PDW
Peter D. Wilson
DURATION ABOUT 40 MINUTES
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 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
PDW
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PDW
 Diminished reactivity owing to
– substantially reduced fissile content much of initial enrichment consumed not entirely compensated by new plutonium
– neutron-absorbing fission products
 Somewhat weakened structure
 Possible pressurisation by fission gases
 Nearly all original fertile content (U-238)
Reasons for discharge
 Minor actinide content (Np, Am, Cm) super-proportional to irradiation
 Continuing heat release from decay of fission products & minor actinides
 Potential for much greater energy generation than already realised
(by up to 2 orders of magnitude)
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(after decay storage)
Direct Disposal
 Minimises operations and cost
 Minimises immediate risk of illicit diversion, but
 Leaves Pu content intact with gradually rising quality and decaying radioactive defence -
“plutonium mine”
 Minimises secondary wastes
 Abandons all remaining energy potential after at best ca. 1% utilisation of mined uranium
(including enrichment tails)
PDW
Reprocessing
 Major industrial operations
 Recovers fissile and fertile materials for further use
 In principle permits near-elimination of fissile content
 Minimises HLW volume, but
 Generates more ILW & LLW
 Operational radiation exposure
 Permits recycling
– potentially 50 - 100% utilisation
– but without fast reactors only
~15-30% improvement over oncethough
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PDW
 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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PDW
Wet
 Water provides cooling and shielding

Dry
Avoids corrosion especially of
Magnox
 Permits direct sight and manipulation
 Avoids need for water purification
 Requires strong structure
 Needs continual purification and leak monitoring
 Allows tighter packing
– less risk of criticality
 Remote manipulation
 Tends to cause corrosion
 Liable to create uncomfortably humid working environment needs good ventilation
 Needs more complex building and equipment
 Requires guided convection or forced-air cooling
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 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 (?)
Nuclear Familiarisation - Reprocessing and Recycling
PDW
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 Operational and environmental safety
– nuclear (avoiding criticality)
– against radiation & contamination
 Product quality - decontamination by10 6 - 10 8
 Manageable wastes
PDW
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PDW
 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
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 Separation of fuel from cladding
PDW
Magnox, peel & dissolve;
Oxide, chop & leach
 Dissolution of fuel substance
 Extraction of uranium and plutonium into solvent
–
1 st Sellafield plant Butex, since 1964 tributyl phosphate (TBP) diluted with e.g kerosene
 Separate backwashing of plutonium and uranium
– plutonium backwash assisted by chemical reduction
 Concentration and storage of wastes (fission products etc)
 Waste conditioning for eventual disposal
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Aqueous
Solvent
Solvent purification
(alkali wash)
PDW
Dissolution
U, Pu,
FPs
Extraction
U, Pu
FPs
Reductive backwash
Pu
U
Highly-active waste
Plutonium purification
Dilute acid backwash
U
Uranium purification
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Loaded solvent
Aqueous feed
Fresh solvent
Depleted aqueous
PDW
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
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Part of mixer-settler bank
Nuclear Familiarisation - Reprocessing and Recycling
PDW


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 lowirradiated fuel
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 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
Nuclear Familiarisation - Reprocessing and Recycling
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PDW
 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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 Combination of radiolysis and acid attack
PDW
 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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 Absolute separation of radioactive from inactive material impossible
– most fission products etc. confined to small volume
– some inevitably emerge in other streams
PDW
 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
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PDW
 High level (HLW) - sufficiently radioactive for heat release to be significant in storage or disposal
 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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 HLW - vitrified fission products, minor actinides and corrosion products mostly from the first cycle raffinate
PDW
 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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 Confine as much as possible of the heatreleasing radionuclide waste to a small volume of glass - HLW
PDW
For eventual deep disposal
 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
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 Finishing - conversion to a form suitable for sale, use or storage
– Uranium
– thermal denitration to UO
3
PDW
– Plutonium
– precipitation as oxalate
– calcination to PuO
2
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 To make the most of a finite resource
PDW
 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
PDW
 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
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PDW
 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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PDW
 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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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
PDW
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Near term
 Reconstitution of oxide fuel for CANDU (Dupic)
– possibly with minimal process to remove volatiles
 Sol-gel vibro-packing route
PDW
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
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