PDW
FAMILIARISATION WITH
NUCLEAR TECHNOLOGY
ADVANCED ISSUES
Peter D. Wilson
DURATION ABOUT 40 MINUTES
Nuclear Familiarisation - Advanced Issues
Page 1
INTERACTIONS BETWEEN TOPICS
PDW
Reactor types
Long-lived
radionuclides
Weapon
proliferation
Closed versus
open cycles
Acceleratordriven systems
Thorium fuels
Connecting lines represent causal interrelations
Proliferation and long-lived nuclides are the driving issues
Nuclear Familiarisation - Advanced Issues
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PDW
WEAPON
PROLIFERATION
Nuclear Familiarisation - Advanced Issues
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PRINCIPAL CONCERNS
PDW
Civil plutonium might be used for weapons though not ideal:
• liable to very slightly premature detonation;
• so unpredictable and probably low yield but still destructive;
• would at the very least make an extremely troublesome mess.
Fear of fissile material falling into wrong hands.
USA has for decades favoured open fuel cycle (no reprocessing)
• tried to convince rest of world likewise;
• now seems to be having second thoughts.
France, Japan, Russia & UK favour reprocessing
• essential for resource conservation;
• existing safeguards under NPT believed adequate against diversion.
Size of civil stockpile arguably irrelevant to proliferation;
• possibility of access to small proportion more important.
Ex-military material and non-nuclear means seem more likely to be attractive
for terrorist purposes.
Hard to stop independent development of weapon sources.
Nuclear Familiarisation - Advanced Issues
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PLUTONIUM SECURITY ISSUES
PDW
Direct disposal of fuel puts plutonium immediately out of reach, but ...
• With time, protective fission products decay
– possibility of “plutonium mine.”
Recycle as fuel would
• degrade Pu quality;
• increase fission product content.
USA therefore started to consider
• separation as a waste-management option;
• not to be confused with reprocessing for utilisation;
distinction lies chiefly in regarding uranium as waste.
Now undertaking a more radical reappraisal
• including advanced fuel cycles, dealing with ...
Nuclear Familiarisation - Advanced Issues
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PDW
LONG-LIVED RADIONUCLIDES
URANIUM, PLUTONIUM, MINOR ACTINIDES
(neptunium, americium, curium)
& SOME FISSION PRODUCTS
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PDW
FORMATION OF MINOR ACTINIDES
Cm-242
Am-241
n
432.7 yr 
Pu-239
n

Np-237
Pu-240
n
n

Cm-243
16.02 hr
Am-242
n
14.4 yr
Pu-241
n
Cm-244
n
Am-243 n
n

Cm-245
10.1 hr
Am-244
Pu-242
2.355 day
Np-239

2.14 M yrs
U-238
n
23.5 min
U-239
Formation of higher nuclides increases disproportionately with irradiation,
basically according to the number of neutrons required but complicated by
decay and consumption.
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CAUSES FOR CONCERN
PDW
Half-lives up to millions of years
Likely to outlast containment or records of repository
~ 1 km
Risk of accidental intrusion - probability unpredictable:
possibly heavy dose to borehole or mining technicians,
significant to local population in case of mining
Deep waste
repository
Nuclear Familiarisation - Advanced Issues
Leaching by ground water - movement can be modelled
though with great uncertainties, especially on geological
movements: very slight addition to ambient radiation
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NUCLIDES CONCERNED
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ACTINIDES:
• Uranium, neptunium, plutonium, americium, curium
• High radiotoxicity ( emitters), generally low mobility
– cf. residues of Oklo natural reactor still nearby after ~2 billion years
– risk of local ingestion in case of mining or drilling
FISSION PRODUCTS:
• Selenium-79, technetium-99, iodine-129, tin-126, caesium-135 etc.
(+ chlorine-36 activation product)
• Lower radiotoxicity (- emitters), some with higher mobility
– risk of widespread low doses through seepage into aquifers
Risks believed very slight, but unquantifiable (like many others)
Hence proposals to separate and destroy the nuclides concerned - P&T
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PARTITION & TRANSMUTATION (P&T)
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Principles
• Separate actinides and long-lived fission products (LLFP) from rest of
high-level waste
• Transmute them into short-lived or stable nuclides by neutron irradiation
Problems
• Difficulty of separating trans-Pu actinides from lanthanides which are
– chemically very similar
– a quarter of fission product atoms
– very much more strongly neutron-absorbing
• Some LLFP may also be difficult to separate from HLW
• Transmutation of particular nuclides not always feasible because of
– insufficient neutron absorption (e.g. Sn-126), or
– faster generation from lower isotopes (e.g. Cs-135)
May therefore be feasible only for
– actinide: neptunium (diverted fairly easily to plutonium product)
– fission products: technetium-99 and perhaps iodine-129
Nevertheless much work done since late 1990s
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MEANS OF TRANSMUTATION
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Requires copious free neutrons
Most plentifully available in fissioning system, i.e. reactor or similar
Uranium-based fuels generate new Pu and MAs
Uranium-free fuels proposed to avoid this, but
• physical characteristics lead to control problems
– impaired self-regulation, possibility of excessive power surge
• reactivity declines rapidly
Call for system that would
• minimise risk of runaway reaction
• tolerate substantial variations in reactivity
Hence interest in ...
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ACCELERATOR-DRIVEN
SYSTEMS
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PRINCIPLE
Accelerator (linear or cyclotron)
Proton beam aim for e.g. 10 mA at 1 GeV
Generalised
without cooling
arrangements wide variety of
specific proposals
Heavy metal target
(source of spallation neutrons 30-40 per proton)
Sub-critical fuel assembly with
multiplication factor ~ 20
Reaction cannot continue without proton drive
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ISSUES
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Such a system
• avoids risks of runaway reaction when reactivity coefficients are adverse,
delayed-neutron fraction small;
• retains graver dangers of decay heating on loss of coolant.
Accelerator drive
• is expensive;
• needs development for
– higher power - maybe achievable
– vastly improved reliability - more difficult - unlikely to reach requirement
as grid supplier;
• could raise extra proliferation issues
– any GeV accelerator could produce plutonium from U-238
or U-233 from thorium.
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PDW
THORIUM FUELS
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THORIUM CYCLE
PDW
Formally analogous to U - Pu cycle
U-238 n U-239    Np-239    Pu-239
23.5 min
2.355 days
Th-232 n Th-233    Pa-233    U-233
22.3 min
27.0 days
Differences in physics:
• High neutron yield of U-233 fission permits near-breeding in thermal reactor
– near-constant reactivity may be maintained after initial drop
• Relatively long half-life of Pa-233 lets parasitic neutron absorption compete
with decay to U-233
– removes both nucleus and neutron from cycle
– minimised by low neutron flux
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THORIUM FUELS
PDW
Usable in any reactor type, but traditionally HTR
• in which absorption resonances of uranium require higher fissile content
• not now a serious consideration
Contamination of U-233 with U-232 by-product & daughters (notably
thallium-208) claimed to resist proliferation
Th-232 / U-233 cycle minimises minor actinide & plutonium production
• but still yields long-lived fission products
Once-through operation favoured by
• near-breeding which allows relatively high burn-up
•difficulties in recycling due to
– chemical inertness to nitric acid
– poor extractability compared with uranium and plutonium
High radiotoxicity of thorium (10  uranium ) discourages practical trials
Little industrial interest outside India, except for ...
Nuclear Familiarisation - Advanced Issues
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RADKOWSKY FUEL
PDW
Elements comprising
• highly reactive seed e.g. plutonium-based
• breeder blanket, mainly thorium
Seed changed every three years; blanket after nine
Dimensions for direct replacement of conventional PWR or VVER fuel, but
Doubts about feasibility of changing seed after distortion in reactor
Trials at Kurchatov Institute, Moscow (no information found)
Claimed proliferation-resistant because
• plutonium too degraded to be worth recovering
• uranium-233 contaminated with U-232 & gamma-emitting daughters
Therefore to be used in open cycle
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OPEN vs CLOSED CYCLE
OPEN
CLOSED
Minimises fuel-cycle operations
Permits maximum resource
utilisation
Raises least public objections
Avoids immediate proliferation risk
but leaves potential “plutonium mine”
Permits Partition &
Transmutation
Probably unavoidable with HTRtype fuel
Generates secondary waste
Wastes resources
Aids dispersion of mobile
nuclides
• 99% of uranium
– including enrichment tails
• probably less waste with thorium
PDW
Much more difficult with thorium
than uranium
Choice depends somewhat on type of reactor and fuel
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REQUIREMENTS OF NEW REACTORS
PDW
Minimum risk from
• runaway reaction
– temperature rise must reduce power (negative feedback)
– true of all designs currently considered
• loss of coolant
– automatic dispersion of decay heat
Reduced capital cost
• most expensive part of cycle
Improved resistance to diversion of fuel material
Tolerance of even extreme operator error
Ease of decommissioning
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REACTOR TYPES
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LWRs industrially dominant
Fast reactors best for burning Pu & minor actinides (all isotopes fissionable)
Widening interest in CANDU
• good neutron economy favouring
– DUPIC - using discharged LWR fuel
– in situ U-233 breeding and burning from thorium
Renewed interest in HTRs
• thermal efficiency - thermodynamic limit (T1-T2)/T1
• open fuel cycle - spent fuel very stable
Special types for developing countries
• fuel for life
• high burn-up in once-through mode
Possibly molten-salt fuels in distant future
•continual reprocessing and replenishment integrated with reactor
• no expensive structure to fabricate, dismantle or suffer failure
• harsh conditions for reactor structure
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Pebble-bed reactor (schematic)
Control rods
moving in reflector
PDW
Pebbles comprise coated fuel microspheres compacted with graphite into
6 cm balls
Graphite
reflector
Pebbles in
Reactor core contains many thousand
pebbles, gradually circulating
Pebbles recycled until exhausted
Continuous addition of fresh pebbles
allows high consumption of fissile
content before discharge while
maintaining mean reactivity
Coolant
out
Monitor & sentence
Reprocessing probably impracticable
Coolant in
Good pebbles to recycle
Early trials used thorium fuel, more
recently uranium
Exhausted pebbles to waste
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GENERAL COMMENTS
PDW
Much interesting work done, though not necessarily for technical reasons
• Politics often important, e.g
– innocent employment for ex-military scientists
– parliamentary demand for action
• Some bandwagon-jumping by laboratories losing military funding
• Claims to disarm opposition to nuclear energy
– “The public will demand .....”
– Misunderstanding opposition mentality
– Generally address rationalisations rather than real grounds
Focus often on individual topics or aspects without regard to broader frame
• e.g. specialists unaware of inherent difficulties in other areas
Some developments could nevertheless prove important in future
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Overheard after an IAEA advisory group meeting:
“Thank God the British are here
to inject some realism.”
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