1. Pebble Bed Reactors for Once Trough Nuclear Transmutation
Pablo T. León, J.M.Martínez-Val, A.Abánades, D.Saphier.
E.T.S.I.Industriales, U.P.M.
11/21/2018
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2. Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004
Contents
Advantages of Pebble Bed Fuel in Once Through Transmutation Scenarios.
High Burn-up.
Description of the nuclear spent fuel.
Once through strategy: Pu242 accumulation.
Pebble Bed transmutator.
Conclusions
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3. >95% 239Pu & >65% all Pu Transmuted
Advantages of PB Fuels
The Triso Coated Fuel Particles can withstand very high burn-ups.
TRU - Fuel Kernel
Porous Carbon Buffer
Silicon Carbide
Pyrolytic Carbon }
TRISO coating
747 MW-days/kg
>95% 239Pu & >65% all Pu Transmuted
Thermal
Spectrum
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4. Description of Spent Fuel
Description of Actinides composition for transmutation.
Isotopic Composition of Actinides in the LWR Discharged Fuel, after 40MWd/kg burn-up and 15y cooling.
Pu Isotopes
Mass %
Pu238
2,27%
Pu239
59,04%
Pu240
25,90%
Pu241
6,81%
Pu242
5,98%
Pu244
0,00%
Element
Mass % Np
5.61% Pu
86.24% Am
7.86% Cm
0.29%
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5. Description of Spent Fuel
The fuel cycle for a LWR park is defined:
Mining
1 ton
Nat U
Depleted Uranium.
0.755 tons.
Fabrication
Enriched (3.64%) Fuel tons.
LWR burn-up
10.7 MWe.
40 MWd/kg.
15 y Storage.
Depleted Uranium
0.8 % Enrichment: Kg
Fission Products: Kg
Pu isotopes: Kg (86,24%)
Minor Actinides: Kg (13,76%)
TRU (Pu+MA) kg (100%)
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6. Description of Spent Fuel
The effective ingestion committed dose of Natural Uranium is 19.7 Sv/kg (ICRP 68.)
The dose for ICRP 72 is 30.8 Sv/kg, due to the 210Po dose increment (56.3% increment.)
All the radiotoxicities results are evaluated taking into account that for 1 ton of Natural Uranium, 2.83 kg of TRUs are generated in the LWR reactor (ICRP 68.)
The radiotoxicity values given in next figures are normalized to the radiotoxicity of 1 ton of Natural Uranium.
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7. Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004
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Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004

8. Description of Spent Fuel
The analysis have identified the Pu isotopes (and direct daughters) as the principle contributors to the effective committed dose.
In a thermal reactor, the behavior of Pu isotopes is as follows: c c c c c { Am Cm
Pu238
Pu239
Pu240
Pu241
Pu242 f f f f f
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9. Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004
Once Through Strategy
The Once Through Strategy limit the maximum burn-up to 700 MWd/kg, approximately. This is the nominal burn-up taken for calculations.
The amount of TRUs mass transmuted to obtain 700MWd/kg is defined by the approximate equation:
Burn-up (MWd/kg)= 975.9(1-RF)
Where RF is the Actinides residual fraction.
The result is, for 700 MWd/kg, a 28.27% of the actinides mass not transmuted.
The isotopic composition of this TRUs mass not transmuted is fundamental for final ingestion effective committed dose calculations.
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10. Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004
Once Through Strategy
As it has been demostrated in previous calculations, the Am and Cm isotopes have a high radiotoxicity level.
Cm isotopes (Cm242 to Cm248) decay by ‘’ disintegration to Pu isotopes, so the final radiotoxicity evolution with time is high (specially for Cm244.)
Am isotopes (Am243 to Am241) behaves differently than Cm isotopes. Am243 and Am241 decay by ‘’ to Np. Np239 decays to Pu239 by ‘-’, and Np237 decays by ‘’. Am242 decays primarily (83%) to Cm242 by ‘-’ disintegration, and the rest to Pu242.
If the % of actinides remnant in the fuel are Am and Cm isotopes, the reduction in radiotoxicity after 700 MWd/kg is not very important as compared to the non-transmutation scenarios.
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11. Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004
Once Through Strategy
One of the isotopes with a lower radiotoxicity level is Pu242. The half life of this isotope is T1/2 =3.7E5 s, and by ‘’ disintegration, it decays to U238, the isotope that starts the nuclear fuel cycle.
If the Pu capture chain during transmutation (86.24% Actinides mass) can be broken in Pu242 isotope, then the final radiotoxicity of the 28.27% of the actinides not transmuted after 700 MWd/kg BUP will be much lower, with a Pu242 mass accumulation. c c c c c { Am Cm
Pu238
Pu239
Pu240
Pu241
Pu242 f f f f f
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12. Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004
Theoretical Analysis:
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14. Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004
Once Through Strategy
To obtain a final accumulation of Pu242 after transmutation, a thermal spectrum is necessary.
If a thermal spectrum is used for transmutation, the neutron flux level is going to be defined primarily by Pu239 fission cross section, for a given transmutator power density.
The spectral index to be studied is then the ratio of Pu242 capture cross section and the Pu239 fission cross section . The smaller the spectral index, the higher the Pu242 accumulation in the spent fuel.
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15. Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004
Once Through Strategy
The minimum of the spectral index is at 0.3 eV neutron energy.
0.3 eV
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16. Pebble Bed Transmutator
Pebble Bed fuel can be designed to obtain different neutron spectra in the fuel region.
An analysis has been done to predict the possibilities of pebble bed fuel to obtain neutron spectrum with a minimum spectral index.
Pebble External Diameter
6 cm.
Active Core Diameter: Rf=f(Mf)
50% mass TRISO
50% Graphite for compactation
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17. Pebble Bed Transmutator
The smaller the amount of mass charged in the pebble, the smaller the radius of the active zone (50%, 50%)
Small active zones give a more thermal neutron spectrum.
To optimize the spectrum for a minimum 242Pu capture, different masses charged per pebble have been analyzed (MCNPX.)
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18. Pebble Bed Transmutator
The thermal spectrum used for transmutation of actinides to a maximum burn-up (700 MWd/kg) without reprocessing permits the burn-up of the fuel in two steps.
Initially, as a critical reactor
Need further safety studies.
When keff<1, the burn-up of the fuel have to continue in a subcritical reactor.
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19. Pebble Bed Transmutator
For the critical reactor burn-up analysis, the following reactor geometry have been adopted.
The active length of the reactor is higher than the active diameter for LOCA cooling of the reactor (radiation is enhanced.)
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Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004

20. Pebble Bed Transmutator
Some preliminary analysis have been carried out for three different active radius (corresponding to 0.25, 0.5 and 1 gr charged per pebble.)
Initially, an infinite array of cells, each one containing a pebble, have been studied.
The active zone can be taken as homogeneous or heterogeneous. Both neutron spectra have been studied.
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Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004

21. Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004
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Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004

22. Pebble Bed Transmutator
For the optimum case (Mf=0.5 gr), the heterogeneous and homogeneous calculation have different results of capture and fission actinides cross sections.
The heterogeneous active zone, with TRISO geometry description, have to be taken into account in the MCNPX calculations.
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Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004

23. Pebble Bed Transmutator
The neutron spectra for the reactor have been also calculated for the clean fuel composition.
A total mean spectrum have been analyzed, and the results are in next figure.
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Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004

24. Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004
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Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004

25. Pebble Bed Transmutator
The final burn-up values of the critical phase have not been calculated, because of some problems in lumped fission products cross sections and fission products characterization (important differences in maximum BUPs for keffcritical = 1.)
Once the problem is solved, as a second step, we will start the subcritical analysis of the reactor.
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26. Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004
Proton Beam
Pb-Bi
Pebbles
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27. Pebble Bed Transmutator
Some thermal-hydraulic analysis have been carried out for the critical phase, with FLUENT.
The reactor parameters for thermall-hydraulic calculations are defined in next table.
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28. Pebble Bed Transmutator
The axial and radial power distribution in the reactor have been calculated, with MCNPX.
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32. Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004H2
Prod
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33. Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004
Conclusions
Pebble Bed Reactors present good characteristics to be used in a “Once-Trough Strategy” transmutation.
The Pu242 accumulation in the final Actinides mass not transmutted after maximum BUP (28.27% for 700MWd/kg) minimize final radiotoxicity for a once-through strategy.
Critical and subcritical transmutation steps are studied.
Heterogeneous analysis of the active zone of the fuel is necessary.
An additional advantage is that it is a non-proliferation strategy (Pu239 BUP.)
High temperature operation can permit H2 production during transmutation.
11/21/2018
Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004