page 1nuclear familiarisation - reprocessing and recycling pdw familiarisation with nuclear...

Nuclear Familiarisation - Reprocessing and Recycling

PDW

FAMILIARISATION WITH

NUCLEAR TECHNOLOGY

REPROCESSING AND RECYCLING

Peter D. Wilson

DURATION ABOUT 40 MINUTES


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


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


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


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


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


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 (?)


PDWPROCESS REQUIREMENTS

Operational and environmental safety– nuclear (avoiding criticality)– against radiation & contamination

Product quality - decontamination by106 - 108

Manageable wastes


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


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


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


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


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


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


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


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


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


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


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


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


PDWPRODUCT FINISHINGFinishing - conversion to a form suitable for sale, use or

storage– Uranium

– thermal denitration to UO3

– Plutonium– precipitation as oxalate– calcination to PuO2


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


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


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


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


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


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

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