Page 1Nuclear Familiarisation - Advanced Issues PDW FAMILIARISATION WITH NUCLEAR TECHNOLOGY ADVANCED...

Page 1Nuclear Familiarisation - Advanced Issues

PDW

FAMILIARISATION WITH

NUCLEAR TECHNOLOGY

ADVANCED ISSUES

Peter D. Wilson

DURATION ABOUT 40 MINUTES


PDWINTERACTIONS BETWEEN TOPICS

Closed versus open cycles

Long-lived radionuclides

Accelerator-driven systems Thorium fuels

Weapon proliferation

Reactor types

Connecting lines represent causal interrelationsProliferation and long-lived nuclides are the driving issues


PDW

WEAPON PROLIFERATION


PDWPRINCIPAL CONCERNSCivil 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.


PDWPLUTONIUM 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 ...


PDW

LONG-LIVED RADIONUCLIDES

URANIUM, PLUTONIUM, MINOR ACTINIDES (neptunium, americium, curium)

& SOME FISSION PRODUCTS


PDWFORMATION OF MINOR ACTINIDES

U-239

Pu-239

U-238

Pu-240 Pu-241 Pu-242

14.4 yr

nnn

n

Am-241

2.355 day

23.5 min

Am-242 Am-243 Am-244nnn

16.02 hr

Cm-242

10.1 hr

Cm-244Cm-243 nn

Np-239Np-237

432.7 yr

Cm-245n

2.14 M yrs

Formation of higher nuclides increases disproportionately with irradiation, basically according to the number of neutrons required but complicated by

decay and consumption.


PDWCAUSES FOR CONCERN

Leaching by ground water - movement can be modelled though with great uncertainties, especially on geological movements: very slight addition to ambient radiation

Risk of accidental intrusion - probability unpredictable: possibly heavy dose to borehole or mining technicians, significant to local population in case of mining

Half-lives up to millions of years

Likely to outlast containment or records of repository

Deep waste repository

~ 1 km


PDWNUCLIDES CONCERNEDACTINIDES:

• 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


PDWPARTITION & TRANSMUTATION (P&T)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


PDWMEANS 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 ...


PDW

ACCELERATOR-DRIVEN SYSTEMS


PDWPRINCIPLE

Generalised without cooling arrangements - wide variety of

specific proposals

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

Reaction cannot continue without proton drive


PDWISSUES

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.


PDW

THORIUM FUELS


PDWTHORIUM CYCLE

Formally analogous to U - Pu cycle

U-238 n U-239 Np-239 Pu-23923.5 min 2.355 days

Th-232 n Th-233 Pa-233 U-23322.3 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


PDWTHORIUM FUELS

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 ...


PDWRADKOWSKY FUEL

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


PDWOPEN 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

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


PDWREQUIREMENTS 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


PDWREACTOR TYPES

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


PDWPebble-bed reactor (schematic)

Good pebbles to recycle

Coolant in

Coolant out

Control rods moving in reflector

Pebbles in

Graphite reflector

Monitor & sentence

Exhausted pebbles to waste

Pebbles comprise coated fuel micro-spheres compacted with graphite into 6 cm balls

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

Reprocessing probably impracticable

Early trials used thorium fuel, more recently uranium


PDWGENERAL 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


PDW

Overheard after an IAEA advisory group meeting:

“Thank God the British are here to inject some realism.”

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