page 1nuclear familiarisation - reprocessing and recycling pdw familiarisation with nuclear...
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Page 1Nuclear Familiarisation - Reprocessing and Recycling
PDW
FAMILIARISATION WITH
NUCLEAR TECHNOLOGY
REPROCESSING AND RECYCLING
Peter D. Wilson
DURATION ABOUT 40 MINUTES
Page 2Nuclear Familiarisation - Reprocessing and Recycling
PDWWHY 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
Page 3Nuclear Familiarisation - Reprocessing and Recycling
PDWDISCHARGED FUEL HAS - 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)
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)
Reasons for discharge
Page 4Nuclear Familiarisation - Reprocessing and Recycling
PDWMANAGEMENT OPTIONS (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)
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 once-though
Page 5Nuclear Familiarisation - Reprocessing and Recycling
PDWPROCEDURE - CLOSED CYCLE
Local storage for decay of heat releaseTransport to reprocessing siteFurther decay storage to limit radiationReprocessing
– separation of uranium & plutonium from each other and from fission products
– finishing U & Pu productspurification and conversion to form for use or storage
– conditioning wastes for disposalRefabrication of U and Pu into new fuel
Page 6Nuclear Familiarisation - Reprocessing and Recycling
PDWDELAY 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
Page 7Nuclear Familiarisation - Reprocessing and Recycling
PDWTRANSPORT 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 (?)
Page 8Nuclear Familiarisation - Reprocessing and Recycling
PDWPROCESS REQUIREMENTS
Operational and environmental safety– nuclear (avoiding criticality)– against radiation & contamination
Product quality - decontamination by106 - 108
Manageable wastes
Page 9Nuclear Familiarisation - Reprocessing and Recycling
PDWBASIS OF SEPARATION PROCESSUranium 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
Page 10Nuclear Familiarisation - Reprocessing and Recycling
PDW
Separation of fuel from cladding
Dissolution of fuel substance
Extraction of uranium and plutonium into solvent– 1st 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
REPROCESSING STAGES
Magnox, peel & dissolve;Oxide, chop & leach
Page 11Nuclear Familiarisation - Reprocessing and Recycling
PDWPUREX PROCESS OUTLINE
U, Pu,
FPsU, 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
Page 12Nuclear Familiarisation - Reprocessing and Recycling
PDWCOUNTERCURRENT OPERATIONFresh solvent
Aqueous feed
Loaded solvent
Depletedaqueous
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
Page 13Nuclear Familiarisation - Reprocessing and Recycling
PDWMIXER-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 throughputBUT 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 fuelPart of mixer-settler bank
Page 14Nuclear Familiarisation - Reprocessing and Recycling
PDWPULSED 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
Page 15Nuclear Familiarisation - Reprocessing and Recycling
PDWREDUCTIVE BACKWASHNecessary 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
Page 16Nuclear Familiarisation - Reprocessing and Recycling
PDWSOLVENT DEGRADATIONCombination 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
Page 17Nuclear Familiarisation - Reprocessing and Recycling
PDWWASTE 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
Page 18Nuclear Familiarisation - Reprocessing and Recycling
PDWSOLID WASTE CLASSIFICATION
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 rubbishbulk less than 4 GBq/m3 beta/gamma
no single item over 40 kBq beta/gamma
Page 19Nuclear Familiarisation - Reprocessing and Recycling
PDWRADIOACTIVE WASTESHLW - 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
Page 20Nuclear Familiarisation - Reprocessing and Recycling
PDWSELLAFIELD WASTE MANAGEMENTConfine 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
Page 21Nuclear Familiarisation - Reprocessing and Recycling
PDWPRODUCT FINISHINGFinishing - conversion to a form suitable for sale, use or
storage– Uranium
– thermal denitration to UO3
– Plutonium– precipitation as oxalate– calcination to PuO2
Page 22Nuclear Familiarisation - Reprocessing and Recycling
PDWWHY 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
Page 23Nuclear Familiarisation - Reprocessing and Recycling
PDW
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 freshbut can be offset by avoiding isotopic enrichment of uranium
FACTORS RELEVANT TO RECYCLING
Page 24Nuclear Familiarisation - Reprocessing and Recycling
PDWDIFFICULTIES IN RECYCLING AS MOXDeleterious 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
Page 25Nuclear Familiarisation - Reprocessing and Recycling
PDWPRACTICAL RECYCLINGUranium
– 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
Page 26Nuclear Familiarisation - Reprocessing and Recycling
PDWFUTURE 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
Page 27Nuclear Familiarisation - Reprocessing and Recycling
PDWFUTURE 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 mediumavoids large volumes of aqueous wastegenerally poorer separations
– as fuel?– symbiosis between pyrochemical reprocessing and molten-salt
reactors