Possible future directions for nuclear power generation

NIKHEF, June 8 2006

G. BonviciniWayne State University,

Detroit MI

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Non renewable Resource lifetime - based on current consumption

Fuel Conv. Res. (EJ)

Unconv. (EJ)

Cons. EJ,1998

Ratio (years)

Oil 12,000 20000 142 85

Gas 17,000 33000 84 200

Coal 200000 92 2200

D2O 3E12 0 1010 *

Actinides

10000**

Up to 106

31 320**

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Peak oil is in the past

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Electricity, energy and GDP in developing countries (very similar

for the US)

Nuclear power

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Nuclear energy as seen today

“not too cheap to meter, but too costly to matter” (The Economist S&T correspondent, on NPR, Marketplace, May 10, 2006)

Really? Consider that

- 1.5% of available energy is currently extracted from uranium, the rest is wasted

- 80% of the uranium is thrown away without burning (so called depleted uranium)

- Costs are dominated by capital costs. If a plant produces twice the energy, the cost is going down by nearly a factor of two. Also, field is immune to lower grade ores. Further, some capital costs may be eliminated or reduced

- There is 3 times as much 232Th as there is 238U

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Properties of nuclear reactions

• Due to underlying conservation laws and nuclear stability laws, there is enormous variation in nuclei’s lifetimes (more than 40 orders of magnitude). Jargon: stable, long-lived, short-lived.

• Due to nuclear stability laws and nuclear states there is enormous variation in the neutron cross section, in the type of (nN) interaction, from isotope to isotope and with neutron energy. Jargon: see next slides, plus resonances

• The basic nuclear fuels are uranium and thorium. Uranium is 99.3% 238U, and 0.7% 235U. Thorium is 100% 232Th.

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Basic fission process

• About 180 MeV per fission are released (0.08% of the mc2 energy), plus about 18 MeV due to radioactive decays of fission products

• 2.3-3.0 neutrons per fission, with energy of order 1MeV are released depending on fissioned isotope and neutron energy. These are fast neutrons

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(nN) scattering

• Most scattering is elastic, with the cross section equal to the geometrical nucleus cross section. Mean free path is of order centimeters.

• Average fractional energy loss is given by

=2A/(A+1)2

Avg. number of collisions from the MeV energies to thermal energies:

- 26 for N=H (=0.5)- 1700 for N=U (=0.008)

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Neutron capture cross section, 238U+n-> 239U+

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Isotope classes

• Fissile: will break up when hit by a thermal neutron. Examples: 233U, 235U (0.7% in nature), 239Pu

• Fissionable (all actinides): will break up when hit by a few MeV neutron. Ex: 238U (99.3% in nature), 232Th

• Fertile: will produce a fissile isotope when absorbing a neutron. Ex: 232Th+n -> 233Th-> 233Pa+() -> 233U+()

238U+n -> 239U -> 239Np+() -> 239Pu+()• Spontaneously fissionable: isotope will break up spontaneously and release neutrons. Ex: 240Pu

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The fundamental parameter of a nuclear reactor

#n produced in generation x

k= ___________________________ #n produced in generation (x+1)

k=1 criticalk<1 subcriticalk>1 supercriticalk is a dynamic quantityGeneration time is 10-4 sec or less

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Nature-given safety features

• Doppler broadening. As the temperature rises, the resonances become broader and more neutrons are captured before thermalizing. This is available only for thermal reactors. Reason why reactors are primarily thermal.

• Delayed neutrons from fission fragments. Flux is 0.65% with a delay of 2 mins. Available to all reactors, if k<1.0065

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Pressurized water reactors

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Basic features of current thermal reactors

• 30% thermal efficiency (=1-T1/T2)

• 4-5% burnup efficiency

• Costs are dominated by capital costs.

• Require enrichment (to 3-3.75%), some fuel is reprocessed, waste disposal

• Enrichment typically throws away 75-80% of the uranium

• Waste is 95% U, 4% fission fragments, 0.8% Pu, 0.2% transuranics

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Breeders

• Burn uranium by means of fast neutrons. They are used to breed Pu

• In principle, need no enrichment - though they have always been enriched

• In principle, could burn most of the uranium (or thorium)

• Beset by many instabilities, their development has been very limited

• They do not need a moderator, or careful core material selection

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Breeder schematic

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The problems with breeders

• No negative feedback due to resonance broadening. This type of reactor is more unstable than a thermal reactor. One still has delayed neutrons.

• Plutonium and other transuranics are produced also in the core, and accumulate. This leads to increased k, and increased spontaneous fission.

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5 parameters formula

• Each parameter is a tuning handle

€

k =ηfpεP = number of fission n (fuel)

f = fraction of thermal n captured (enrichment level)

= fraction of fast n correction (moderator)

p = fraction of thermal n captured (moderator, materials)

P = fraction of neutrons that did not leak out (n reflector)

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Open cycle (US), a.k.a. Once Through

• The uranium is enriched to about 3% 235U• Manufactured into ceramic pellets (UO2 ), the pellets assembled into 12-ft Zr rods

• Burned once (about 2.4% 239Pu is produced during the burn, and 1.5% is burned, extending the reactor efficiency)

• And then, ideally, put into a geological repository (these days, stored for decades in guarded open pools at the plant)

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Closed cycle (F/UK), a.k.a. Twice Through, a.k.a. reprocessing

• First cycle goes as above• Zr rods are chopped and the various chemical species separated

• Fission fragments are either glassified or (Iodine) dumped at sea (much to Holland’s chagrin)

• Plutonium and other actinides are recovered, made into ceramics (Mixed OXides) and burned once more

• Geological repository/open pool steps as above

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Proliferation: 235U/enrichment

• k=r/• Bomb trivial to make, if given highly enriched

235U• Very low activity (20mCi, 30/sec at surface) makes it portable

• Low enrichment is already 65% of the way to weapon grade

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SVETLANA

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Proliferation: Pu/reprocessing

• Pu is harder to fashion into a bomb, due to 240Pu

• It is also plentiful in spent fuel, and relatively easy to separate chemically

• Bomb has to be built as implosion device

=1/, =Nr-3, k=r/ =r-2

• Pu is also not portable due to high activity (400000n/sec at surface)

• Reprocessing eliminates one fundamental barrier to proliferation, 137Cs

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Radioactive Waste from LWRsHow much? ≈1000 Ton/RQ/30 years (RQ: Reactor quantum: 1 GWatt-electric)

France : 54 RQ’s, Germany 26 RQ, Spain 8 RQ, World : 330 RQ (≈ 6% world’s energy)

at end of present nuclear deployment: 330 x1000 = 0.33 Million ton

How radio-active? ≈ 108 Sv/ton ≈105 times the initial Uranium used. It decays back to radio toxicity of initial Uranium only after ≥ 106 years (geologic times)

at end of present nuclear deployment: 0.33 106 x 108 = 3.3 1013 Sv[recommended ICRP max. dose to radiation workers 2.0

mSv/year]

Main concerns Leaks in the environment (biosphere)Proliferation (Pu239): world’s waste stockpile about 5 ÷ 10

times the military Pu stockpile

at end of present nuclear deployment:600’000 critical Pu masses

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Radioactive Waste from LWRs (2)

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The challenges for the next generation of reactors

• Safety• Economy • Proliferation resistance• Waste reduction• Fuel economy

Two ideas I likeGen IV(US main research

program) reactors are good but not good enough. The

most advanced is the Pebble Bed concept, which is safe and scalable, but burns

uranium only slightly more efficiently than LWR,

requires enrichment, waste disposal, and substantial

manufacturing.

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The Energy Amplifier concept (1)

• European Collaboration initiated by C. Rubbia• It consists of a subcritical breeder-like reactor with constant neutron input from accelerated protons

• The energy of the neutrons liberated by proton collisions with the core is amplified by a factor 1/(1-k)

• It could conceivably burn unenriched, and even depleted, uranium

• Given a negative political climate, it has been repackaged as a subsidiary system to existing LWR, burning efficiently the most toxic, most dangerous part of nuclear waste (Pu and other transuranics)

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The Energy Amplifier Concept (2)

Method: A high energy proton beam interacts in a molten lead (Pb-Bi) swimming pool. Neutrons are produced by the so-called spallation process. Lead is “transparent” to neutrons. Single phase coolant, b.p. ≈ 2000 °C

TRU: They are introduced, after separation, in the form of classic, well tested “fuel rods”. Fast neutrons, both from spallation and fission, drift to the TRU rods and fission them efficiently. A substantial amount of net power is produced (up to ≈ 1/3 of LWR), to pay for the operation.

LLFF: Neutrons leaking from the periphery of the core are used to transmute also LLFF (Tc99, I129 ....)

Safety: The sub-criticality (k ≈ 0.950.98) condition is guaranteed at all times.

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The Energy Amplifier Concept (3)

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Transmutation Before Storage by EA

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Accelerator issues

• The Energy Amplifier is also an Energy Attenuator when energy is sent back to beam (0.33 thermal efficiency, 0.1 wall-to-beam (WtB) efficiency). This means k>0.99.

• 900 MeV protons will damage/destroy windows

• Accelerating d-t at 6-9 GeV would improve window lifetime and energy efficiency by factor 6-9

• Patent US 6326861 (F. Villa, A. Luccio), Fourier driven accelerator, shows 1 GeV/m, expects 3GeV/m, potentially high efficiency (perhaps 0.3 WtB ultimately)

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The EA future

• Transuranics incineration is good but…

• The crux of the matter is: how stable or manageable is k under operation? If it can be made very stable, then the EA might very well be the future of energy

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The CAESAR concept (C. Filippone, Maryland)

• A new type of reactor, with dual operation capabilities (breeder and thermal)

• Burning unenriched fuel• Starting as a subcritical breeder with external energy source (an accelerator is a possibility)

• Switching to thermal mode once accumulated transuranics make operation difficult

• Continuing to oscillate between the two modes

• Innovative thermal-hydraulics through the use of heat cavities

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Heat cavity

• A way to extract a lot of heat with relatively little water

• By not allowing a vapor film to develop the cavity manages to turn all the water going through the sleeve into vapor in a single pass

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The CAESAR concept (courtesy C. Filippone)

Fuel rods

Rod bundle Pressu

re vessel

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The challenge of CAESAR

• Core materials should satisfy the requirements of both thermal and breeder reactors (similar to Gen IV concepts in this respect)

• Core design is more complex• Steam jets presents new mechanical challenges

• CAESAR is, in the end, a safer, much cleaner breeder. It is still harder to operate than a thermal reactor because of the lack of Doppler broadening

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Our current proposal

• Given the currently negative political climate, we started with something that is related to a subproblem of CAESAR

• We requested funds to study the feasibility of diamond coating the structure of a reactor core (Maryland, Wayne State, INL Collaboration)

• If funded, we would also study the neutronics of CAESAR and find suitable working points

• We would also have a chance to explore the possibility of diamond growth under intense radiation

• Future ramifications include the possibility of slow diamond growth on reactor elements using nuclear waste heat (1m/hr a possibility)

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CAESAR vs 5 points program

• Safety: expected to be safer than a breeder• Proliferation: eliminates enrichment, reprocessing, and transportation of fissile materials

• Cost: minimizes cost per kWh by producing more energy per plant (more efficient burnup), eliminating enrichment or reprocessing, and reducing waste storage costs

• Natural resources: more efficient burnup• Nuclear waste: poor in transuranics, with expected cooling times similar to ADS

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Conclusions

• Operating a subcritical breeder is unsafe, but not operating subcritical breeders is more unsafe

• We do not have now the technologies to maintain civilization beyond a few decades within the constraints given by global warming. Some risk taking is necessary