeam, sample losses are far less than when the sample isbombarded outside the ion source and then transferred to themass spectrometer. We can also analyze smaller particles andfibers with this system than we can with a standard bench-topion trap.

This new system has numerous applications. One possibleuse is to provide a chronological record of chemical exposuresby analyzing hair, vegetation, and other materials. For

F

ForensicScienceCenterUpdate

ForensicScienceCenterUpdate

All of the travelingcomponents of LLNL’sminiaturized gaschromatograph/massspectrometer can fitinto a metal travelcase. Included are (ontable) the GC/MS andlaptop computer and(on floor) the pumpingunit and generator.

2726


turbomolecular pumping station refreshes the vacuum andother systems in the unit for another 12 hours of operation.

And how have they made this unit so small? When LLNLfirst took on the job of making a portable GC/MS system, veryfew off-the-shelf parts were available that, when assembled,would fit into anything the size of a suitcase. Almost all of the pieces that went into the first 68-kg unit were thereforedesigned and manufactured at LLNL. Meanwhile,miniaturization began to catch on in the GC/MS industry, so many of the components of the 32-kg version could bepurchased from outside sources. While a few components ofthe latest 20-kg model had to be produced here, most havebeen purchased commercially, modified as necessary, andfitted together.

The unit’s hydrogen supply for the portable gaschromatograph is typical of the shrinking components. Thehydrogen supply in the 68-kg model weighed 14 kg. Today itweighs just 0.4 kg and still operates at 250 psi, just like itsbigger bench-top brother.

The Center also has produced another unit whose parts canbe replaced in the field. Parts are fitted together with O-rings,which facilitates repair, but more pumping capacity is neededto hold the desired vacuum. So there is still much work to do.

Counter-Forensic InspectionIn the summer of 1994, DOE asked the Forensic Science

Center to perform a preliminary “counter-forensic” analysis tohelp the government investigate vulnerabilities of two

gaseous-diffusion, uranium-enrichment plants that will besubject to international inspections. Although inspections ofthe plants are expected to be visual only, DOE wanted toknow whether a hypothetical inspector with a differentagenda, while walking through one of its plants, couldsurreptitiously collect samples of material, take them home,examine them, and replicate the enrichment process. TheCenter’s mission was to examine the similar samples and learn critical details of the enrichment process.

In the gaseous diffusion enrichment process, uraniumhexafluoride passes through a series of semipermeablebarriers, the number of barriers being determined by theenrichment required. Uranium used in power reactors requiresless enrichment than weapons-grade uranium, which is highlyenriched.

The Center used for its analysis a variety of materialscollected from different areas in the plant. With minutequantities of these materials and state-of-the-art analyticalequipment, our chemists, engineers, and metallurgists wereable to determine whether or not various aspects of theenrichment process are vulnerable to surreptitious collections.We expect these results to be useful in determining futureinspection protocols.

Forensic Science Center Update

For further information contact Brian Andresen (510) 422-0903([email protected]).

Awards and Patents

PatentsPatent issued to Patent title, number, and date of issue Summary of disclosure

Raymond P. Mariella, Jr. Infrared-Sensitive Photocathode A single-crystal, multilayer device incorporating an IR absorbing layer that is Gregory A. Cooper U.S. Patent 5,404,026 compositionally different from the GaxAl1-xSb layer, which acts as the electron

April 4, 1995 emitter. Different IR absorbing layers can be used, limited only by the ability to grow quality material on a chosen substrate.

Arnold C. Lange Base Drive Circuit Electronic circuitry for controlling bistable switching circuits, particularly high-U.S. Patent 5,404,052 power circuits for an electron-beam gun or electric motor. The switching control April 4, 1995 circuit ensures that bipolar power sources are not shorted together. A pair of

solid-state output devices are connected to a level shifter through a pair of nonlinear delays.

Blake Myers Ceramic Tile Expansion Engine Housing A high-temperature engine housing, including interlocking ceramic tiles that form U.S. Patent 5,404,793 an expandable ceramic housing; a pressurizable external metal housing that provides April 11, 1995 a support for the ceramic tiles; and means for thermally insulating the metal housing

from the ceramic housing.

Stanley W. Thomas Image Intensifier Gain Uniformity Improvements A microchannel-plate image intensifier having a photocathode that is damaged to in Sealed Tubes by Selective Scrubbing reduce high-gain areas. The high-gain sections are selectively scrubbed with a U.S. Patent 5,408,087 controlled bright light source.April 18, 1995

Ronald E. English, Jr. Optical Power Splitter for Splitting A prism segmenter having a plurality of prisms arranged about a central axis John J. Christensen High Power Light for forming a plurality of divided beams of light from a single beam of light

U.S. Patent 5,408,553 without using complex optical pathways or sensitive components.April 18, 1995

Thomas M. Tillotson Method for Producing Metal A two-step hydrolysis-condensation method to form metal-oxide aerogels of John F. Poco Oxide Aerogels any density, including densities of less than 0.003g/cm3 and greater thanLawrence W. Hrubesh U.S. Patent 5,409,683 0.27g/cm3. A condensed metal intermediate is formed and can be stored for Ian M. Thomas April 25, 1995 future use.

John S. Toeppen Method and Apparatus for Holographic Wavefront A wavefront diagnostic apparatus and method for determining the parallelism Diagnostics of the rays of light within a beam of light by comparing projected and reference U.S. Patent 5,410,397 holographic images. Other geometric parameters can be measured using these April 25, 1995 diffractive optical elements.

Mark A. Rhodes Magnetron Cathodes in Plasma An electro-optic switch using magnetron cathodes as plasma electrodes. Electrode Pockels Cells A low-pressure ionized gas is formed on both sides of the crystal. A magnetic fieldU.S. Patent 5,410,425 is produced by permanent magnets or electromagnets near the surface of the April 25, 1995 cathode.

Awards

The 1995 R&D Awards were given to five Laboratory groups for 1995 achievementsin scientific and technological breakthroughs. Given by the Chicago-based R&DMagazine, 100 of these prestigious awards are presented each year. Following are thetechnologies and LLNL personnel involved.

• Sealed Tube Electron Beam Guns for Material Processing: Booth Myers, Hao-LinChen, James Davin, and Glenn Meyer, and George Wakalopulos and Peter Bond,American International Technologies, Inc.

• Miniature Ion Cyclotron Resonance Mass Spectrometer: Daniel Dietrich andRobert Keville.

• All-Solid-State Laser with Diode Irradiance Conditioning: Raymond Beach,Christopher Marshall, Mark Emanuel, Stephen Payne, William Benett, BarryFreitas, Steven Mills, Scott Mitchell, Charles Petty, John Lang, and Larry Smith.

• High Average Power Solid-State Laser with High Pulse Energy and Low BeamDivergence: Clifford Dany, Lloyd Hackel, and Mary Norton.

• Aerogel Process Technology: A shared award for two different processes (1)Injection Molding Process for Rapid Production of Net-Shaped Aerogels: LawrenceHrubesh, Paul Coronado, and John Poco. (2) Capacitive Deionization with CarbonAerogel Electrodes: Joseph Farmer, Richard Pekala, David Fix, Gregory Mack,John Poco, William Grant, and Charles Pomernacki.

The Laboratory received a 1995 National DOE Pollution Prevention Award forachievement in recycling hazardous materials. Specifically recognized were eightemployees in Plant Operations that received the National Award forRadioactive/Hazardous Waste Recycling. They are Keith Gilbert, Mike Hayes, RodHollister, Charlie Patterson, Linda Souza, and Robert Wiebers. The Laboratory’sHazardous Waste Management’s CHemical Exchange Warehouse (CHEW) received aspecial mention, as did its developers Marjorie Gonzalez and Mike DeMicco.


28


U.S. Government Printing Office: 1995/683-074-20017

Scanning Tunneling Microscopy: Opening aNew Era of Materials Engineering

Incorporating an STM into an ultra-high-vacuumenvironment that contains a suitable combination of facilities for sample preparation, material deposition, andcomplementary diffraction-based surface diagnostics hasenabled us to analyze the atomic details of the growth of avariety of thin films for diverse applications. We are currentlyapplying this combination of techniques to evaluate howprocessing parameters, such as substrate temperature and filmdeposition rate, affect the atomic structures of interfaces in thefabrication of multilayers for x-ray optics, microelectronics,and magnetic recording devices. The results of these studiesallow us to identify the surface defects that have a criticalinfluence on film growth, to investigate their origins, andultimately to control their occurrence. By doing so, we canimprove new materials and devices and give them betterperformance characteristics.

Contact: Peter J. Bedrossian (510) 423-5938 ([email protected]).

Risk Assessments: From Reactor Safety toHealth Care

The Laboratory’s Fission Energy and Systems SafetyProgram (FESSP) performs engineering risk assessments tostudy and assess the safety, reliability, and effectiveness ofvarious products, processes, and facilities. Evolving methodsand techniques are discussed in the context of four cases: ananalysis to develop seismic criteria for siting and design ofnuclear power plants, risk analysis of reactor coolant pipingsystems to establish new piping design objectives andincrease nuclear power plant safety, study of risks involved inthe transport of spent reactor fuel to determine the level ofsafety provided during transport and the adequacy of existingtransport regulations for such material, and development ofan approach to identify human-initiated risks in the use ofnuclear medical devices such as the Gamma Knife.

Contact:Edwin Jones (510) 422-8259 ([email protected]).

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