INTRODUCTION TO NUCLEAR TECHNOLOGY

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Transcript INTRODUCTION TO NUCLEAR TECHNOLOGY

PDW
FAMILIARISATION WITH
NUCLEAR TECHNOLOGY
REPROCESSING AND RECYCLING
Peter D. Wilson
DURATION ABOUT 40 MINUTES
Nuclear Familiarisation - Reprocessing and Recycling
Page 1
WHY REPROCESS?
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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
Nuclear Familiarisation - Reprocessing and Recycling
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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
Reasons for
discharge

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)
Nuclear Familiarisation - Reprocessing and Recycling
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MANAGEMENT OPTIONS
(after decay storage)
Direct Disposal
Reprocessing

Minimises operations and cost

Major industrial operations

Minimises immediate risk of
illicit diversion, but

Recovers fissile and fertile materials
for further use

Leaves Pu content intact with
gradually rising quality and
decaying radioactive defence “plutonium mine”

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

Minimises secondary wastes

Abandons all remaining energy
potential after at best ca. 1%
utilisation of mined uranium
(including enrichment tails)
Nuclear Familiarisation - Reprocessing and Recycling
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PROCEDURE - CLOSED CYCLE
 Local
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storage for decay of heat release
 Transport
 Further
to reprocessing site
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
Nuclear Familiarisation - Reprocessing and Recycling
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DELAY STORAGE
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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
Nuclear Familiarisation - Reprocessing and Recycling

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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

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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
Nuclear Familiarisation - Reprocessing and Recycling
PDW
Photo copyright BNFL (?)
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PROCESS REQUIREMENTS
PDW
 Operational
and environmental safety
– nuclear (avoiding criticality)
– against radiation & contamination
 Product
quality - decontamination by106 - 108
 Manageable
wastes
Nuclear Familiarisation - Reprocessing and Recycling
Page 8
BASIS OF SEPARATION PROCESS
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 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
Nuclear Familiarisation - Reprocessing and Recycling
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REPROCESSING STAGES
 Separation
of fuel from cladding
 Dissolution
of fuel substance
 Extraction
Magnox, peel & dissolve;
Oxide, chop & leach
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
 Waste
and storage of wastes (fission products etc)
conditioning for eventual disposal
Nuclear Familiarisation - Reprocessing and Recycling
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PUREX PROCESS OUTLINE
Aqueous
Solvent
Solvent purification
(alkali wash)
U, Pu,
Dissolution
Extraction
FPs
FPs
Highly-active
waste
Nuclear Familiarisation - Reprocessing and Recycling
U, Pu
Reductive
backwash
Pu
Plutonium
purification
U
Dilute acid
backwash
U
Uranium
purification
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COUNTERCURRENT OPERATION
Loaded solvent
Aqueous
feed
PDW
Fresh 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
Nuclear 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 lowirradiated fuel

Part of mixer-settler bank
Nuclear Familiarisation - Reprocessing and Recycling
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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
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BUT

Performance depends on conditions
– limited range of throughput

Prediction largely empirical and approximate
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Needs sophisticated operational control
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Height requires tall buildings, seismic
qualification expensive
Nuclear Familiarisation - Reprocessing and Recycling
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REDUCTIVE BACKWASH
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 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
Nuclear Familiarisation - Reprocessing and Recycling
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SOLVENT DEGRADATION
 Combination
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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
Nuclear Familiarisation - Reprocessing and Recycling
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WASTE MANAGEMENT PRINCIPLES
PDW
 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
Nuclear Familiarisation - Reprocessing and Recycling
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SOLID WASTE CLASSIFICATION
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/m3 beta/gamma
no single item over 40 kBq beta/gamma
Nuclear Familiarisation - Reprocessing and Recycling
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RADIOACTIVE WASTES
PDW
 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
Nuclear Familiarisation - Reprocessing and Recycling
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SELLAFIELD WASTE MANAGEMENT
as much as possible of the heatreleasing radionuclide waste to a small
volume of glass - HLW
PDW
 Confine
 Immobilise
other substantially radioactive
waste (without troublesome heat release)
with cement - ILW
For eventual
deep disposal
 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
Nuclear Familiarisation - Reprocessing and Recycling
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PRODUCT FINISHING
 Finishing
PDW
- conversion to a form suitable for sale, use or
storage
– Uranium
– thermal denitration to UO3
– Plutonium
– precipitation as oxalate
– calcination to PuO2
Nuclear Familiarisation - Reprocessing and Recycling
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WHY RECYCLE?
 To
PDW
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
Nuclear Familiarisation - Reprocessing and Recycling
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FACTORS RELEVANT TO 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 fresh
but can be offset by avoiding isotopic enrichment of uranium
Nuclear Familiarisation - Reprocessing and Recycling
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DIFFICULTIES IN RECYCLING AS MOX
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
Nuclear Familiarisation - Reprocessing and Recycling
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PRACTICAL RECYCLING
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
Nuclear Familiarisation - Reprocessing and Recycling
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FUTURE REPROCESSING
PDW
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
Nuclear Familiarisation - Reprocessing and Recycling
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FUTURE RECYCLING
PDW
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
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