Familarization with Nuclear energy supply

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

PDW
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

PDW
DISCHARGED 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

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MANAGEMENT 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

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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

PDW
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 by106 - 108
 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

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 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

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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

PDW
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

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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

PDW
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

PDW
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

PDW
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

PDW
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

PDW
SOLID 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 rubbish
bulk less than 4 GBq/m3 beta/gamma
no single item over 40 kBq beta/gamma

PDW
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

PDW
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

PDW
PRODUCT FINISHING
 Finishing - conversion to a form suitable for sale, use or
storage
– Uranium
– thermal denitration to UO3
– Plutonium
– precipitation as oxalate
– calcination to PuO2

PDW
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

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 fresh
but can be offset by avoiding isotopic enrichment of uranium
FACTORS RELEVANT TO RECYCLING

PDW
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

PDW
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

PDW
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

PDW
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

Familarization with Nuclear energy supply

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