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FAMILIARISATION WITH NUCLEAR TECHNOLOGY

Nuclear Familiarisation - Advanced Issues

ADVANCED ISSUES

Peter D. Wilson

DURATION ABOUT 40 MINUTES Page 1

INTERACTIONS BETWEEN TOPICS

Reactor types Weapon proliferation Long-lived radionuclides

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Closed versus open cycles Accelerator driven systems Thorium fuels Connecting lines represent causal interrelations Proliferation and long-lived nuclides are the driving issues Nuclear Familiarisation - Advanced Issues Page 2

Nuclear Familiarisation - Advanced Issues

WEAPON PROLIFERATION

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PRINCIPAL CONCERNS

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

• tried to convince rest of world likewise; • now seems to be having second thoughts.

(no reprocessing) France, Japan, Russia & UK favour

reprocessing

• essential for • existing

resource conservation; 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.

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PLUTONIUM SECURITY ISSUES

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 ...

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LONG-LIVED RADIONUCLIDES

URANIUM, PLUTONIUM, MINOR ACTINIDES (neptunium, americium, curium) & SOME FISSION PRODUCTS

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FORMATION OF MINOR ACTINIDES

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Cm-242

Am-241

n  n Cm-243 16.02 hr Am-242 n Pu-239

Np-237

432.7 yr

2.14 M yrs

U-238  n  Pu-240 2.355 day Np-239 n n 14.4 yr Pu-241  23.5 min U-239 n n Pu-242 Cm-244 Am-243 n  n

Cm-245

10.1 hr Am-244 Formation of higher nuclides increases disproportionately with irradiation, basically according to the number of neutrons required but complicated by decay and consumption. Nuclear Familiarisation - Advanced Issues Page 7

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CAUSES FOR CONCERN

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 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

PDW ACTINIDES

• High : • Uranium, neptunium, plutonium, americium, curium

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 Nuclear Familiarisation - Advanced Issues Page 9

PARTITION & TRANSMUTATION (P&T)

PDW 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

– chemically

very similar

from

lanthanides

– – a

quarter of fission product

very much more strongly atoms

neutron-absorbing

• Some

LLFP

may also be difficult to separate from HLW • Transmutation of particular nuclides

not always feasible

– insufficient

neutron absorption

(e.g. Sn-126), or – faster

generation

from lower isotopes (e.g. Cs-135) which are because of 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 Nuclear Familiarisation - Advanced Issues Page 10

MEANS OF TRANSMUTATION

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 Heavy metal target (source of spallation neutrons 30-40 per proton) Sub-critical fuel assembly with multiplication factor ~ 20

Generalised without cooling arrangements wide variety of specific proposals

Reaction cannot continue without proton drive Nuclear Familiarisation - Advanced Issues Page 13

ISSUES

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.

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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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Nuclear Familiarisation - Advanced Issues

THORIUM FUELS

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THORIUM CYCLE

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    22.3 min

27.0 days

Pa-233    U-233

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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 Nuclear Familiarisation - Advanced Issues Page 16

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THORIUM FUELS

Usable in

any reactor

type, but traditionally • in which absorption resonances of uranium require higher fissile content • not now a serious consideration

HTR

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

• but still yields

long-lived fission products

production Once-through operation favoured by • near-breeding which allows relatively

high burn-up

•difficulties in

recycling

due

to

– chemical

inertness

– poor

extractability

to nitric acid compared with uranium and plutonium High radiotoxicity of thorium (10  uranium ) discourages practical trials Little industrial interest outside India, except for ...

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RADKOWSKY FUEL

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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 Nuclear Familiarisation - Advanced Issues Page 18

OPEN vs CLOSED CYCLE

OPEN

Minimises fuel-cycle

operations

Raises least public

objections

Avoids immediate

proliferation

risk but leaves potential “plutonium mine” Probably

unavoidable

with

HTR

type fuel Wastes resources • 99% of uranium – including enrichment tails • probably less waste with thorium

PDW CLOSED

Permits maximum

resource utilisation

Permits

Partition & Transmutation

Generates

secondary waste

Aids dispersion of mobile nuclides Much more difficult with thorium than uranium Choice depends somewhat on type of reactor and fuel Nuclear Familiarisation - Advanced Issues Page 19

REQUIREMENTS OF NEW REACTORS

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 PDW

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REACTOR TYPES

PDW LWRs

industrially dominant

Fast

reactors best for burning

Pu & minor actinides

(all isotopes fissionable) Widening interest in

CANDU

• good –

neutron economy

in situ

favouring – DUPIC - using discharged LWR fuel U-233 breeding and burning from thorium Renewed interest in

HTRs

• thermal efficiency - thermodynamic limit (T 1 -T 2 )/T 1 • 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 Nuclear Familiarisation - Advanced Issues Page 21

Pebble-bed reactor (schematic)

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Pebbles comprise coated fuel micro spheres compacted with graphite into 6 cm balls Control rods moving in reflector Pebbles in Graphite reflector Reactor core contains many thousand pebbles, gradually circulating Coolant out Monitor & sentence Coolant in Good pebbles to recycle Pebbles recycled until exhausted Continuous addition of fresh pebbles allows high consumption of fissile content before discharge while maintaining mean reactivity Reprocessing probably impracticable Early trials used thorium fuel, more recently uranium Exhausted pebbles to waste Nuclear Familiarisation - Advanced Issues Page 22

GENERAL COMMENTS

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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