Possible future directions
for nuclear power
generation
NIKHEF, June 8 2006
G. Bonvicini
Wayne State University, Detroit MI
Non renewable Resource lifetime - based on
current consumption
Fuel
Oil
Conv.
Unconv.
Res. (EJ) (EJ)
12,000
20000
Cons.
EJ,1998
142
Ratio
(years)
85
Gas
17,000
84
200
Coal
200000
92
2200
D2O
3E12
0
1010 *
33000
Actinides 10000** Up to 106 31
320**
2
Peak oil is in the past
3
Electricity, energy and GDP in developing
countries (very similar for the US)
4
Nuclear power
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
6
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, longlived, 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.
7
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
8
(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)
9
Neutron capture cross section, 238U+n-> 239U+
10
11
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
12
The fundamental parameter of a nuclear reactor
#n produced in generation x
k= ___________________________
#n produced in generation (x+1)
k=1 critical
k<1 subcritical
k>1 supercritical
k is a dynamic quantity
Generation time is 10-4 sec or less
13
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
14
Pressurized water reactors
15
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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18
Breeder schematic
19
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  fpP
= 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)
21
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)
22
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
23
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
24
SVETLANA
25
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
26
Radioactive Waste from LWRs
How 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
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Two ideas I like
Gen 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.
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
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transuranics)
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 ≈ 0.950.98)
condition is guaranteed at all times.
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The Energy Amplifier Concept (3)
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Transmutation Before Storage by EA
34
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)
35
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
36
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
37
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
38
The CAESAR concept (courtesy C. Filippone)
Rod
bundle
Pressure
vessel
Fuel
rods
39
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
40
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 (1m/hr a possibility)
41
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
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