652 atw 57. Jg. (2012) Heft 11 | November

New Reactor Design Developments

New perspectives for nuclear reactor designEmilio Baglietto, Cambridge, MA/USA

DieEntwicklungneuerReaktorlinienistunteranderemgeprägtvomEinsatzneuarti-gerVerfahrenbeiEntwicklungundKonstruk-tion,die zudeutlich flexiblerenundange-passtentechnischenLösungenführen.DabeiermöglichtdieIntegrationvonAnalyse-undSimulationstechnikenunter anderem eineweiteroptimierteEffizienzderReaktor-undAnlagentechniksowiedesKernbrennstoffein-satzesalsaucheineWeiterentwicklungderSicherheitüberdasschonerreichteunder-forderlicheNiveauimauslegungsüberschrei-tendenBereichhinaus.DieErgebnisseumfangreicherForschungs-

arbeitendervergangenenJahrzehntestellenheute präziseAnalyse- undEntwicklungs-toolszurVerfügung,dieeinenmaßgeblichenEinflussaufdenweiterenZubauvonKern-kraftwerkenhattenundhabenwerden.Einedieser Entwicklungen betrifft die Anwen-dungundIntegrationdernumerischenStrö-mungsmechanik. Einzeln, oder inVerbin-dungmitNeutronen-undSystemanalysen,hatesaufderStrömungsmechanikbasierteSoftwareermöglicht,dieKapazitätundVer-fügbarkeitderlaufendenAnlagenerheblichzuoptimieren.DiegleichenAnalysetoolsunterstützendie

EntwicklungderkommendenKernreaktor-Generation.Modellierung,SimulationundBerechnungwerden insgesamtzurErmitt-lungdesReaktor-undKomponentenverhal-tenseingesetzt.AufgrunddernotwendigenInformationenzumSystemverhaltenderIV.GenerationvonKernkraftwerkenhatdienu-merischeStrömungsmechanikalsAnalyse-tooleinewichtigeRolle.Insgesamt tragen Simulationen zudem

dazubei,denAufwandfürsonsterforderli-che und teils kostenintensive ExperimenteoderprototypischeAnlagenzuminimierenunddadurchfreiwerdendeKapazitätenfürzielgerichteteexperimentelleUntersuchun-genoderPilotanlageneinzusetzen.

Anschrift des Verfassers: Emilio Baglietto,

MIT77 Massachusetts Avenue

Cambridge, MA 02139-4307, USA

The story of nuclear reactors design is a progression from a reliance on inflexible-empirically derived guidelines to the more versatilenumerical analyses of computa-tional fluid dynamics and next-generation tools. The adoption of the new analysis and simulation techniques ispushing the opera-tional limits of reactors and fuel, with great impact on the economics of the plants, while improving the safety standards.

Less than 2 years after the Fukushimaevent data show that nuclear power capac-ity is growing steadily worldwide, with over 60 reactors currently under construc-tion in 14 countries. The growth of the nu-clear power is fundamentally related to the improvements in the technology, which have made the plants more economical and reliable, and have greatly enhanced the safety standards. Modern plants Proba-bilistic Risk Assessment (PRA) results show that the core damage frequency (CDF) is at least 2 orders of magnitude lower than the operating fleet.

The extensive research in the last dec-ades has identified refined and created analysis and design tools that have been crucial to the growth and advancement of the nuclear power industry.One of the most recent and important developments has been the application and adaptation of computational fluid dynamics (CFD). Used alone or in combination with neutronics and system analysis tools, the analysis soft-ware has enabled nuclear operators and vendors to greatly enhance the capacity and availability of current reactors. In the 1980s, nuclear plants produced only 60 to 65 % of the electric power they were li-censed to produce, assuming they operat-ed continuously. Today, they are well above 90 %, with the upper quartile being nearly 97 % of capacity. This achievement is espe-cially remarkable because the plants must shut down on average every 18 months for refueling. To maintain 97 % availability, plant operations must be near perfection.

The same analysis technology is also helping in the development of the next generation of nuclear reactors. The designs proposed for Generation IV reactors repre-sent a large technologystep for which ex-isting experimental data and operating ex-

perience is limited. As a result, CFD and other advanced technologies are playing a greater role. Vendors and operators will re-ly to a much greater extent on modeling, simulation, and computation to determine how reactors and their components will perform, and to characterize and measure the conditions inside the reactors. One of the side effects of this trend will be the transition to reliance on smaller, less ex-pensive experiments that confirm the ac-curacy of the advanced tools rather than the large, expensive prototypic experi-ments of the past that can often be applied to only one plant design.

The starting point

Currently, energy companies operate about 435 nuclear reactors worldwide. The aver-age age of the world fleet is 25 years and in Western Europe, 75 percent of the plants are in the last half of their operating life. In the U.S., the largest operator with 104 plants, 17 reactors are between 40 and 50 years old; 49 are between 30 and 40 years old; 36 are between 20 and 30 and only 2 are less than 20 years old.

Economic and environmental forces are driving the optimization of these facilities, but an equally important underlying factor is the age of the nuclear plants. The reac-tors were initially licensed to operate 40 years, so most have reached – or will soon reach – an age where their operating life must be extended. The U.S. has clearly been the first to face the issue of aging of the fleet, and one of the most important economic initiativeshas been to demon-strate to the U.S.NuclearRegulatoryCom-mission that the reactors could operate safely for at least another 20 years, in a process known as life extension.

A number of issues fall under scrutiny during the life-extension process of prov-ing safe operation. Most of these are relat-ed to the conventional components of a nu-clear plant – the wear and tear on turbines, condensers, and seawater systems. You al-so have the nuclear steam supply system, which is not only subject to high pres-sures, temperatures, and flows, but also

653atw 57. Jg. (2012) Heft 11 | November

New Reactor Design Developments

high radiation.Life extension requires that plants look at and reassess the effect of en-vironmental elements on the material properties and integrity of the reactor ves-sels, piping, insulation, and cable supports. So they must conduct comprehensive re-views of the systems and structures to make sure that they will perform as origi-nally intended. Now that over half of the U.S. reactors have gone through a 20-year life extension, there is a clearer roadmap of the process. There are, however, deci-sions to be made on a plant-by-plant basis.

Whether some components and systems will be able to withstand the new 40-year extension target; which ones must be re-placed or refurbished; and if, on balance, it makes economic and environmental sense.Advanced simulation approaches are a critical tool to answer these fundamental questions. In particular CFD combined with other advanced modeling technolo-gies have shown to be a fundamental tool to enable more informed decisions. The predictive capabilities of CFD and the fun-damental insights into complex turbulence phenomena allow the analyst to predict the precise operational environment and as a consequence to better predict the times and modes of failures of systems and components. Common examples of the use of CFD in life extensions are predictions of local flow and temperature distributions inside the reactor vessel and primary sys-tem in order to predict the effect of vibra-tion, erosion, thermal striping and thermal cycling that are the main cause of failures. The recent OECD/NEA sponsored CFD benchmark exercise on thermal fatigue in a T-Junction was a clear demonstration of the importance of these tools and of the maturity of the technology.

The importance of fuel reliability

In addition to life extension of current reac-tors, operators are also concerned with achieving power uprates – increasing the power produced by a plant. Unfortunately, power uprates increase the demand on fuel materials. Current plants use about 3 to 5 % of the available energy in the fuel before it’s removed from the core, and an obvious target is to extend the life of the fuel in or-der to extractmore energy.

The economic viability of a reactor is greatly influenced by the reliability of the fuel, which for the current technology means being able to insert the fuel and run the reactor for cycles of 18 to 24 months. The operators of electric utilities work on design-ing guidelines that can improve the reliabili-ty of the plants and the efficiency of the fa-cilities. So they have looked at the most common causes of fuel failure and support-ed research to eliminate these causes.

The shortcomings of legacy tools

Designing guidelines and identifying the causes of failurerequire an understanding of conditions in the reactors. The legacy tools provide bounding conservative condi-tions for safety analyses however do not de-liver data of high enough resolution, accu-racy, and scale to better understand the causes of failure.

Industry’s system codes have played and continue playing a very fundamental role in the operation of reactors. Nevertheless the tools are basically one-dimensional and strongly based on experimental correlation-srather than first-principle.They, therefore, have limited applicability in predicting any deviation from optimal design conditions. The toolsare very accurate when they oper-ate in their regime, but engineerscannot extrapolate them outside of that range. This approach does not allow gaining in-sights into the fundamental behavior of a specific fuel design and therefore greatly limits the capability of developing new and better solutions. To remedy the situation, research laboratories around the world have been investigating the feasibility of higher fidelity approaches. In particular theU.S.DepartmentofEnergy(DOE)has strongly encouraged the national laborato-ries and nuclear power industry vendors and operators to identify or develop new computational tools to realistically model and simulate reactors and the conditions within them.

Improving fuel reliability

In 2002, the ElectricPowerResearchInsti-tute (EPRI), an independent, nonprofit company in the electric power sector, launched the Fuel Reliability Action Plan.This program began efforts to develop guidelines and tools forpredicting and pre-venting fuel failures.

To this end, the program focused on a common cause of fuel failure known as crud, a problem particular to light-water reactors in the current fleet and Genera-tion III, Advanced Evolutionary Reactors. In these cases, water is used as the coolant. The problem arises when the water re-moves oxides from the structural materials and deposits them on the fuel rods. That usually occurs in locations where you have sub-cooled boiling – where the water tem-perature is below the boiling point, but lo-cally, near the heated surface, the temper-atures are elevated enough that you have local boiling. The bubbles generated by this process stay near the surface of the fu-el rods. When the bubbles collapse, they deposit solutes from the coolant on the surface of the rods. The presence of the

solutes, called crud, can reduce the heat transfer coefficients and coolability of the rods and lead to failure.

A few rods could fail without affecting the plant’s operation, but when more than a few failures occur, the plant is forced to shut down the reactor and replace the failed rods.These types of unplanned shut-downs translate into considerable losses for plant operators.

New tools

One of the early efforts of the program was the Numerical Nuclear Reactor (NNR), which was sponsored by DOE and EPRIand carried out byArgonneNationalLaboratory,as the principalinvestigator, in collabora-tion with a commercial software vendor CD-adapco and others. The NNR was to provide a high-fidelity tool to help manage the formation of tenacious crud in light-wa-ter reactors.The high fidelity was achieved by integrating neutronic and thermal-hy-draulic modules. Each module used meth-ods and models formulated on first princi-ples, real geometry, and constituents.

The critical analysis elements of the cou-pled code were a whole-core neutron trans-port solution and a computational fluid dy-namics (CFD)/heat transfer solution. Mas-sively parallel computers provided the com-putational resources required for such high-ly refined modeling. Essentially, the project looked at the various challenges of coupling CFD simulation with neutronics calcula-tions.

The project brought forward a funda-mental innovation represented by the col-laboration with the private industry in the development of advanced simulation tools. CD-adapco, one of the chief analysis tool vendors involved in the project, provided STAR-CD as the CFD engine for the analy-sis. The software was the foundation for calculating the thermo-hydraulic condi-tions in the reactor core. STAR-CD was cou-pled to the DeCARTneutronics code and Ar-gonne developed the data exchange be-tween the 2 codes.

The project also developed an advanced boiling framework to extend the capability of CFD to the boiling and 2-phase flow con-ditions of a boiling water reactor (BWR). To develop the framework, STAR-CD soft-ware incorporated the ability to handle multi-phase flows, boiling heat transfers, and flow regime transitions that occur within a boiling reactor environment.The development of an advanced and flexible boiling framework brought forward a unique new level of realism in multiphase flow simulations.

The major outcome of the project was the demonstration that engineers could now manage crud by better predicting

654 atw 57. Jg. (2012) Heft 11 | November

New Reactor Design Developments

boiling onset and locations of hotspots. The NNR simulations incorporated the modeling of sub-cooled boiling, 2-phase flow, and influence on the flow distribu-tion, and could then be coupled to specific crud deposition models.

Grassroots collaboration

Individual collaborative initiatives also supported EPRI’s fuel reliability program and the NNR large-scale modeling effort. In these cases, major industry players ex-amined situations where tenacious crud had been observed to see what might cause it to form in some locations but not others.

In one collaborative initiative, Areva and CD-adapco sought to investigate the rela-tionship between thermal-hydraulic condi-tions and the distribution of tenacious crud in BWRs. To do this, they used STAR-CD to model portions of a plant core, where tena-cious crud had been observed. In doing so, they were able to show that variations in thermal-hydraulic conditions in reactor cool-ant channels account for the deposition of te-nacious crud. The predicted pattern of ther-mal-hydraulicfeatures thought to be impor-tant to crud formationwas remarkablysimi-lar to crud patterns observed in a plant.

In collaboration, CD-adapco worked with Westinghouseto develop a methodology to evaluate the propensity for crud forma-tion in pressurized water reactors (PWRs). The methodology used STAR-CD to show that the higher resolution achieved with CFD could predict the likelihood of crud forma-tion more reliably than conventional sub-channel methods. Sub-channel methods, be-ing too coarse cannot resolve the local condi-tions and azimuthal variations necessary to predict the onset of crud. As a result of this work, Westinghouse developed techniques using CFD to assess the crud formation risk for a PWR core design and has incorporated it in their reloading design process.

So collectively, Areva,Westinghouse, and CD-adapco have contributedto the fu-el reliability program by pushing ahead the creation of guidelines for the use of differ-ent screening levels and particularly ad-vanced CFD based methods for crud pre-diction. EPRI fuel reliability guidelines specify a hierarchy of measures that fuel designers should use to manage crud. One of those is the use of CFD in designs that are not otherwise supported by extensive operating experience. Today, both Arevaand Westinghouse are implementing these guidelines, including the use of CFD.

Grid-to-rod fretting

Another issue of paramount importance to fuel reliability is grid-to-rod fretting. Tak-

ing the U.S. as an example, about 80 % of the fuel failures that occur in operating re-actors occur as a result of grid-to-rod fret-ting. The fretting is caused by the relaxa-tion of the grid spacer springs as a conse-quence of irradiation, which coupled with the high-speed coolant flow it causes the rod to vibrate. As a result of the vibration, the contact points eventually wear through the outer cladding of the fuel, leading to a detectible fuel failure.

CFD allows predicting the detailed flow conditionsin reactor cores as a means of

understanding grid-to-rod fretting. Reac-tor vendors and operators can construct analyze high-fidelity CFD models contain-ing each single fuel rodin one-quarter sym-metry of a reactor core, as well as the sur-rounding reactor vessel and internals. These models usuallyexceeded 200 million cells and can efficiently be run on HPC sys-tems in few hours. Following the full core analysis CFD provides the capability to look at the interaction between the flowing coolant and the structural materials of the grid and the rod and predict what sort of

Fig. 1. Quarter core model of PWR and detailed velocity distribution in a typical 17x17 fuel assembly [Karoutas, 2010].

Fig. 2. Representative computational model of a fuel rod bundle.

655atw 57. Jg. (2012) Heft 11 | November

New Reactor Design Developments

vibration modes you will have in that par-ticular design. Spacer grid designscan be compared and optimize to reduce the like-lihood of grid-to-rod fretting failures. (Fig-ures1 to 3)

The virtual reactor

Up to this point, the push to develop analy-sis tools for the nuclear power industry has taken a very focused approach. Analy-sis has been compartmentalized. Engineers would look at the performance of the reac-tor core, the steam generator, or the tur-bine separately. In reality, though, they are integrated. To provide a broader per-spective, the U.S.DepartmentofEnergy in-stituted a near-term development program called CASL, which is meant to develop a toolset that will allow operators to opti-mize the entire system at once, on a com-ponent wise and a system wise basis.

The consortium was tasked with devel-oping computer models that simulate nu-clear power plant operations, forming a “virtual reactor” for the predictive simula-tion of light-water reactors. The partici-pants of the consortium were to develop ways of using computer models to reduce capital and operating costs per unit of en-ergy, extend the lifetime of the current U.S. reactor fleet, and reduce nuclear waste by enabling higher fuel burnups.

In order to reach the next incremental improvement in power, it’s recognized that newer and better computational methods and modeling capabilities will be needed to determine where efforts should be made to increase power and how much margin there is. Some of the margin is in the inter-play between what was previously consid-ered separately. Thermal-hydraulics, for example, was largely considered separate

from neutronics, or coupled in a very loose fashion. A tighter coupling will not only demonstrate that there is more margin of safety to increase power, but also may give insight into issues beginning to emerge due to longer operations, such as crud and grid-to-rod fretting. It has helped that the advanced simulation tools can provide a predictive means of avoiding these prob-lems in the design phase. The net result is going to be a new generation of analy-sis capability that can provide better in-sight into how to improve designs and how to introduce new materials that will yield benefits in plant upgrading and life exten-sions.

CASL’s efforts are focused on issues con-fronting the existing fleet of reactors, but industry leaders expect the same issues will be relevant to future generationsof re-actors. Operators and vendors will also use the virtual reactor to accelerate the deploy-ment of next-generation reactor designs, particularly advanced nuclear fuel technol-ogies and structural materials within the reactor core.Thus, the enhanced capabili-ties brought to the table by CFD will bene-fit Generation III and beyond. The role of CFD has been defined not only by special-ists from the national labs but also from leading technology providers. (Figure4)

Generation IV Reactors

The next generation of nuclear plants is pushing the limits of operating tempera-tures and materials used. In the case of the high-temperature gas reactors, tempera-tures are around 1,000 °C. In the case of the sodium reactor, the liquid sodium is heated over 500 °C. These reactors have complex, large geometries, with complex 3D flow behavior, which are necessary to

Fig. 3. Validation of fuel rod acceleration vibration amplitude: Test data vs. STAR-CCM+/VITRAN simulation [Elmahdi, 2011].

reach the required improvements in eco-nomics, safety and fuel utilization.

To provide the necessary design and analysis tools for this technology, the Nu-clear Energy Advanced Modeling and Sim-ulation (NEAMS) Program aims to pro-vide the next step in computational simu-lation capacity, which will rely much more heavily on the 3D analysis provided by CFD to evaluate the large components in a plant.

The CFD tools contribution is necessary to predict the conjugate heat transfer be-tween the solid components and the cool-ant that enables accurate predictions of the structural response. NEAMS, as was the case for NNR will rely on a mix of in-house and the commercial tool STAR CCM+, to allow evaluating potential designs and also to begin to develop guidelines for how these tools can be used in future reactor design.

The future of reactor design

The evolution of analysis tools has been driven by development projects such as EPRI’s fuel reliability program, CASL, and NEAMS. Over a 20-year period, the indus-try will move from the application of con-ventional methods that rely on experimen-tal correlations to using CFD on small com-ponents, larger components, and full-plant

Fig. 4. 1.035 Billion Cells Virtual PWR Model [Popov, 2012].

656 atw 57. Jg. (2012) Heft 11 | November

New Reactor Design Developments

analysis. CFD will be a fundamental com-plement to operators’ system tools, and new technology developed by NEAMS.

One critical factor in determining the pace of this evolution will be the comput-ing capacity. High-performance comput-ing will need to keep growing and sup-port the efficient applications of the simu-lation tools. If you were to sum up the progress that has been made in the devel-opment of tools for the nuclear industry, it would be fair to say that the innovation is as much in the application as it is in the technology.

References

E.Popov,J.Yan,Z.Karoutas,J.Gehin,R.Brew-ster,E.Baglietto, 2012: PWR Internal Flow Modeling With Fuel Assemblies Details, Proceedings of ICAPP’12, Chicago, Illinois, USA.

A.M.Elmahdi,R.Lu,M.E.Conner,Z.Karoutas,E.Baglietto,2011: Flow Induced Vibration Forces on a Fuel Rod by LES CFD Analysis. Proceedings of the NURETH14 Conference, Toronto, Ontario, Canada.

Z.Karoutas,K.Lang,P. Joffre,E.Baglietto,R.Brewster,E.Volpenhein,2010: Evaluating

PWR Fuel Performance Using Vessel CFD Analysis. Proceedings of 2010 LWR Fuel Performance, Orlando, Florida, USA.

M.E.Conner,E.Baglietto,A.M.Elmahdi, 2010: CFD Methodology and Validation for Sing-le-Phase Flow in PWR Fuel Assemblies. Nuclear Engineering and Design, 240 (2010) 2088-2095.

D.P.Weber,T.Sofuat al., 2007: High-Fidelity Light Water Reactor Analysis with the Nu-merical Nuclear Reactor – Nuclear Science and Engineering, 155, 395-408.

http://www.ne.doe.gov/AdvModelingSimulati-on/program.html

http://www.casl.gov/

_____________________________________

Planungen für Personalkapazitäten

Vorgehensweise zur Bestimmung von Personalkapazitäten am Beispiel des Kernkraftwerks UnterweserKarl Ramler, Andreas Auffarth und Hue-Minh Trinh, Stadland Bernd Vollmüller, Dortmund

AufgrundderNovellierungdesAtomge-setzesmusstendieBetreiberderKernkraft-werke inderBundesrepublikDeutschlanddenLeistungsbetriebfürausgewählteAnla-genbeendenundbefindensich jetzt inderNachbetriebsphase.FürdieBetreiberbedeu-tetdies,dassdiePersonalkapazitätenindenKernkraftwerkenzuüberprüfenundggf.an-zupassensind.HierzuwurdevondemKern-kraftwerkUnterweser(KKU)einekennzah-lenbasierte Analogie-Methode eingeführtundmitderzuständigenAufsichtsbehördeabgestimmt.DerEinsatzderMethodehatvalideErgebnissegeliefertundgewährleisteteinensicherenundwirtschaftlichenNicht-leistungsbetrieb.DieVorgehensweiseunddieErfahrungenmitdieserMethodewerdenindiesemFachartikelbeschrieben.

Anschriften der Verfasser: Karl Ramler, Andreas Auffarth und

Hue-Minh TrinhKernkraftwerk Unterweser

Dedesdorfer Straße 226935 Stadland

Bernd VollmüllerDr. Kalaitzis & Partner GmbH

Rheinlanddamm 19944139 Dortmund

Unternehmen

Das Kernkraftwerk Unterweser(KKU) ist eine Betriebsstätte, die zu 100 % zurE.ONKernkraft gehört. Im Jahr 1979 wurde der kommerzielle Leistungsbetrieb der Druckwasserreaktoranlage mit einer ge-nehmigten thermischen Reaktorleistung von 3.900 MW aufgenommen. Dies ent-spricht einer elektrischen Leistung von 1.345 MW. Am 1. Oktober 2010 erreichte das KKUals erste Einzelblockanlage welt-weit eine Gesamterzeugung von 300 TWh.

Im Leistungsbetrieb waren ca. 300 Mit-arbeiter im technischen Bereich beschäf-tigt. Zurzeit befindet sich das Kernkraft-werk im Nichtleistungsbetrieb.

Ausgangssituation

Aufgrund der Novellierung des Atomgeset-zes (AtG, 13. AtG-Novelle) mussten die Be-

treiber der Kernkraftwerke in der Bundes-republik Deutschland den Leistungsbetrieb in 2011 für ausgewählte Kernkraftwerke beenden und bereiten sich jetzt auf die Nachbetriebsphase und den Rückbau vor. Für die Betreiber bedeutet dies, neben den wirtschaftlichen und technischen Heraus-forderungen, dass die Personalkapazitäten in den Kernkraftwerken zu überprüfen und ggf. anzupassen sind.

Zurzeit sind in dem für Kernkraftwerke relevanten Regelwerk jedoch keine Metho-den vorgegeben, mit denen der Personal-bedarf in Kernkraftwerken quantitativ zu bestimmen ist. Für die Betreiber ist jedoch die exakte Ableitung der Personalkapazi-tät je Betriebsfunktion wesentlich, um auch weiterhin einen sicheren und wirt-schaftlichen Nichtleistungsbetrieb gewähr-leisten zu können.

Zur Ableitung der Personalkapazitä-ten je Betriebsfunktion stehen verschiede-ne analytische/arbeitswissenschaftliche,