Needs of external neutrons for fission based reactors

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


World electricity generation World GreenHouse Gas emissions

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

2050 Factor 1 to 10

2100 Factor 5 to 40

TWh/y

Uranium, thorium and minor actinides


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, …

5

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

6

Breeding


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)

7

Minor Actinides Transmutation


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


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


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


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


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


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


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


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?

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Needs of external neutrons for fission based reactors