needs of external neutrons for fission based reactors
TRANSCRIPT
S. David, Needs of external neutron source for fission-based reactors, ICAN workshop, Ecole Polytechnique, 2014 1
Needs of external neutrons for fission based reactors Waste transmutation and thorium cycle S. David, CNRS/IN2P3/IPN Orsay [email protected]
World Energy / Climate Context
S. David, Needs of external neutron source for fission-based reactors, ICAN workshop, Ecole Polytechnique, 2014 2
World electricity generation World GreenHouse Gas emissions
Nuclear power in the coming century
S. David, Needs of external neutron source for fission-based reactors, ICAN workshop, Ecole Polytechnique, 2014 3
Uncertainty about the deployment or not of nuclear power in the world in the coming century
Long term strategies : waste transmutation and breeding
2050 Factor 1 to 10
2100 Factor 5 to 40
TWh/y
Uranium, thorium and minor actinides
S. David, Needs of external neutron source for fission-based reactors, ICAN workshop, Ecole Polytechnique, 2014 4
Only 3 natural actinides 238U, 235U et 232Th
Only 235U (0,7% of natural uranium) is fissile : can fission with low energy neutrons Present reactor use enriched Uranium : 4%235U + 96%238U (UOX fuel) UOX fuel produces Pu (250kg/GWe/an) and minor actinides Np, Am, Cm 238U and 232Th are fertile : they produce a fissile nucleus after a neutron capture
238U + n → 239U → 239Np (2j) → 239Pu 232Th + n → 233Th → 233Pa(27j) → 233U
Heavy minor actinides Am, Cm, …
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Present situation
S. David, Needs of external neutron source for fission-based reactors, ICAN workshop, Ecole Polytechnique, 2014
Present nuclear reactors based on fission of 235U, essentially enriched uranium fuels
Once-through cycle
Reprocessing strategy
Water reactor
Enriched U Spent-fuel
(U, Pu, M.A., F.P.)
Waste
Water reactor
Enriched U
reprocessing
U,Pu
Waste = M.A., F.P.
Pu MOX MOX spent fuel (U,Pu, M.A., F.P.) Valuable material
U Re-Enriched U spent fuel
(U,Pu, M.A., F.P.) Valuable material
1 GWe : 200 tons of natural U → 30 tons of enriched U → 1 ton of fissions
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Breeding
S. David, Needs of external neutron source for fission-based reactors, ICAN workshop, Ecole Polytechnique, 2014
Present nuclear reactors based on fission of 235U, essentially enriched uranium fuels
Fast reactors
Depleted U
Waste Fission products Minor actinides
U,Pu
• In a breeder reactor, all the fisionning material is replaced by neutron capture on the fertile • The mass of fissile is constant, only fertile is consumed, ie 1 ton/Gwe/y • Energy production during more than 20000 years (idem for Li and fusion reactors) • In fast neutron reactors, breeding os possible with the neutrons produced by the fission (critical systems)
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Minor Actinides Transmutation
S. David, Needs of external neutron source for fission-based reactors, ICAN workshop, Ecole Polytechnique, 2014
Fast reactors
Depleted U Waste
Only Fission products
U,Pu + MA
All the cycle is « poluted » by the minor actinides
Double strata strategy
U, Pu
FR
MA
Dedicated reactors (Subcritical)
Minor actinides are concentrated in
dedicated reactors
Waste transmutation
× 60
Comparison U/Pu cycle in fast reactor with and without MA transmutation
× 10
× 60
8 S. David, Needs of external neutron source for fission-based reactors, ICAN workshop, Ecole Polytechnique, 2014
Long term radiotoxicity of final waste – homogeneous or ADS transmutation
Accelerator-Driven Systems
S. David, Needs of external neutron source for fission-based reactors, ICAN workshop, Ecole Polytechnique, 2014 9
Neutronic behaviour of Minor Actinides fuels is not compatible with critical systems • Not enough delayed neutrons (reactor too nervous) • Positive void or temperature coefficients (more fissions →T increases → more fissions) • Subcriticlity is needed : 1 fission → k → k2 … = 1/(1-k) • Chain reaction is not sustained
• External neutron source is needed to continuously feed the finite chain reaction • More efficient neutron source : spallation p + Pb @1 GeV produce ~30 neutrons • 1 neutron produces 1/(1-k) neutrons, which produce 1/(1-k) k/ν fissions • The fissions transmute the minor actinides, this gives the thermal power of ADS • The beam intensity is simply related to the thermal power of the subcritical core.
Beam intensity
S. David, Needs of external neutron source for fission-based reactors, ICAN workshop, Ecole Polytechnique, 2014 10
U, Pu
FR
MA
1 GWe = 2,5 GWth
45 kg/an
45kg * 200MeV/fission Pth (ADS) = 0,1 GWth
Nn = spallation neutrons for 1p@1GeV ~30 k = multiplication factor ~0,95 ν = number of neutrons produced per fission ~3 εf = energy delivered per fission 200MeV Ep = proton energy 1 GeV
French case, 60 GWe (load factor=80%)
= ADS 5,28 GWth ~16 * 400 MWth ~8 * 800 MWth
Beam power
S. David, Needs of external neutron source for fission-based reactors, ICAN workshop, Ecole Polytechnique, 2014 11
The beam power depends on the subcriticality level
0
2
4
6
8
10
12
14
0,8 0,85 0,9 0,95 1
beam
inte
nsity
mA@
1GeV
k multiplication factor
Typically k = 0,95
I = 2,5 mA
French case : 8 ADS * 800 MWth ↔ 8 proton beams of 20mA = 160 mA
Myrrha project
S. David, Needs of external neutron source for fission-based reactors, ICAN workshop, Ecole Polytechnique, 2014 12
Main features of the ADS demo
50-100 MWth power
keff around 0.95
600 MeV, 2.5 - 4 mA proton beam
Highly-enriched MOX fuel
Pb-Bi Eutectic coolant & target
Myrrha project (SCK@Mol) is a prototype not so far from what could be an industrial MA burner of 500-1000 MWth
External neutrons for thorium cycle
S. David, Needs of external neutron source for fission-based reactors, ICAN workshop, Ecole Polytechnique, 2014 13
Breeding is possible with U/Pu cycle in Fast Spectrum (238U +n → 239Pu) Standard Water Reactors are not able to be both critical and breeder with U/Pu cycle For thorium cycle (232Th + n → 233U), breeder capacity is better in standard reactors, but very difficult to achieve
U/Pu cycle
Water reactors 1000 Pu/y 750 kg/y
Th/U cycle 1000 233U/y 900 kg/y
Needs 250kg/y
Needs 100kg/y
Problematics : can external neutrons sources compensate the under-breeding mode? This could make the present water reactors sustainable for thousands of years
External neutrons for breeding
S. David, Needs of external neutron source for fission-based reactors, ICAN workshop, Ecole Polytechnique, 2014 14
For breeding use Subcritical reactors with neutron source is equivalent to critical reactor and
external neutron source
U/Pu cycle 1000 233U/y 900 kg/y
Only Th
100 kg/y
1000 kg/y
1p@1GeV produces 30 neutrons, then 30 233U 1mA@1GeV produces 3 kg/an 100 kg/an needs 33mA, for each 1GWe reactor, beam 33 MW = 100 Mwelec (if ηacc=33%) French case total intentisty of the beams >1500 mA !!
p+
Conclusion
S. David, Needs of external neutron source for fission-based reactors, ICAN workshop, Ecole Polytechnique, 2014 15
External neutron source for fission based reactor is needed to transmute minor actinides in dedicated reactors • Safety reasons : MA fuels cannot be used alone in critical systems • Competition with homogeneous transmutation (MA diluted in Fast Reactors) • Specificity for accelerator
o High power 10-50 mA o Reliability
Avoid no beam cut > 3sec Number of beam cut (< 1sec) < 10 per 3 months For thorium cycle and breeding application • External neutrons could be used to compensante the negative neutron balance of present reactors • Competition with fast reactors which can be critical and breeder (U and Th cycles) • This strategy seems to be very difficult and very expensive • More than 1500mA would be required to make the french park breeder with water reactors • Could a very cheap accelerator change this conclusion?