external neutrons sources for fission - based reactors

S. David,external neutron source for fission-based reactors, IZEST, Orsay, Nov 2017 1

External neutrons sources for fission-based reactors S. David, CNRS/IN2P3/IPN Orsay [email protected]

World Energy / Climate Context


World electricity generation World GreenHouse Gas emissions

Per capita

Target « GIEC 2°C » = 1,5 t/an/hab

0

5

10

15

20

25

0 1 2 3 4 5 6 7 8 9

tCO

2/ha

b

tep/hab

trajectories 1980 - 2014 tep vs tCO2 / cap

Inde

Chine France

Germany

USA

Sweden

Japan

scénario 2°C 2050 : émission moyenne 1,3 tCO2/hab

Carbon-free sources

sobriety

Trajectoires tep – tCO2 par habitant 1980 - 2014


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

5

Present situation

S. David,external neutron source for fission-based reactors, IZEST, Orsay, Nov 2017

Present nuclear reactors based on fission of 235U (0,7% of natural U)

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 = minor actinides fission products

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

Sustainable nuclear = breeding


Problematics Today, we use 235U = 0,7% of natural uranium It is possible to use 238U by breeding 239Pu, and also Thorium

238U + n → 239U → 239Np (2j) → 239Pu

232Th + n → 233Th → 233Pa(27j) → 233U

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 = σcap/ σfis

1 + α neutrons captured on the fertile nucleus (fissile regeneration)

+ -

ν – 2 ( 1 + α) > 0 ⇒ regeneration possible < 0 ⇒ regeneration impossible

7

The number of available neutrons only depends on fissile material

Na (E) = ν - 2(1+ α(E))

fissionfissile

capturefissile

σσ

α =

Thermal spectrum

Th/U Na > 0 U/Pu Na < 0

Fast spectrum

Th/U Na > 0 U/Pu Na > 0

Sustainable nuclear power : breeding principle


8

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 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 20000 years (idem for Li and fusion reactors) • In fast neutron reactors, breeding os possible with the neutrons produced by the fission (critical systems)

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 (U or Th cycle) But external neutron sources can compensate the negative neutron balance

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


Order of magnitude for thorium cycle

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 10% of the electricity produced

p+

11

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

12 S. David,external neutron source for fission-based reactors, IZEST, Orsay, Nov 2017

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

Pth = 100 MWth

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


If the external neutron has an energy cost - Need to multiplicate the neutrons by fission - Production of waste still occurs - Transmutation

A few tens of years

1/σaΦ

𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑

= 𝑃𝑃 − 𝜎𝜎𝜎𝜎𝑑𝑑 ( ) (1 e )a t

a

PN t σ φ

σ φ−= −

Waste are never 0 phase out or not really effective If neutrons are produced by fusion, the problem is totally different

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

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?

external neutrons sources for fission - based reactors

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