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316 Journal o Chemical Education Vol. 86 No. 3 March 2009 www.JCE.DivCHED.org Division o Chemical Education

Reprinted romJournal of Chemical Education, Vol. 86, pp 316323, March 2009.

Copyright 2009 by the D ivision o Chemical Education o the American Chemical Society. Reprinted by permission o the copyright owner.

Expanded use o nuclear power is currently being con-sidered as a no-carbon-emission energy source to reduce theharmul eects o climate change (1). Currently nuclear powersupplies 19.4% o U.S. electricity (2) and 14% globally (3).Many nations, including the United States, are consideringthe construction o new nuclear acilities to both expand andmaintain generating capacity. Some environmentalists whopreviously opposed the nuclear option have now embraced it,but opposition remains. Any substantive discussion o the prosand cons o nuclear power as a source o electricity requires someknowledge o the nuclear uel cycle (Figure 1).

While many general chemistry texts have materialon nuclear energy, this topic often receives a low priorityrelative to others owing to constraints of time and its usual

position near the end of those texts. Given the current in-terest and importance of nuclear power, it is appropriate tohave an up-to-date treatment of this subject centered on thenuclear fuel cycle. The goal of this article is to provide anintroduction to the current fuel cycle with suggestions forcase studies and problems as well as material with referencesfor teachers and students wishing to pursue the subject ingreater depth.

Uranium to Electricity: The Chemistry of the Nuclear Fuel CycleFrank A. Settle

Department o Chemistry, Washington and Lee University, Lexington, VA 24450; [email protected]

Figure 1. The nuclear uel cycle (withpermission rom InormationskreisKernEnergie, Berlin) (4).

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Division o Chemical Education www.JCE.DivCHED.org Vol. 86 No. 3 March 2009 Journal o Chemical Education 317


Copyright 2009 by the Division o Chemical Education o the American Chemical Society. Reprinted by permission o the copyright owner.

Te uel cycle consists o a series o industrial processes thatproduce uel or the production o electricity in nuclear reactors,use the uel to generate electricity, and subsequently managethe spent reactor uel. Because the majority o the worlds com-mercial reactors and all commercial U.S. reactors are light-waterreactors, this article will address light-water nuclear reactors,which require enriched uranium uel. In contrast, heavy-waterreactors do not require enriched uranium; instead, natural ura-nium can be used as uel. Electricity is created by using the heatrom controlled ssion in the reactor to produce steam, whichdrives a turbine connected to a generator. Spent uel is removedrom the reactor and either reprocessed to make new uel orstored or uture disposal. Each process in the cycle has saety,environmental, and economic issues. Prolieration o nuclearweapons is also an important consideration, as weapons-gradessile materials can be prepared and diverted rom either theenrichment or spent-uel processes.

Te uel cycle is divided into three components: the rontend, which encompasses all activities prior to placement o theuel into the nuclear reactor; the service period, which includesthe conversion o uel to energy in the reactor; and the back end,

which incorporates all processes involving spent uel. Te ateo the spent uel determines whether the uel cycle is closedoropen. In the closed cycle, the spent uel is reprocessed, whereasin the open cycle, it is sent to storage. o date, the United Stateshas opted or the open cycle, while France, the United Kingdom,Russia, China, and Japan reprocess spent uel.

While the physics and engineering o the service periodare central to reactor processes, chemistry dominates the otherprocesses o the nuclear uel cycle. Chemical reactions transormuranium ore into reactor uel and then convert the spent uelrom the reactor into disposable waste and recyclable uel. Tisarticle provides the chemical background essential to under-standing the important role o the uel cycle in any expanded use

o nuclear power. It also provides perspectives on the economicso nuclear uel and a comparison o the relative costs associatedwith producing electricity rom other sources o energy.

The Front End of the Fuel Cycle

Uranium occurs in rocks, soil, and bodies o water. Whileit is 500 times more prevalent than gold and about as abundantas tin, it is usually ound dispersed in trace quantities. Mosto the radioactivity associated with uranium comes rom itsdaughters1 such as radon. Pitchblende is the common mineralorm o uranium, composed largely o UO2 with smaller con-centrations oUO3. I its concentration is large enough to beextracted economically, the material is known as an ore. Te

cost o recovering uranium is determined by the concentrationo ore: the lower the concentration the higher the cost. Depositscontaining 0.120% pitchblende are economically viable. Oresin Canada, Australia, and Kazakhstan accounted or more thanhal the worlds uranium production in 2007 (5).

Uranium Ore Dressing:Mining and Initial Concentration

raditionally, uranium ore is extracted by either open pitor underground mining, depending on the location o the ore.Recovery o uranium rom the ore is oen dicult, and theprocedures vary with the geological environment. Te ore isrst milled (crushed and ground) to liberate mineral particlesand then exposed to a leaching solution whose composition is

determined by the geochemistry o the deposit. Ores containinghigh concentrations o silicates are leached with suluric acid toproduce the water soluble UO2

2+ ion. Tose with high concen-trations o carbonates are converted by a similar process using(NH4)2CO3 to orm the water soluble UO2(CO3)3

4 ion.wo methods are used to extract the uranium rom the

leach solutions: solvent extraction and ion exchange. Solventextraction, the more common method, employs tertiary aminesdissolved in kerosene in a continuous extraction process. First,the amines, R3N, react with suluric acid:

2R3N(org)

(R3NH)2SO4(org)+ H2SO4(aq)(1)

Te resulting amine sulate extracts the uranyl ions into the or-ganic phase, while the impurities remain in the aqueous phase.In the case o UO2

2+, the ollowing reaction occurs:

2(R3NH)2SO4(org)

(R3NH)4UO2(SO4)22+(org)+ UO2

2+(2)

he solvents are removed by evaporation, and ammoniumdiuranate, (NH4)2U2O7, is precipitated by adding ammonia toneutralize the solution. Finally, the diuranate is heated to yielda concentrated solid, U3O8, known as yellowcake.

Ion exchange extraction involves passing the uranium insolution through a resin bed, which exchanges the uraniumcarbonate ion with a negative ion such as chloride. Te uraniumcomplex concentrated on the resin is then removed with a saltsolution and precipitated as diuranate, which when heated yieldsyellowcake.

Recently, another technique or extraction and concentra-tion o uranium ore has gained popularity, in situ leaching(6).Tis technique involves circulating oxygenated groundwater

through a porous ore body to dissolve the uranium and bring itto the surace. Te composition o the leaching solution is deter-mined by the geology and groundwater o the ore deposit. I thedeposit contains signicant quantities o calcium, an alkaline so-lution is employed. Otherwise, a slightly acidic leach is used. Tesolution also contains a complexing agent to hold the extracteduranium in solution. At the surace, the uranium is recoveredusing ion exchange columns and then oxidized to U3O8.

Te global 2007 gures or extraction methods are under-ground mining and open pit mining combined, 62%, in situleaching, 29% and by product,2 10% (7). Te solutions andmaterials (tailings) remaining aer uranium extraction containboth chemically toxic and radioactive substances that require

secure containment.Uranium Ore Dressing: Rening Yellowcake to UO3

In the second component o ore dressing, yellowcake,containing 7090% by mass U3O8, is dissolved in nitric acid ata renery. Te resulting solution o uranyl nitrate is ed into acontinuous solvent extraction process. Te uranium is extractedinto an organic phase (kerosene) with tri-n-butyl phosphate,and the impurities remain in the aqueous phase. Aer this purication, the uranium is washed out o the kerosene withdilute nitric acid and concentrated by evaporation to pureUO2(NO3)26H2O, which upon heating yields pure UO3. Teprocesses involved in dressing the ore generate large volumes oacidic and organic waste.

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Conversion to Uranium Hexafuoride, UF6It is necessary to increase the isotopic concentration o

the 235U isotope rom its natural composition in uranium(0.7% 235U, 99.3% 238U) or use as uel in light-water reactors.ypically, reactor uel contains 3.0 to 5.0% 235U and is knownas low-enriched uranium (LEU).3 Because the uranium isotopeshave identical chemical properties, the processes employed or

enrichment must use physical techniques that take advantage othe slight dierences in their masses. Te two enrichment meth-ods currently used, centriugation and diusion, require theuranium to be in a gaseous orm, uranium hexauoride, UF6(g).Although the enrichment involves physical processes, chemistryplays an important role in synthesizing UF6 and returning theUF6 enriched in

235U to the solid, UO2.Preparation o UF6 rom UO3 involves a multistep chemi-

cal process. Te concentrated UO3 extracted rom the yellow-cake is reduced with hydrogen in a kiln:

UO2(s) + H2O(g)UO3(s) + H2(g ) (3)

Te uranium dioxide is then reacted with hydrogen uoride toorm uranium tetrauoride:

UF4(s) + 2H2O(g)UO2(s) + 4HF(g) (4)

Finally, the uranium tetrauoride is ed into a uidized bedreactor and reacted with gaseous uorineto obtain the uraniumhexauoride:

UF6(g )UF4(s) + F2(g ) (5)

In an alternative process, yellowcake is uoridated and theresulting UF6 isseparated by ractional distillation. Te uraniumhexauoride rom either process is now suitable eedstock orenrichment by either gaseous diusion or centriugation.

Uranium Enrichment with Gaseous Diusion

he United States has relied on the gaseous diusionprocess to enrich uranium or both nuclear weapons and uelor its eet o light-water reactors. Te U.S. government builta large gaseous diusion plant at Oak Ridge, ennessee duringWorld War II and added other plants in the post-war period.Te gaseous diusion process is based on molecular eusion,a phenomenon that occurs whenever a gas is separated rom avacuum by a porous barrier that contains microscopic holes. Agas ows rom the high-pressure side to the low-pressure side;it passes through the holes because there are more collisionswith holes on the high-pressure side than on the low-pressureside. Tomas Graham, a Scottish chemist, observed that therate o eusion o a gas through a porous barrier was inverselyproportional to the square root o its molecular mass. Tus,lighter molecules pass through the barrier aster than heavierones (Figure 2).

Te ratio o times it takes the equal quantities o two gases,A and B, to euse through a barrier is

rate A

r

eff( )

aate Beff( )=

B

A

M

M(6)

Applying this equation to uranium hexauoride where there isonly one common isotope o uorine, 19F, the ratio o rates orthe uranium hexauoride rom the two isotopes is

rate UF

rate

eff235

6( )eeff

2386UF( )

= =

352

3491 004. (7)

Tis small dierence in rates means that many eusion

barriers (stages) are necessary or enrichment by passage o theenriched stream through successive stages. Te gaseous diu-sion plant at Oak Ridge, called K-25, required 4000 stages toassist in the production o the highly enriched uranium (HEU),approximately 90% 235U, or the weapon used on Hiroshima.4Te size o this plant, one-hal mile long and six stories high,indicates the magnitude o this industrial-scale process. Teproduction o a suitable barrier was the key to successul separa-tion. Te microscopic holes were approximately one-milliontho an inch in diameter and uniorm in size. In addition the mate-rial had to be porous enough to maintain gas ow and remainchemically resistant to the corrosive uranium hexauoride. Itwas ound that nickel and aluminum oxide are best suited orbarrier materials.

Figure 2. Gaseous diusion (with permission rom InormationskreisKernEnergie, Berlin) (8).

Figure 3. Centriugation (with permission rom InormationskreisKernEnergie, Berlin) (10).

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Diusion equipment is large and consumes signicantquantities o energy. Furthermore, the entire system must beleak ree; no air can be allowed in and no uranium hexauorideallowed out. Most o the worlds gaseous diusion plants havebeen shut down with the advent o the more ecient, less ex-pensive centriugation process. oday, China, France, and theUnited States still have operating diusion acilities (9).

Uranium Enrichment with Centriugation

Te United States explored centriugation o UF6 as ameans o enriching235U in the Manhattan Project during WorldWar II, but ound that the technology required was not robustenough or large-scale production. Since that time, new technol-ogy has allowed industrial-scale production o both LEU andHEU. oday, lower energy consumption (5% o that requiredor gaseous diusion), short equilibrium (separation) time, andmodular design make centriugation the preerred method ouranium enrichment.

In centriugation, gaseous UF6 is ed into a centriuge unitconsisting o a cylindrical rotor spinning at a high speed inside

an evacuated casing (Figure 3). Te centriugal orce in the rap-idly spinning rotor causes a partial separation o the UF6, withthe heavier 238U molecules becoming slightly more concentratedaround the outside walls, while the concentration o the lighter235U molecules increases around the middle o the tube. Teseparation is acilitated by a relatively slow axial countercurrentow in the rotor that moves the molecules enriched in 235U toone end and the depleted molecules containing increased con-centration o 238U to the other. Te separation can be enhancedurther by heating the lower end o the casing, creating convec-tion currents that move the 238U down and the 235U up (11).Although it is possible to obtain signicantly more enrichmentrom a single centriuge than rom a single gaseous diusion

stage, this process must be repeated in a series o connectedcentriuges known as a cascade to obtain the desired concentra-tion o enriched uranium. Te slightly enriched stream is ed tothe next higher stage, while the depleted stream is recycled backto the preceding stage. Cascades containing several hundred oreven thousands o units are the basic component o a centriuge-enrichment acility. Cascades o centriuges operating in parallelto produce LEU or reactors can be easily recongured to orma single unit to produce HEU, or the LEU can be returned tothe parallel cascade to increase the concentration o235U. Tus,any centriuge-enrichment acility has the potential to produceHEU or weapons, a major concern with respect to nuclear weapon prolieration.

Equipment or centriuge enrichment requires complexmetallurgy and precise engineering. Materials used or tubesmust be able to withstand mechanical stress and contact withcorrosive UF6 gas. Current materials include high-strengthalloys o steel or aluminum or ber-resin composites. Tesematerials must be precisely machined to maintain balance at thehigh speeds necessary or separation o the isotopes. Te abilityo a single centriuge tube to separate uranium isotopes dependson the outside speed o the rotor raised to the 4th power. Tus,doubling the speed increases the eciency o separation bya actor o 16. Te degree o enrichment also depends on thelength o the tube, with the latest 1012 meter tubes obtainingan enrichment actor 100 times greater than a single gaseousdiusion stage.

Te production o 1.0 kg LEU requires approximately11 kg uranium or the enrichment process while 1.0 kg HEUrequires about 176 kg o uranium. he production o thisquantity o HEU requires 33 times the energy required or thecorresponding quantity o LEU. Gaseous diusion consumesabout 40 times as much energy to produce equivalent quantitieso enriched uranium as does centriugation (12).

Fuel Fabrication

Te enriched UF6 gas is transported in tanks rom the en-richment plant to the abrication acility, where it is convertedinto uranium dioxide powder. Te powder is then transormedinto cylindrical, ceramic pellets that are ground to a uniormsize. Tese pellets are sealed into tubes made o zircaloy, an alloyo zirconium containing small quantities o tin and other metals.In addition to its corrosion-resistant property, zirconium alsohas a low cross-section or thermal neutron absorption, makingit an ideal cladding material or the tubes. Tese tubes, known asuel rods, are bundled together into uel assemblies or insertioninto the reactor core. Depending upon the reactor type, a uel

assembly may contain up to 264 uel rods in a 59 inch squarethat is about 12 eet in length.

In the closed uel cycle, plutonium oxide reprocessedrom spent reactor uel or dismantled nuclear weapons (13) iscombined with depleted uranium oxide to orm mixed oxide,MOX,5 with a composition o 37% PuO2 and the rest depletedUO2.

6 Te MOX is then mixed with ordinary LEU in a ratioo MOX and LEU or ueling light-water reactors (see thereprocessing section below).

Consumption of Fuel in the Reactor

Te reactor uel burnupis an important parameter in the

nuclear uel cycle. It determines the uel requirements andthe quantity o waste produced. It is usually dened as theenergy obtained rom a given mass o uel expressed in units ogigawattdays7 per tonne8 o heavy metal.9 A reactor producingelectricity at a rate o 1000 megawatts (MWe) and operatingat a thermal eciency10 o 32% produces a total quantity othermal energy at the rate o 3125 megawatts (MWt). Tus,a typical 1000 MWe reactor generating one gigawattyear(GWeyear) o electric power produces a thermal output o1141 gigawatt thermaldays (GWtdays).

e e1 GW year 365 GW day=

GW day

GW year GWday

GWday eeet

365 0 32 1141. =GWday

tGW yeare

(8)

hereore, i the average burnup o uel in a reactor is40 GWtdaystonne, 28.5 tonnes o enriched uranium are re-quired to produce one GWeyear.

1141GW day

GW year40

GW day

tonnet

e

t= 228 5.

tonne

GW yeare(9)

Te value o the burnup depends on the enrichment o the uel,the power density in the reactor, and the length o time the uelremains in the reactor (14). High values or burnup reduce thequantity o waste.

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The Back End of the Fuel Cycle

Current reactors are typically shut down or 1 month out oevery 18 months or reueling and maintenance. Approximatelyone-third o the spent uel is removed and replaced with reshuel assemblies. ypical resh and spent uels11 removed aer3 years in a light-water reactor are compared in able 1 (15).

Initially, the level o radioactivity o spent uel is extremelyhigh owing to emission rom short-lived ssion products andtransuranium elements ormed by neutron capture. It is alsothermally hot due to the thermal energy released with the radio-active emissions. Tus, the rst step in processing the weapons-grade uel is to allow the radiation rom the short-lived isotopesto dissipate and to permit the uel to cool thermally. Tis occursin water-lled cooling pools at the reactor site.

he components o spent uel determine the methodsemployed or reprocessing and storage. High-level waste iscomposed o the radioactive products resulting rom the ssiono235U in the reactor and some short-lived transuranic elementsproduced by absorption o neutrons by 238U. Although the highlevels o radiation rom many o these relatively short-lived iso-

topes and their daughters decrease rapidly, radiation rom thelong-lived products will remain or thousands o years. In theinitial decay period, most o the radiation is due to 137Cs, 90Sr,and their short-lived daughters. High-level waste comprises only3% o the volume o radioactive waste worldwide, but it contains95% o the radioactivity. ransuranic (RU) waste containsalpha-emitting isotopes o transuranic elements with hal-livesgreater than twenty years and a combined activity o greaterthan 100 nanocuries per gram o waste, a relatively low level oradioactivity. However, many o the isotopes in RU waste havelong hal-lives and remain radioactive or thousands o years.

Open Cycle Interim Storage

Te ultimate orm o the spent uel depends on whetherthe uel cycle is open or closed. In the open cycle, because nocountry has a ully unctioning, permanent high-level nuclearwaste repository, spent uel remains untreated in the uel as-semblies, which are placed in pools or stored in dry casks atthe reactor site. In the closed cycle, the waste is the materialremaining aer the uel rods have been processed to removethe uranium and plutonium. In the open cycle, the spent-uelassemblies are initially placed in pools under at least 20 eet owater at the reactor site to provide shielding rom radiation andto aid in thermal cooling o the waste. Current regulations per-mit reconguration o the rods to increase the quantity o uelthat can be stored in a given pool. Dry-cask storage is also used

to increase spent-uel storage at reactor sites and or removal toother locations. In this method, spent uel that has been cooledin the spent-uel pool or at least one year is placed in a cylinderrom which water and air are removed and replaced with aninert gas. Te cylinder is sealed and then encased in steel andconcrete to contain the radiation and to provide security ortransportation or storage.

Closed Cycle ReprocessingReprocessing employs chemical reactions (redox, precipi-

tation, and extraction) to separate the uranium and plutoniumrom the other components o spent uel. Te extracted uraniumcan be converted into UF6 to produce enriched uranium or ad-ditional reactor uel, while recovered plutonium can be mixedwith enriched uranium to produce MOX uel. Te relativelysmall volume o remaining high-level radioactive waste can bestored in liquid orm and then solidied.

Fuel assemblies are placed in storage pools and allowed tocool and decrease in radioactivity or 5 to 25 years aer removalrom the reactor. Tey are then put into secure containers andtransported to the reprocessing plant. Here the assemblies are

removed and the uel rods chopped to expose the spent uelor leaching with nitric acid. Te cladding is insoluble in theacid and is removed as waste. Te leaching occurs in containersdesigned to resist the corrosive nitric acid and to ensure that nocritical mass o ssile 235U or 239Pu can accumulate.

The PUREX Process

Te plutonium recovery by extraction (PUREX) processused by all commercial reprocessing plants separates the uraniumand plutonium rom the ssion products and other transuranicelements using redox and nonaqueous chemistry. Te recoveryo relatively small quantities o plutonium in the presence olarge quantities o uranium depends upon the relative ease o

the redox reactions involving plutonium. Te spent uel is thentaken rom the pool or reprocessing, the outer cladding is re-moved either chemically or mechanically and the uel dissolvedin nitric acid. Te pH o the resulting aqueous solution is raised,and then this solution is equilibrated with an immiscible solu-tion o tri-n-butyl phosphate (BP) in rened kerosene. TeBP solution extracts the UO2

2+ and Pu4+, leaving the othercomponents o the spent uel in the aqueous phase. Te pluto-nium is then reduced to the 3+ state by a solution containinghydroxylamine (NH2OH), extracted into a resh aqueous phase,and sent to the plutonium purication section. Te uranium inthe 6+ state remains in the organic phase. It is then extractedinto an aqueous solution o nitric acid and removed or urther

Table 1. Composition of Fresh and Spent Fuel

Material Fresh Fuel (%) Spent Fuel (%) Product

Transuranic Elementsa 0.000 0.065 Transuranic waste236U 0.02 0.56 Uraniumisotope produced

Pu Isotopes 0.000 0.89 Transuranic waste

Fission Products 0.000 5.11 High-Level waste235U 4.0 0.81 Unreacted 235Ub238U 95.95 92.3 Unconverted 238Uc

a Excluding plutonium. b 235U that did not undergo fssion prior to removal o uel rods. c 238U that did not convert toPu via neutron absorption.

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purication.

(Pu4+, UO22+)(org)

Pu4+ + UO22+

TBP

Pu3+(aq) + UO22+(org)

NH2OH(10)

Final puriication removes lingering ission products,other impurities, as well as trace quantities o plutonium romthe puried uranium and trace quantities o uranium rom theplutonium. All plutonium is treated with ascorbic acid to ensurethat it is in the 3+state and then precipitated as PuF3. Uraniumis converted to the oxide or storage. Te solid metal orm oboth elements can be produced by reacting their uorides withcalcium or magnesium at elevated temperature in a sealed con-tainer. Te availability o plutonium rom reprocessing is a majorconcern in the prolieration o nuclear weapons (16).

Reprocessing must occur in isolated environments withrobots or humans using remote devices to manipulate thehighly radioactive materials and toxic chemicals. Nitric acid andBPkerosene solvents are recycled, gaseous efuents ltered

and scrubbed beore discharge, and liquid waste concentratedor solidied or storage. All materials are monitored to protect workers and to prevent the accidental discharge o harmulagents into the environment.

The UREX Process

Te uranium extraction (UREX) process, currently underdevelopment, uses acetohydroxamic acid to suppress the extrac-tion o plutonium and promote the separation o uranium along with technetium. Te highly radioactive technetium is thenseparated rom the uranium, which can be disposed o as low-level waste or stored in an unshielded acility or re-enrichment.Tis process produces ve output streams: uranium at a purity

o 99.999%; plutonium with other transuranic elements, whichis suited or MOX reactor uel but not or weapons; long-livedssion products, mostly 129I and 99c, which can be stored ordestroyed in a reactor; and the short-lived ssion products,predominately 137Cs and 90Sr, which can be kept on site inpools or rapid radioactive decay. In addition to producing uelto burn in reactors and reducing the threat o prolieration, thisprocess also reduces the quantity o waste sent to storage. I theUREX process proves easible, it could replace the PUREXprocess (17).

Fuel rom Reprocessing

Te uranium oxide rom reprocessing is reintroduced into

the uel cycle or conversion into UF6. Te conversion o plu-tonium into reactor uel is more complicated. Te plutonium isconverted into PuO2 and mixed with the UO2 extracted romthe spent uel to orm a mixed oxide (MOX) uel. In the dry pro-cess, the oxides are mixed to yield a composition o 37% PuO2and the remainder UO2. An alternative wet process uses a basesuch as ammonia to convert a mixture o uranyl nitrate and plutonium nitrate into ammonium diuranate and plutoniumhydroxide, which, when heated, becomes a mixture o uraniumdioxide and plutonium dioxide. Te dierence in nuclear prop-erties o 239Pu and 235U dictates a ratio o MOX, LEU asuel or most light-water reactors. MOX uel is also used todispose o plutonium rom dismantled nuclear weapons.

Immobilizing High-Level Waste

High-level waste can be transormed into solid, glassy mate-rials or saer storage. Tese materials can be mixed with wasteso dierent compositions and cast into usable orms that con-duct heat well and are chemically stable in storage environments.he liquid high-level waste rom reprocessing is convertedinto solids by evaporation, mixed with a sodium borosilicate

composite, and heated to produce a molten mixture containingapproximately 25% waste. Te molten mixture is poured into astainless-steel container and welded shut. Tese containers canthen be moved to temporary or permanent storage sites.

The Economics of the Nuclear Fuel Cycle

While chemistry plays a critical role in the nuclear uelcycle, no discussion would be complete without presenting theeconomics o each component o the cycle, the costs o produc-ing and disposing o uel relative to the other costs associatedwith nuclear power, and a comparison o nuclear power withother sources o electricity. In the nal analysis, these economics

will be critical actors in determining the role o nuclear powerin the global energy portolio. Te data presented here relateto estimated U.S. costs. Current costs associated with the rontend and the back end o the nuclear uel cycle are provided inables 2 and 3 (18).

Tese data indicate the cost associated with the productionand disposition o uel to be $2000(kg235U) or LEU uel and$5.0(megawatthour) o electricity. Tese costs are comparedwith other costs associated with nuclear power and the othersources o electricity in able 4. It indicates that the cost oproducing uel and handling spent uel is only 7% o the totalcost o producing nuclear electricity. Te data also show that a$100ton carbon emission tax on coal and natural gas could

make nuclear energy competitive with the alternatives.

Table 2. Front-End Costs

Process Cost/[$/(kg 235U)] Cost/[$/(MW h)]

Mining and Milling 500 1.3

Conversion 50 0.1

Enrichment 600 1.5

Fabrication 250 0.6

Total 1400 3.5

Note: Assumes uel enriched to 4.4% 235U by centriugation, a burnupo 50 MWtday/kg, a tails assay (concentration o 235U in depletedmaterial) o 0.3%, and an efciency o 33%.

Table 3. Back-End Costs

Processa Cost/[$/(kg 235U)] Cost/[$/(MW h)]

Dry Storage 200 0.5

Geological Disposalb 400 1.0

Total 600 1.5aCost o wet storage on the reactor site is included in the capital,

operational, and maintenance costs. bThese values are estimated.

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Te energy requirements and carbon emissions associatedwith production, transport, and waste associated with nuclearuel compared with those or coal and natural gas must also beconsidered in the role o nuclear power in reducing greenhouseemissions. ypical data on the energy requirements or theentire uel cycle rom mining to storage and decommissioningindicate 5.7% o thermal energy output (MWt) o a reactor igaseous diusion is used or enrichment and 1.7% i the uel is

enriched by centriugation (20). Te cost associated o obtain-ing hydrocarbon uels are 2 to 10 times greater than those ornuclear uel (19). Carbon dioxide emission data in g CO2 perkilowatthour indicate nuclear at 1026 compared with 894975 or coal and 450608 or natural gas (20). Tese data avorproduction o electricity rom uranium over production romhydrocarbon uels.

However, even with the low cost o nuclear uel, lowergreenhouse emissions, and the possibility o a carbon emissionstax increasing the economic competitiveness o nuclear power,the environmental, saety and nuclear prolieration issues asso-ciated with the uel cycle remain. Furthermore, repositories orthe long-term storage o waste must be developed and placed

in service. I nuclear power is to be a signicant contributor tothe global energy portolio, these issues must be addressed withstrict regulations, vigilant monitoring, and strong enorcemento all aspects o the uel cycle. It will also require educating citi-zens, policy makers, and nancial analysts about the benets andrisks o the nuclear uel cycle to provide a realistic assessment othis energy source.

Use of This Material with Classes

Tis material can be used with introductory and inorganicchemistry courses as well as courses in other disciplines thataddress environmental science and energy issues. Tis article

supports the scientic and technical aspects o the ollowingactivities, which may be assigned as individual or group pre-sentations.

Comparethescience,technology,economics,andenvironmen-tal impacts o producing nuclear uel with those or coal andnatural gas.

Comparethescience,technology,economics,andenvironmen-tal impacts o the waste rom nuclear reactors with those orcoal and natural gas power plants.

Perform the same comparisons for nuclear power with renewableenergy sources (wind, solar, hydro, and biomass).

Table 4. Costs of Nuclear vsersus Alternative Sources of Energy

Areaa Nuclear/[$/(MW h)] Coal/[$/(MW h)] Gas/[$/(MW h)] Wind/[$/(MW h)] Solar/[$/(MW h)]

Capital 50 30 12 60 250

Operation and Maintenance 15 5 3 10 5

Fuel 5 10 2550 0 0

Total 70 45 4065 70 255

Carbon Tax at $100/tonb

0 25 12 0 0New Total 70 70 5277 70 255aData are rom re 19. According to the U.S. Energy Inormation Agency, in 2006, only 2% o U.S. electricity was produced rom petroleum (2). bTon (also

called the short ton) can be converted to metric tons (or tonne) by multiplying by 0.907184.

Comparetheadvantagesanddisadvantagesoftheopenandclosed nuclear uel cycles.

GiventhepercentageUO2 in a given ore deposit, calculate howmany tons (short ton) o ore would have to be processed to

produce 100 pounds o nuclear uel (uranium metal containing3.5% 235U).

Howmanypoundsofspentfuelfromalight-waterreactor

would have to be processed to produce 4.2 pounds o 239Purequired or a nuclear ssion weapon?

Calculatethemassoffuelrequiredfora1500megawatt(MWe)reactor to operate or 6 months i the average burnup o uel is40 gigawatts (GWt)days/tonne.

Amajorcomponentofthessionproductsinspentnuclearuel is 137Cs. I it has a hal-lie o 30 years, what percent o theoriginal 137Cs remains in the uel aer 90 years?

Americium-241withahalf-lifeof432yearsisfoundintran-suranic waste. What percent 241Am remains aer 864 years?

Whatistherelationshipbetweenthehalf-lifeandtheintensity

o radiation emitted by radioactive isotopes? How does thisapply to the ssion products and transuranic elements in spentuel?

Explainhownuclearweapons-gradematerialcanbeobtainedrom the nuclear uel cycle.

Useul Web Sites (all sites accessed Nov 2008)

Beckjord, E. S. Te Future o Nuclear Power: An Interdisciplin-ary MI Study. http://web.mit.edu/nuclearpower/

Cameco Corp. Uranium 101: Fact Sheets. http://www.cameco.com/uranium_101/act.php

Gonyeau, J. Te Virtual Nuclear ourist. http://www.virtual-nucleartourist.com/

he Keystone Center. Nuclear Power Joint Fact Find-ing. http://www.keystone.org/spp/documents/FinalReport_

NJFF6_12_2007(1).pd

Settle, F.; Blackmer, E.; Strang, J.; Whaley, . Alsos DigitalLibrary or Nuclear Issues, Nuclear Fuel Cycle. http://alsos.wlu.edu/qsearch.aspx?browse=science/Nuclear+Fuel+Cycle

Settle, F. Concept Map or Nuclear Power. http://alsos.wlu.edu/conceptmaps/nuclearpower/main/index.html

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Acknowledgments

Tis work was unded by an educational grant rom theLenest Foundation. Elizabeth Blackmer and Charles Fergusonprovided extensive suggestions and edits.

Notes

1. Daughters are radioactive elements produced by radioactivedecay o uranium.

2. Uranium is oen obtained as a by product rom the processesinvolved in mining other metals.

3. Some research reactors use uel around the 20% enrichmentlevel, the upper limit or LEU.

4. Te 235U or the rst atomic bomb was enriched to 7% at thegaseous diusion plant and then used to eed the electromagnetic units(calutrons) or nal enrichment to approximately 90%.

5. Te United States and Russia are investigating the use o ura-nium and plutonium rom dismantled nuclear weapons as a source oMOX and are planning to dispose o up to 68 metric tons o weapon-origin plutonium as MOX.

6. Depleted uranium contains less than 0.72% uranium-235.Tus, it is less reactive in the nuclear reaction sense than natural or en-riched uranium, which have greater concentrations o the ssile isotope235U.

7. One gigawattday = 8.64 1013 joules.8. One tonne or metric ton = 1000 kilograms.9. Tere is a dierence between the mass o heavy metal and the

mass o uel. In the case o uranium oxide uel, uranium comprises 88%o the mass o the uel.

10. Te thermal eciency is limited by the second law o thermo-dynamics. Tus 3200 megawatts total (thermal) is required to produce1000 megawatts electrical.

11. Te composition o spent uel is determined by the 235U con-

centration o the resh uel, the time uel remains in the reactor, andthe neutron ux within the reactor core.

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SupportingJCE OnlineMaterialhttp://www.jce.divched.org/Journal/Issues/2009/Mar/abs316.html

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