Familarization with Nuclear energy supply

Familarization with Nuclear energy

1. Page 1Nuclear Familiarisation -Reprocessing and Recycling
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FAMILIARISATION WITH
NUCLEAR TECHNOLOGY
REPROCESSING AND RECYCLING
Peter D. Wilson
DURATION ABOUT 40 MINUTES

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WHY REPROCESS?
Originally
–To obtain plutonium for military use
Currently
–To ease storage problems
especially Magnox -cladding corrodes easily
–To concentrate high-level waste
–To recover clean plutonium and uranium
–As a business opportunity

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DISCHARGED FUEL HAS -
Diminishedreactivityowingto
–substantiallyreducedfissilecontent
muchofinitialenrichmentconsumed
notentirelycompensatedbynewplutonium
–neutron-absorbingfissionproducts
Somewhatweakenedstructure
Possiblepressurisationbyfissiongases
Nearlyalloriginalfertilecontent(U-238)
Minoractinidecontent(Np,Am,Cm)super-proportionaltoirradiation
Continuingheatreleasefromdecayoffissionproducts&minoractinides
Potential for much greater energy generation than already realised
(by up to 2 orders of magnitude)
Reasons for
discharge

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MANAGEMENT OPTIONS
(after decay storage)
Direct Disposal
Minimisesoperationsandcost
Minimisesimmediateriskof
illicitdiversion,but
LeavesPucontentintactwith
graduallyrisingqualityand
decayingradioactivedefence-
“plutoniummine”
Minimisessecondarywastes
Abandons all remaining energy
potential after at best ca. 1%
utilisation of mined uranium
(including enrichment tails)
Reprocessing
Majorindustrialoperations
Recovers fissile and fertile materials
for further use
In principle permits near-elimination
of fissile content
Minimises HLW volume, but
GeneratesmoreILW&LLW
Operationalradiationexposure
Permitsrecycling
–potentially 50 -100% utilisation
–but without fast reactors only
~15-30% improvement over once-
though

5. Page 5Nuclear Familiarisation -Reprocessing and Recycling
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PROCEDURE -CLOSED CYCLE
Local storage for decay of heat release
Transport to reprocessing site
Further decay storage to limit radiation
Reprocessing
–separation of uranium & plutonium from each other
and from fission products
–finishing U & Pu products
purification and conversion to form for use or storage
–conditioning wastes for disposal
Refabrication of U and Pu into new fuel

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DELAY STORAGE
Wet
Water provides cooling and
shielding
Permits direct sight and
manipulation
Requires strong structure
Needs continual purification and
leak monitoring
Tends to cause corrosion
Liable to create uncomfortably
humid working environment -
needs good ventilation
Dry
Avoids corrosion especially of
Magnox
Avoids need for water
purification
Allows tighter packing
–less risk of criticality
Remote manipulation
Needs more complex building
and equipment
Requires guided convection or
forced-air cooling

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TRANSPORT FLASK REQUIREMENTS
Shielding appropriate
to radioactive content
(gamma, neutron)
Heat dispersion
adequate for maximum
thermal load
With customary water
coolant, robust
containment of
activated corrosion
products
Structural integrity
maintained against
worst credible impact
or fire
Photo copyright BNFL (?)

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PROCESS REQUIREMENTS
Operational and environmental safety
–nuclear (avoiding criticality)
–against radiation & contamination
Product quality -decontamination by10
6
-10
8
Manageable wastes

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BASIS OF SEPARATION PROCESS
Uranium and plutonium in their most stable chemical states
are readily soluble in both nitric acid and certain organic
solvents immiscible with it
Fission products generally are at most very much less so.
–iodine (a major exception) is largely boiled off during
dissolution
Equilibrium distribution depends on e.g. acidity
Uranium and plutonium can therefore be extracted from a
fuel solution and then taken back into clean dilute acid

10. Page 10Nuclear Familiarisation -Reprocessing and Recycling
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Separationoffuelfromcladding
Dissolutionoffuelsubstance
Extractionofuraniumandplutoniumintosolvent
–1
st
Sellafield plant Butex,
since 1964 tributyl phosphate (TBP) diluted with e.g kerosene
Separatebackwashingofplutoniumanduranium
–plutonium backwash assisted by chemical reduction
Concentrationandstorageofwastes(fissionproductsetc)
Wasteconditioningforeventualdisposal
REPROCESSING STAGES
Magnox, peel & dissolve;
Oxide, chop & leach

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PUREX PROCESS OUTLINE
U, Pu,
FPs
U, Pu
FPs
Highly-active
waste
Pu
Plutonium
purification
U
U
Uranium
purification
Solvent purification
(alkali wash)
Extraction
Reductive
backwash
Dilute acid
backwash
Dissolution
Aqueous
Solvent

12. Page 12Nuclear Familiarisation -Reprocessing and Recycling
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COUNTERCURRENT OPERATION
Fresh solvent
Aqueous
feed
Loaded solvent
Depleted
aqueous
Required separation factors need many stages of equilibrium or
equivalent in partial equilibrations
Loaded solvent meets the most concentrated aqueous solution
Fresh solvent meets depleted aqueous feed
Thus extraction and loading are maximised
Similar principles apply in reverse to backwashing
Design challenge is to maximise local inter-phase contact without
excessive longtitudinal mixing
Contact between solvent and aqueous may be continuous or stagewise

13. Page 13Nuclear Familiarisation -Reprocessing and Recycling
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MIXER-SETTLER
Physical & theoretical stages
very nearly equivalent
Simple to design and operate
–can be set up effectively with
beakers and bent tubes on a bench
Tolerates variable throughput
BUT
Large settler volume at each
stage
Therefore long residence time,
high process inventory and
solvent degradation
Poor geometry for high
plutonium content
NEVERTHELESS
Adequate for uranium and low-
irradiated fuel
Part of mixer-settler bank

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PULSED COLUMN
Multiple stage equivalence with settler
volumes only at top and bottom
Tall, thin profile -good for nuclear safety
Gamma loss & short residence time reduce
solvent degradation
Therefore satisfactory for plutonium and
fairly high-irradiated fuel
BUT
Performance depends on conditions
–limited range of throughput
Prediction largely empirical and approximate
Needs sophisticated operational control
Height requires tall buildings, seismic
qualification expensive

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REDUCTIVE BACKWASH
Necessary for clean separation of plutonium from uranium
–Pu(III) very much less extractable than Pu(IV)
Magnox plant uses ferrous sulphamate
–leaves salt residue (ferric sulphate)
corrosive
limits volume reduction -intended for discharge after
decay storage, so
must be kept free from major contamination
–therefore U/Pu split in second cycle
Thorp uses uranous nitrate
–waste contains no residual salts
–can be greatly concentrated by evaporation
–therefore acceptable in first cycle (early split)
nearly didn’t work -unexpected complications from technetium

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SOLVENT DEGRADATION
Combination of radiolysis and acid attack
Short-term, i.e. within cycle (chiefly TBP extractant)
–forms (a) dibutyl and (b) monobutyl phosphates
–(a) impairs backwash
–(b) forms precipitates
–removed by alkaline wash
Long-term(largely diluent)
–forms acids, alcohols, ketones, nitro-compounds etc.
–impair decontamination and settling
–only partly removed by washing
–require gradual or complete solvent change
–waste solvent needs disposal

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WASTE MANAGEMENT PRINCIPLES
Absolute separation of radioactive from inactive material
impossible
–most fission products etc. confined to small volume
–some inevitably emerge in other streams
Radioactive content confined as far as practicable to
eventually solid forms for disposal
Some very difficult to confine reliably, e.g. iodine, krypton
–very small dose to everyone preferred to risk of local
accidental high dose
–therefore dilution & dispersion rather than concentration

18. Page 18Nuclear Familiarisation -Reprocessing and Recycling
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SOLID WASTE CLASSIFICATION
Highlevel(HLW)-sufficientlyradioactiveforheatreleaseto
besignificantinstorageordisposal
Low level(LLW) -no more than
4 GBq alpha per tonne or
12 GBq beta/gamma per tonne
Intermediate level(ILW) -higher than LLW but not
significantly heat-releasing
Very low level(VLWW) -disposable with ordinary rubbish
bulk less than 4 GBq/m
3
beta/gamma
no single item over 40 kBq beta/gamma

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RADIOACTIVE WASTES
HLW -vitrified fission products, minor actinides and
corrosion products mostly from the first cycle raffinate
ILW-cladding fragments, plutonium-contaminated
materials, resins & sludges from effluent treatment,
scrapped equipment
LLW-e.g. domestic-type rubbish from active areas, mildly
contaminated laboratory equipment
Low-level liquid-treated effluents from ponds,
condensate from evaporators, etc.
Gaseous-filtered and treated ventilation air from cells
and working areas

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SELLAFIELD WASTE MANAGEMENT
Confine as much as possible of the heat-
releasing radionuclide waste to a small
volume of glass -HLW
Immobilise other substantially radioactive
waste (without troublesome heat release)
with cement -ILW
Pack and encapsulate low-level solid waste in secure
containers for near-surface burial
Discharge hard-to-confine species e.g. iodine, krypton
Otherwise discharge as little as reasonably achievable in
liquid and gaseous effluents
For eventual
deep disposal

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PRODUCT FINISHING
Finishing -conversion to a form suitable for sale, use or
storage
–Uranium
–thermal denitration to UO
3
–Plutonium
–precipitation as oxalate
–calcination to PuO
2

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WHY RECYCLE?
To make the most of a finite resource
To reduce short-term need for fresh mining
–Most environmentally damaging part of industry
To reduce storage or disposal requirements for materials
with little or no other legitimate use
–e.g. over a million tonnes depleted uranium world-wide
plutonium from decommissioned weapons
To put fissile material out of reach of potential terrorists

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Uranium
–recovered from oxide still has more than natural enrichment
could be used “as is” in CANDU
–also has U-232 (radiation hazard from daughters) and
–U-234 & U-236 (neutron absorbers) -though U-234 fertile
Plutonium
–contains
–Pu-238 (heat & neutron emission)
–Pu-240, Pu-241 (parent of Am-241 -radiation hazard) & Pu-242
–as well as desirable Pu-239
–only odd-numbered isotopes fissile
Current reactors take at most a partial load of plutonium-enriched fuel;
newer types designed for full load
Refabricating recycled civil material more expensive than fresh
but can be offset by avoiding isotopic enrichment of uranium
FACTORS RELEVANT TO RECYCLING

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DIFFICULTIES IN RECYCLING AS MOX
Deleterious isotopes in uranium
–U-236; unproductive neutron absorber
–U-232; extremely energetic -emitting daughter Tl-208
Requirement for intimate mixing, ideally solid solution
–to avoid hot spots weakening cladding
–achievable but difficult in solid state
–co-precipitation tends to some segregation
–sol-gel process may be preferable in future
Plutonium oxide very hard to dissolve in pure nitric acid
–a mixed product from a future reprocessing plant would
be more tractable

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PRACTICAL RECYCLING
Uranium
–1600 te AGR fuel produced from re-enriched recovered
uranium
–manufacture essentially as from fresh material
–generally cheaper to use fresh -but for how long?
Plutonium
–used in about 2% of current fuel manufacture
–~2000 tonnes fuel so far
–in UK as powder dry-blended with uranium dioxide,
formed into loose aggregates, pressed into pellets,
sintered, ground to size and packed into tubes
–elements distinguished only by identification markings

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FUTURE REPROCESSING
Aim to simplify, reduce waste arisings and costs at source
Single-cycle flowsheet?
–increased cycle decontamination, or
–reduced (more realistic) specification
Intensified process equipment
–continuous dissolver
–centrifugal solvent-extraction contactors
(essentially short-residence mixer-settlers)
Different (e.g. pyrochemical) processes for special fuels
Waste partitioning (e.g. for transmutation)
–currently seems an unjustifiable complication

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FUTURE RECYCLING
Near term
Reconstitution of oxide fuel for CANDU (Dupic)
–possibly with minimal process to remove volatiles
Sol-gel vibro-packing route
Distant
Molten salts
–as process medium
avoids large volumes of aqueous waste
generally poorer separations
–as fuel?
–symbiosis between pyrochemical reprocessing and molten-salt
reactors