External neutrons sources for fissionbased

Transcription

1 External neutrons sources for fissionbased reactors S. David, CNRS/IN2P3/IPN Orsay S. David,external neutron source for fission-based reactors, IZEST, Orsay, Nov

2 World Energy / Climate Context World electricity generation World GreenHouse Gas emissions Per capita Target «GIEC 2 C» = 1,5 t/an/hab S. David,external neutron source for fission-based reactors, IZEST, Orsay, Nov

3 Trajectoires tep tco2 par habitant trajectories tep vs tco2 / cap 20 sobriety USA 15 tco2/hab 10 Japan Germany Carbon-free sources Chine France Sweden 5 scénario 2 C 2050 : émission moyenne 1,3 tco2/hab 0 Inde tep/hab S. David,external neutron source for fission-based reactors, IZEST, Orsay, Nov

4 Nuclear power in the coming century Uncertainty about the deployment or not of nuclear power in the world in the coming century Long term strategies : waste transmutation and breeding TWh/y 2100 Factor 5 to Factor 1 to 10 S. David,external neutron source for fission-based reactors, IZEST, Orsay, Nov

5 Present situation Present nuclear reactors based on fission of 235 U (0,7% of natural U) 1 GWe : 200 tons of natural U 30 tons of enriched U 1 ton of fissions Once-through cycle Enriched U Water reactor Spent-fuel (U, Pu, M.A., F.P.) Waste Reprocessing strategy Enriched U Water reactor reprocessing U,Pu Pu MOX MOX spent fuel (U,Pu, M.A., F.P.) Valuable material U Waste = minor actinides fission products Re-Enriched U spent fuel (U,Pu, M.A., F.P.) Valuable material S. David,external neutron source for fission-based reactors, IZEST, Orsay, Nov

6 Sustainable nuclear = breeding Problematics Today, we use 235 U = 0,7% of natural uranium It is possible to use 238 U by breeding 239 Pu, and also Thorium 238 U + n 239 U 239 Np (2j) 239 Pu 232 Th + n 233 Th 233 Pa(27j) 233 U Breeding needs neutrons For one fission + ν neutrons are produced - 1 neutron is used to induce a new fission (chain reaction) α neutrons captured on fissile nucleus = σ cap / σ fis 1 + α neutrons captured on the fertile nucleus (fissile regeneration) ν 2 ( 1 + α) > 0 regeneration possible < 0 regeneration impossible S. David,external neutron source for fission-based reactors, IZEST, Orsay, Nov

7 Sustainable nuclear power : breeding principle The number of available neutrons only depends on fissile material α = σ σ capture fissile fission fissile Thermal spectrum Na (E) = ν - 2(1+ α(e)) Fast spectrum Th/U Na > 0 U/Pu Na < 0 Th/U Na > 0 U/Pu Na > 0 7 S. David,external neutron source for fission-based reactors, IZEST, Orsay, Nov 2017

8 Breeding Present nuclear reactors based on fission of 235 U, essentially enriched uranium fuels Depleted U Fast reactors Waste Fission products Minor actinides U,Pu In a breeder reactor, all the fissionning 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 years (idem for Li and fusion reactors) In fast neutron reactors, breeding os possible with the neutrons produced by the fission (critical systems) S. David,external neutron source for fission-based reactors, IZEST, Orsay, Nov

9 External neutrons for thorium cycle Breeding is possible with U/Pu cycle in Fast Spectrum ( 238 U +n 239 Pu) Standard Water Reactors are not able to be both critical and breeder (U or Th cycle) But external neutron sources can compensate the negative neutron balance Water reactors U/Pu cycle 1000 Pu/y 750 kg/y Needs 250kg/y Th/U cycle U/y 900 kg/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 S. David,external neutron source for fission-based reactors, IZEST, Orsay, Nov

10 External neutrons for breeding Order of magnitude for thorium cycle U/Pu cycle U/y 900 kg/y 1000 kg/y p+ 100 kg/y Only Th produces 30 neutrons, then U 1mA@1GeV produces 3 kg/an 100 kg/an needs 33mA, for each 1GWe reactor, beam 33 MW = 100 MWelec (if η acc =33%) French case total intentisty of the beams >1500 ma 10% of the electricity produced S. David,external neutron source for fission-based reactors, IZEST, Orsay, Nov

11 Minor Actinides Transmutation Depleted U Fast reactors Waste Only Fission products U,Pu + MA All the cycle is «poluted» by the minor actinides Double strata strategy FR Minor actinides are concentrated in dedicated reactors U, Pu MA Dedicated reactors (Subcritical) S. David,external neutron source for fission-based reactors, IZEST, Orsay, Nov

12 Waste transmutation Comparison U/Pu cycle in fast reactor with and without MA transmutation Long term radiotoxicity of final waste homogeneous or ADS transmutation S. David,external neutron source for fission-based reactors, IZEST, Orsay, Nov

13 Accelerator-Driven Systems 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 k 2 = 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 + 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. S. David,external neutron source for fission-based reactors, IZEST, Orsay, Nov

14 Beam intensity FR 1 GWe = 2,5 GWth U, Pu MA 45 kg/an French case, 60 GWe (load factor=80%) 45kg * 200MeV/fission P th (ADS) = 0,1 GWth = ADS 5,28 GWth ~16 * 400 MWth ~8 * 800 MWth 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 S. David,external neutron source for fission-based reactors, IZEST, Orsay, Nov

15 Beam power The beam power depends on the subcriticality level beam intensity Pth = 100 MWth ,8 0,85 0,9 0,95 1 k multiplication factor Typically k = 0,95 I = 2,5 ma French case : 8 ADS * 800 MWth 8 proton beams of 20mA = 160 ma S. David,external neutron source for fission-based reactors, IZEST, Orsay, Nov

16 Myrrha project Myrrha project is a prototype not so far from what could be an industrial MA burner of MWth Main features of the ADS demo MWth power k eff around MeV, ma proton beam Highly-enriched MOX fuel Pb-Bi Eutectic coolant & target S. David,external neutron source for fission-based reactors, IZEST, Orsay, Nov

17 If the external neutron has an energy cost - Need to multiplicate the neutrons by fission - Production of waste still occurs - Transmutation dddd P dddd = PP σσσσnn σa φt Nt ( ) = (1 e ) σφ a Waste are never 0 phase out or not really effective A few tens of years If neutrons are produced by fusion, the problem is totally different 1/σ a Φ S. David,external neutron source for fission-based reactors, IZEST, Orsay, Nov

18 If neutrons are produced by fusion process Neutron is the vector of the energy produced by fusion There is no more energy cost of the neutron produced The question of transmutation of nuclear waste becomes different Possibility to reduce a inventory : Fission phase-out strategy Possibility to revisit the question of transmutation of fission products (neutron consumers) Possibility to deeply burn Pu But One p+t fusion produces 17 MeV and 1 neutron Needs of ~1.2 neutrons at least to breed tritium Fusion process surounded by a highly subcritical core k 0,5 This produces k/nu 1/(1-k) fissions = 0,33 fissions = 70 MeV >> 17 MeV To burn 1 t of fissile material / y (typical order of magnitude) 3GWth from fission means 750 MWth from fusion machine S. David,external neutron source for fission-based reactors, IZEST, Orsay, Nov

19 Conclusion 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 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? S. David,external neutron source for fission-based reactors, IZEST, Orsay, Nov