Science of Nuclear Energy and Radiation - nuceng.ca · world: ~ 30% of world uranium market. l...

Science of NuclearEnergy and Radiation

Ben Rouben

1998 June

CNS

The Nuclear Fuel Cycle

Ben RoubenManager, Reactor Core Physics

AECL

Topic of Discussion

Nuclear fuel cycle. Will covervarious phases in use of nuclear fuel, frommining to disposal, and look at differentforms that nuclear fuel can take in areactor. Specific emphasis on possibleapplications to CANDU reactors.

Opportunities to ponder issues withimportant social impact: use of resources,efficiency and economy of utilization,“throw away or recycle”, possibleproliferation of prescribed materials, andhow to deal with fuel after it’s irradiated.

Front End: Mine to Fuel

Front end: production of nuclear fuel.First phase obviously mining of uranium.Canada (Sask.) has large deposits ofuranium ore, biggest uranium producer inworld: ~ 30% of world uranium market.

l uranium leached out of uraninite, givesyellowcake, U3O8

l U3O8 reduced with H to UO2 powder,compacted into pellets, sintered to a hardceramic, ground to 12-mm diameter, 16-mm length

Pellets

From Mine to Fuel (Cont’d)

A number of pellets are encased in~50-cm-long “elements” made of zircaloy,which are then assembled into bundleswith 28 or 37 elements per bundle.

A 37-element bundle contains about20 kg of uranium.

Fuel Enrichment

Above sequence applies to natural-uranium fuel, as used in CANDU reactors.

For other reactors, uranium must beenriched in 235U. In this case yellowcakeis first converted to gaseous uraniumhexafluoride, UF6, then subjected toenrichment (by gaseous diffusion orcentrifuge process).

Enrichment increases manufacturingcost of nuclear fuel significantly.

Once-Through Fuel Cycle

Only new fuel enters reactor, and onceused up, is removed permanently forstorage and eventual disposal.

Main energy source in uranium fuel isinitially fission in 235U. However,plutonium is produced from neutronabsorption in “fertile” 238U. 239Pu, 241Puboth fissile, contribute to energy released:plutonium is both created and burned in-situ. In CANDU, half total energyoriginates in Pu fissions.

Once-Through Fuel Cycle (Cont’d)

As fuel is burned in CANDU, the 235Uis depleted, but there is a net increase ofplutonium with time.

Fissile component starts at 0.71%(235U concentration in natural uranium). Atdischarge, total fissile component still0.50% (0.23% 235U, 0.27% fissile Pu):much energy left in irradiated fuel.Discharged not because fully used-up, butneutron-absorbing fission products havebuilt up, become a net load on system.


Typical light-water fuel, initial fissilecontent is 3.5% 235U, at discharge ~1.5%(0.9% 235U, 0.6% fissile Pu): > twicefissile content of natural uranium!

The once-through cycle is simple, but“spent” fuel really far from spent! Simply“throwing away” energy in discharged fuelis in fact extremely wasteful.

CANDU and PWR Fuel

Natural Uranium

0.7% U-235

Spent NU CANDU Fuel

0.23% U-2350.27% Puf

Enriched Uranium

3.5% U-235

Spent PWR Fuel

0.9% U-2350.6% Puf

PWR

CANDU


Once-through nuclear fuel cycle notcompatible with “reduce, reuse, recycle”philosophy. Two reasons why once-through cycle continues to be used:

l economics: price of natural uranium issufficiently low to make cost of recyclingused fuel unattractive.

l politics: some countries (U.S.) havebanned recycling (reprocessing) ofcommercial fuel, to set example anddiscourage proliferation of nuclear-weapons material (plutonium).

Alternative Fuel Cycles for CANDU

Neutron economy of CANDU permitsnatural uranium + gives great flexibilityfor application of other fuels. Somealternatives described here. Idea is toextend resources by increasing uraniumutilization, i.e., energy per kg mined U.

Some concepts described here alsoapplicable, sometimes already in use (tovarying degrees) in other reactor types,whereas others are exclusive to CANDU.

Slightly Enriched Uranium

Slightly enriched uranium (SEU)contains greater concentration of 235U thannatural uranium.

Concentrations in range 0.9-1.2% (cf0.71% in natural U) can be used inCANDU without changes in reactor, andwith optimum uranium utilization.

Light-water reactors (LWR) useenriched U, but because of poorer neutroneconomy, need enrichments of ~ 3% andgreater.

Uranium Utilization

3.25

%, 3

3 M

Wd/

kg

218

192 188

157

119 114

100

150

200

Mg

U /G

We

yr

3.25%33 MWd/kg

3.7%43 MWd/kg

4.4%53 MWd/kg

NU7.5 MWd/kg

0.9% SEU14 MWd/kg

1.2% SEU21 MWd/kg

PWR

CANDU

Recovered Uranium

Fuel discharged from LWR contains~0.9% 235U. If fuel reprocessed so that Puand U are separated, then recovereduranium ~ equivalent to SEU, could beused in CANDU.

Recovered uranium available fromcommercial fuel reprocessors, but notusable as is in other reactors - enrichmenttoo low: Use in CANDU would in effectreduce amount of fuel waste from otherreactors: synergism between CANDU andother reactors.

Mixed-Oxide Fuel (MOX)

If fuel is reprocessed, then fissile Pualso available: in PuO2 form, could bemixed with “virgin” UO2 to make mixed-oxide (MOX) fuel to be burned inCANDU.

With sufficient enrichment, MOX canalso be used in LWR - already used inEurope, but limited to ~1/3 core. U.S.does not burn MOX.

Fuel cycle aims at > total energy fromoriginal mined U, + amount of waste perunit of electricity produced much reduced.

Weapons-Derived Pu

MOX can also be made starting withPu derived from weapons. U.S., Russiahave agreed to reduce nuclear arsenals,>100 tonnes of Pu from weapons.

This use of military Pu reactors isultimate swords-to-plowsharesopportunity: a useful commodity beingcreated at the same time as a threat toworld peace is significantly reduced!

DUPIC

Chemical (wet) reprocessing separatesU, Pu - considered by some asproliferation risk.

DUPIC: Direct Use of PWR Fuel inCANDU. Research Canada-Korea project- alternative to chemical reprocessing.

“Spent” PWR fuel first mechanicallydecladded, then treated by dry oxidation-reduction process to remove volatilefission products. Yields powder which canbe pressed into pellets again.

DUPIC (Cont’d)

DUPIC does not involve chemicalseparation of U, Pu - superiorsafeguardability. Fuel will have a fissilecontent of ~ 1.5%, so cannot be used inPWRs.

But in CANDU, would yield ~ twiceas much energy again asproduced inoriginal PWR cycle - ideal synergismbetween CANDU and PWR: Again, totalwaste much reduced.

Research now: some DUPIC fuelelements have been produced, testirradiations to be conducted.

CANDU / PWR Synergism

Spent PWRFuel

0.9% U-2350.6% Pu-fissile

- MOX - 0.9% U-235 - Actinides

EnrichedFuel

3.5% U-235

CANDUCANDUDry ProcessPWRPWR

Reprocessing Plant

0.9% U-2350.6% Pu

DUPICFuel

“Unconventional” reprocessing . . .“Advanced” recycling . . .

Fast Breeder Reactor

Most reactors operating today arethermal reactors. However, fast (i.e., fast-neutron) reactors are also possible, and infact prototypes have been built. Whileprobability of fission is much smaller athigh neutron energies, number of neutronsproduced per fission is higher, and extraneutrons can be used to produce more Pufrom fertile 238U. In fact, more fissilematerial can be produced than isconsumed! Fast breeder reactor (FBR)creates its own fuel, potential of extendingutilization of U resources to centuries.

Thorium Cycle

There is another fertile isotope besides238U: 232Th, on neutron absorption and β-decay, yields 233U, a fissile isotope. Thusthorium can be used to produce 233U,which can then be burned just as 235U or239Pu.

Since there is approximately threetimes as much thorium as uranium in theworld, this would be another way ofextending precious uranium resources.

Back End - Bays

No matter which fuel cycle is used,there is eventually fuel to be disposed of:“back end” of fuel cycle.

Spent-Fuel Bays

Fuel which comes out of reactor is“hot” - both temperature-hot andradioactivity-hot. First step in dealingwith fuel is storage in water-filled spent-fuel bays. There, water provides cooling,and shielding against the radioactivity.

Back End - Air Storage

After several years in spent-fuel bay(typically 6 years for CANDU natural-Ufuel), fuel has cooled sufficiently to bemoved out.

Can then be stored in air in dry-storagemodules above ground. Modulesconstructed of concrete, which providesshielding, while air provides cooling. Canprovide dry storage for decades - even 50years, until a permanent disposal facility isavailable.

MACSTOR at Gentilly 2

Back End - Permanent Disposal

Ultimate step is permanent disposal ofeither the irradiated fuel as is, or of wastesarising from reprocessing.

Aim is to permanently and safelydispose of radioactive material so it isisolated from biosphere for an appropriatelength of time - say, 10,000 years.

Method under consideration in manycountries is geologic disposal - describedin a separate lecture.

Conclusion

Expanding world population, strongdesire in developing countries for standardof living equal to the West’s, pollution andconcern for the environment, threat ofglobal warming, what is most important toyou? To future generations?

A reliable energy source? Orconservation, to cut down on pollution?Price of energy? Fuels which “guarantee”non-proliferation of sensitive material? Orextension of precious natural resources?

Conclusion (Cont’d)

Energy has become an importantcomponent of our way of life. More andmore in the future, societies will have toface choices on how much to use, andwhere to get it.

Science of Nuclear Energy and Radiation - nuceng.ca · world: ~ 30% of world uranium market. l...

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