CEA - The nuclear fuel cycle · From the uranium mine to waste disposal 7> The nuclear fuel cycle 4 > UPSTREAM THE REACTOR: PREPARING THE FUEL 5 AFTER MINING, THE URANIUM IS PURIFIED,

> INTRODUCTION

From the uranium mine to waste disposal 7> The nuclear fuel cycleFrom the uranium mine to waste disposal 7> The nuclear fuel cycle

FROM RESEARCH

TO INDUSTRY

> From the uranium mine to waste disposal

UPSTREAM THE REACTOR: PREPARING THE FUELIN THE REACTOR: FUEL CONSUMPTIONDOWNSTREAM THE REACTOR:REPROCESSING NUCLEAR WASTENUCLEAR WASTE

THE COLLECTION

1> The atom2> Radioactivity3> Radiation and man4> Energy5> Nuclear energy: fusion and fission6> How a nuclear reactor works7> The nuclear fuel cycle8> Microelectronics9> The laser: a concentrate of light10> Medical imaging11> Nuclear astrophysics12> Hydrogen

THE COLLECTION

1 > The atom2> Radioactivity3> Radiation and man4> Energy5> Nuclear energy: fusion and fission6> How a nuclear reactor works7> The nuclear fuel cycle8> Microelectronics9> The laser: a concentrate of light10> Medical imaging11> Nuclear astrophysics12> Hydrogen

© Commissariat à l’Énergie Atomique et aux Energies Alternatives, 2005Communication DivisionBâtiment Siège - 91191 Gif-sur-Yvette cedexwww.cea.fr

ISSN 1637-5408.

>The nuclearfuel cycle7 >The nuclearfuel cycle

FROM RESEARCH

TO INDUSTRY

© Commissariat à l’Énergie Atomique et aux Energies Alternatives, 2005Communication DivisionBâtiment Siège - 91191 Gif-sur-Yvette cedexwww.cea.fr

ISSN 1637-5408.

UPSTREAM THE REACTOR: PREPARING THE FUELIN THE REACTOR: FUEL CONSUMPTIONDOWNSTREAM THE REACTOR:REPROCESSING NUCLEAR WASTENUCLEAR WASTE

From the uranium mine to waste disposal 7> The nuclear fuel cycle

> CONTENTS 32

introductionFuel is a material that can be burnt to pro-

vide heat. The most familiar fuels are wood,coal, natural gas and oil. By analogy, the ura-nium used in nuclear power plants is called“nuclear fuel”, because it gives off heat too,although, in this case, the heat is obtainedthrough fission and not combustion.After being used in the reactor, spent nuclearfuel can be reprocessed to extract recyclableenergy material, which is why we speak of thenuclear fuel cycle. This cycle includes all thefollowing industrial operations:• uranium mining,• fuel fabrication,• use in the reactor,• reprocessing the fuel unloaded from thereactor,

• waste treatment and disposal.

“The nuclear fuel cycle includes an erray of industrial operations, from uraniummining to the disposal of radioactivewaste.”

Per unit or mass (e.g. per kilo), nuclear fuelsupplies far more energy than a fossil fuel (coalor oil). When used in a pressurised water reactor,a kilo of uranium generates 10,000 times moreenergy than a kilo of coal or oil in a conven-tional power station. Also, the fuel will remainin the reactor for a long time (several years),unlike conventional fuels, which are burnt upquickly. Nuclear fuel also differs from others inthat uranium has to undergo many processesbetween the time it is mined and the time itgoes into the reactor. For the sake of simplicity, the following pageswill only look at nuclear fuel used in pressurisedwater reactors (or PWRs), because nuclearpower plants consisting of one or more PWRsare the most widely used around the world (seeHow a nuclear reactor works booklet).

UPSTREAM THE REACTOR: PREPARING THE FUEL 4Extracting uranium from the ore 5Concentrating and refining uranium 6Enriching uranium 6Enrichment methods 8

IN THE REACTOR: FUEL CONSUMPTION 9Preparing fuel assemblies 10Uranium-235 consumption 10Fuel degradation 11

DOWNSTREAM THE REACTOR:REPROCESSING NUCLEARWASTE 12The purpose of reprocessing 13Extracting fission products 13Recycling fuel materials 14

NUCLEAR WASTE 16Nuclear waste production in France 17Sorting and disposing of radioactive waste 18Research on long-lived waste 19

> INTRODUCTION 3

The nuclear fuel cycle

Spent fuel is stored in a pool on the site, where it willremain for three years.

Uranium ore isextracted fromopen-pit mines –such as theMcClear mines inCanada seen here– or undergroundworkings.


Designed and produced by Spécifique – Cover photo by © CEA/Roquemaure - Illustrations by YUVANOE - Printed by Imprimerie de Montligeon - 03/2005

© Cogema

© Cogema/P. Lesage


> UPSTREAM THE REACTOR: PREPARING THE FUEL 54

AFTER MINING, THE URANIUM IS PURIFIED,CONCENTRATED AND ENRICHED.

EXTRACTING URANIUM FROM THE OREUranium is a relatively common metal in theEarth’s crust (it is 50 times more common thanmercury, for example). Like most metals, it can-not be mined directly in its pure form, becausein its natural state it is found in rocks com-bined with other chemical elements. The rocks


Upstream thereactor: preparingthe fuel

with the highest uranium content are known asuranium ores, which often include uraniniteand pitchblende.The nuclear fuel cycle thus begins at the open-pit mines or underground workings where theuranium ore is mined. The largest known oredeposits are in Australia, the United States,Canada, South Africa and Russia.

© Cogema/P. Lesage

Simplified diagram of the fuel cycle in France today

Ore extraction

Natural uranium

Spent UO2 fuel

Spent MOX fuel

PWRs with thermalneutrons

Storage

Storage

Storage

Reprocessingplants

Fuel fabrication

UO2 + PuO2

Ultimate disposal3% of spent fuel0.5% of natural uranium mined

Waste

Recycleduranium

Pure naturaluranium

Plutonium

U-235depleteduranium

U-235enricheduranium

Enrichment

Conversion

Concentration

UO2 fuel

MOX fuel

Upstream thereactor: preparingthe fuel

CONCENTRATING AND REFININGURANIUMThe ore generally has a rather low uranium con-tent. For example, in France, one tonne of orecontains between 1 and 5 kg of uranium(between 0.1 and 0.5%). This makes it essen-tial to concentrate the uranium in these ores,a job usually carried out on the spot.First of all, the rocks are broken up and finelycrushed. Then various chemical processes areused to extract the uranium.The resulting concentrate looks like a yellowpaste and is called yellow cake. It contains about75% uranium oxide, i.e. 750 kg per tonne.Uranium is a metal that oxidises very quickly when it comes intocontact with oxygen in the air and changes into uranium oxide.


> UPSTREAM THE REACTOR: PREPARING THE FUEL 76 > UPSTREAM THE REACTOR: PREPARING THE FUEL

nium-235, making only 0.7% of fissile ura-nium-235. The process of increasing the pro-portion of uranium-235 is called enrichment.This is a difficult operation because, like all theisotopes of the same element, uranium-235 anduranium-238 are very similar and have almostidentical chemical properties (see The Atom

booklet). They can be distinguished, however,due to their slightly different mass, uranium-235being just a little lighter than uranium-238.This is why the current uranium enrichmentprocess used is based on the difference inmobility caused by this slight difference inmass. Of all the enrichment processes studiedso far, only two have been developed on anindustrial scale: gaseous diffusion and theultracentrifuge process.

ENRICHMENT METHODSGaseous diffusionBefore being enriched via this process, the ura-nium tetrafluoride obtained after extractionfrom the ore and refining will be transformed

However, this uranium concentrate cannot beused in a nuclear reactor as it is. The uraniumoxide must first go through various stages ofpurification (or refining) to get rid of any impu-rities. Once it is very pure, it is converted intouranium tetrafluoride (UF4), which is composedof four fluorine atoms and one uranium atom.

ENRICHING URANIUMThe fuel used in a PWR must contain between3 and 5% uranium-235, because this is theonly uranium isotope that can withstandenergy-releasing nuclear fission (see How anuclear reactor works booklet). The problemis that 100 kg of natural uranium contains99.3 kg of uranium-238 and 0.7 kg of ura-

“Before it can be usedas nuclear reactorfuel, natural uraniummust be enriched withuranium-235.”

COMINAL ore processing plant in Niger.Raise boring in frozen ground in the McArthur mine (Canada).

© Cogema/Cameco

“In order to increase uranium content, ore rocks are broken up and finely ground.The resulting concentrate is called yellow cake.”

© Cogema/O. M

artel

enough 235U-enriched uranium for use innuclear power plants.

Ultracentrifuge processAnother uranium enrichment process is usedon a smaller scale by the European Urenco group(Germany, Netherlands, United Kingdom). It isknown as the ultracentrifuge process.This separation process uses a centrifuge which,acting like a high-speed salad spinner, projectsthe uranium-238 hexafluoride molecules to itsouter edge more quickly than those of uranium-235 hexafluoride, which remain nearer the centre.The very slight difference in mass between thetwo molecules gradually increases the uranium-235concentration. This process also requires manystages to obtain sufficiently enriched uranium.

FUEL IS USED IN A NUCLEAR REACTORFOR THREE OR FOUR YEARS.

In the reactor: fuel consumption

© Médiathèque EDF/C. Pauquet

> UPSTREAM THE REACTOR: PREPARING THE FUEL


98

into uranium hexafluoride (UF6), which becomesa gas when heated to 56°C.The gaseous diffusion process consists in passinggaseous UF6 through a long series of “barriers”formed by membranes with microscopic pores.Uranium-235 hexafluoride molecules are slightlylighter than uranium-238 hexafluoride mole-cules and cross each barrier a little faster, grad-ually enriching the uranium as they do so.However, as the difference in mass between thetwo isotopes is very small, the uranium-238travels hardly slower than the uranium-235. Forthis reason, the operation has to be repeated1,400 times at the uranium enrichment plantin France (the Eurodif plant in Tricastin in theRhone valley, which produces more than a thirdof the world’s enriched uranium) to obtain

Diffusers at Eurodif’sGeorges Besse plant.©

Cogema/P. Lesage

> IN THE REACTOR: FUEL CONSUMPTION


> IN THE REACTOR: FUEL CONSUMPTION 1110

FUEL DEGRADATIONLittle by little, the fuel’s performance deterio-rates as it undergoes a number of transforma-tions, including:• the gradual consumption of uranium-235,• the appearance of fission products (whichabsorb neutrons and disturb the chain reac-tion).After a certain period of time, the fuel musttherefore be removed from the reactor, even ifit still contains large amounts of retrievableenergy material, in particular uranium and plu-tonium. This spent fuel is also highly radioac-tive because it contains fission products. The

radiation emitted by these radioactive atomsgives off a great deal of heat. For this reason,once it has been removed from the core, spentfuel is stored for three years in a special poolnear the reactor to lose some of its radioactivity(see Radioactivity booklet).

“In a nuclear power plant, more than 40,000“rods” are prepared and grouped together in“bundles” with a square cross-section calledfuel assemblies.”

“Spent fuel is stored in a pool on the site,where it will remainfor three years.”

PREPARING FUEL ASSEMBLIES Following enrichment, uranium hexafluoride istransformed into a black uranium oxide pow-der. This is compressed, then sintered (bakedin a furnace) to make “pellets”, which are smallcylinders about 1 cm long and as thick as asmall piece of chalk. Each pellet weighs only7 g but can release as much energy as a tonneof coal (1 million grams).The pellets are inserted into four metreslong tubes made of zirconium alloy. These“claddings” are sealed at both ends to makefuel “rods” In a nuclear power plant, more than

Each fuel assembly contains 264 “rods” that containuranium oxide “pellets”.

40,000 of these rods are prepared and groupedtogether in “bundles” with a square cross-section. These are called “fuel assemblies.”There are 264 rods in each assembly. It takes157 fuel assemblies containing a total of11 million pellets to fuel a 900 MW nuclearreactor (1 MW = 1 million watts).

URANIUM-235 CONSUMPTIONThe reactor core is made up of fuel assemblies,arranged in a precise geometrical pattern.Each one remains in the core for three or fouryears. During this period, uranium-235 fission provides the heat required to generate watersteam, then electricity.This is possible because uranium-235 is fis-sile, which means that when its nucleus col-lides with a neutron, it splits (hence the termfission) into radioactive fission products,

releasing energyas it does so. Ura-nium-238, eventhough it repre-

sents 97% of the mass of nuclear fuel, doesnot split when a neutron is absorbed. However, some uranium-238 nuclei cap-ture a neutron and are transformed into plu -tonium-239, which is fissile like ura nium-235. That’s why we say that uranium-238 isfertile. Some of the plutonium-239 can generate energy through nuclear fission. Asmall fraction is also transformed into other plutonium isotopes by neutron capture mechanisms.

Atoms with unstable nuclei are saidto be radioactive. These nuclei arenaturally transformed into othernuclei, emitting radiation as they doso (see Radioactivity booklet).

© CEA/M. Faugère

Preparing fuel assemblies

Height: 4 metres

Welded plug

Uranium oxidepellet

Spacer grid

Fuel assembly

FUEL ROD

Welded plug

> DOWNSTREAM THE REACTOR: REPROCESSING NUCLEAR WASTE


1312

THE PURPOSE OF REPROCESSINGReprocessing involves:• retrieving material that can still be used –plutonium and uranium– to produce more elec-tricity, in other words, recycling energy mate-rials found in the spent fuel,¥ sorting radioactive waste that cannot be recy-cled.Some countries, such as Sweden and the UnitedStates, have not opted for reprocessing. In thesecountries, spent fuel is considered as waste andis stored after removal from the reactor awaitingdirect disposal. France, the United Kingdom,Russia and Japan have chosen to build repro-cessing plants. Other countries, like Germany,Switzerland and Belgium have their spent fuelreprocessed in other countries (particularly inFrance).

EXTRACTING FISSION PRODUCTSWhen they arrive at the reprocessing plant,spent fuel assemblies are again stored in spentfuel pools. They are then cut into small piecesand placed in a chemical solution that dissolvesthe fuel but leaves the metal parts (cladding,etc.) intact. These are then stored as nuclear

waste. The fuel solution undergoes a series ofchemical processes to separate the plutoniumand uranium from the fission products. The fis-sion products are embedded in special glass(this is the vitrification process) and stored asnuclear waste. Uranium and plutonium, whichaccount for 96% of the total, are isolated andconditioned separately.

Downstream the reactor: reprocessingnuclear waste

REPROCESSING INVOLVES RETRIEVING RECYCLABLEMATERIAL – PLUTONIUM AND URANIUM – ANDISOLATING NON-RECYCLABLE RADIOACTIVE WASTE.

Once separated, fission products are embedded in specialglass and disposed of as nuclear waste.

“Some countries reprocess their spent fuel themselves, while otherssubcontract the job to other countriessuch as France.”

© Cogema/P. Lesage

© Cogema

Downstream the reactor: reprocessingnuclear waste

> DOWNSTREAM THE REACTOR: REPROCESSING NUCLEAR WASTE


> DOWNSTREAM THE REACTOR: REPROCESSING NUCLEAR WASTE 1514

Loading the reactor core in the Daya Bay reactor (China).

Coloured cross-section of MOX fuel.

“Recovered uranium can be enriched to more than 3% and follow a pathsimilar to that of ordinary fuel.”

© Cogema/G. Liesse

© CEA

RECYCLING FUEL MATERIALSHow to use plutonium after reprocessing is thesubject of many studies, especially at the CEA.New fuels made from a mixture of uraniumoxide and plutonium oxide (called MOX from“Mixed Oxides”) are already in use in someEDF reactors (PWRs). In addition, the uraniumrecovered during reprocessing is still slightlyricher in uranium-235 than natural uranium(about 1% uranium-235), so it can beenriched again to more than 3% and follow apath similar to that of ordinary fuel.

Nuclear waste

> NUCLEAR WASTE


1716

NUCLEAR WASTE PRODUCTION IN FRANCEAll human activities generate waste. With pop-ulation growth and industrial developmentcomes an increasing volume of waste to betreated, conditioned, recycled or disposed ofwhen recycling is impossible.The nuclear industry is no exception to the rule.This waste, however, only represents a tiny frac-tion of the total amount that society produces.

NUCLEAR WASTE DISPOSAL METHODSDEPEND ON HOW LONG THE WASTEREMAINS RADIOACTIVE.

For the sake of comparison, France produces2,500 kg of industrial waste per capita everyyear (including 100 kg of toxic waste) comparedto 1 kg of nuclear waste of which only 10 g ishigh-level waste. Quantity is not, however, theonly factor to be considered; toxicity is also veryimportant. This is why a great deal of researchfocuses on waste treatment and disposalmethods. Nuclear waste is produced at everystage of the nuclear fuel cycle: uranium mining

© CEA/E. Joly

Industrial waste per capitaper year: 2,500 kg

Less than 1 kg (0.04%)of nuclear waste

Less than 100 g of B + C waste

Including less than10 g of C waste

THE THREE CATEGORIES OF RADIOACTIVE WASTE

Category A• Short-lived (half-life less than 30 years) low- andintermediate-level waste.

– “Beta” and “gamma” radiation.

– Radioactivity comparable with naturally occurringradioactivity after 300 years.

> Origin: laboratories, nuclear medicine, industry (food processing, metallurgy, etc.), nuclear plants(contaminated items: gloves, filters, resins, etc.).

Category B• Long-lived (half-life several tens of thousands of years)low- and intermediate-level waste.

– “Alpha” radiation.

Category C• Long-lived, high-level waste, giving off heat for severalhundred years.

– “Alpha”, “beta” and “gamma” radiation.

> Origin: reprocessing of spent fuel from nuclear powerplants (combustion ashes).

> NUCLEAR WASTE


19


> NUCLEAR WASTE18

and enrichment, fuel fabrication, reactor oper-ation and reprocessing. Dismantling nuclearfacilities also creates waste. Radioactive wasteis also produced by research centres (such asthe CEA) as well as industries and hospitalsusing radioactive elements.

SORTING AND DISPOSING OFRADIOACTIVE WASTEAs not all radioactive waste is the same, it isclassified according to two criteria for disposalpurposes:• activity level, i.e. the radiation intensity, whichdetermines the degree of radiological protec-tion required,• radioactive half-life, which determines how

long the waste maybe harmful.Waste is thereforedistinguished accor -ding to its lifetime

and activity as follows.¥ Short-lived low- and intermediate-level waste.This accounts for 90% of radioactive waste pro-duced in France. After 300 years, it has lost almostall its activity (see Radioactivity booklet). It iscompacted in steel or concrete containers thatare disposed of in surface repositories. There aretwo of these in France, one in La Hague (Manche),the other in Soulaines (Aube). They are managedby Andra, the French national agency for radioac-tive waste management.¥ Long-lived and/or high-level waste (10% ofthe total volume). The radioactive decay of this

waste spans thousands, or even hundreds ofthousands, of years. It is embedded in bitumen,cement or glass. In France, a law was passedin 1991 to determine what should be done aboutthis type of waste. Deep geological disposal isone option considered. One underground lab-oratory has been built to study this option. Otheroptions are transformation into shorter-livedwaste in a nuclear reactor (this is known as trans-mutation), studies of new conditioning processesand long-term, surface or sub-surface storage(sub-surface means several tens of metres belowthe surface). Until a final decision is reached,this waste is being held in surface facilities inLa Hague and Marcoule.

RESEARCH ON LONG-LIVEDWASTEReducing the volume and activity of solid andliquid waste is among the top priorities of current research and development work, whichincludes:• CEA research on the separation and trans-mutation of long-lived radioactive elementscontained in this waste,• the CEA’s study of conditioning and long-term, surface and sub-surface storageprocesses,• the study of reversible or irreversible disposaloptions in deep geological formations, work thatis largely supported by the underground labo-ratory built by Andra.Safeguarding humans and their environmentis a prominent part of the work of CEA

“After 300 years, 90% of waste is no longer radioactive.”

“In 2006, the Frenchparliament willannounce its decisionconcerning theprefered managementscheme for long-livednuclear waste.”

Conditioning waste in a concrete matrix.

Shielded vitrification line for fission products.

© CEA

© CEA/Foulon

Radioactive half-life of aradioelement: the time requiredfor half the atoms initially foundin the radioelement todisappear due to adisintegration process.

researchers and engineers, who take specialcare to develop processes and technologiesaimed at constantly reducing risks relating toradioactivity. On a day-to-day basis, they takethe same care in managing the waste producedby their own research work.

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CEA - The nuclear fuel cycle · From the uranium mine to waste disposal 7> The nuclear fuel cycle 4 > UPSTREAM THE REACTOR: PREPARING THE FUEL 5 AFTER MINING, THE URANIUM IS PURIFIED,