Pebble Bed Reactors for Once Trough Nuclear Transmutation

Pebble Bed Reactors for Once Trough Nuclear Transmutation Pablo T. León, León J. M. Martínez-Val, A. Abánades, D. Saphier. E. T. S. I. Industriales, U. P. M. 9/29/2020 Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 1

Contents • Advantages of Pebble Bed Fuel in Once Through Transmutation Scenarios. – High Burn-up. • Description of the nuclear spent fuel. • Once through strategy: Pu 242 accumulation. • Pebble Bed transmutator. • Conclusions 9/29/2020 Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 2

Advantages of PB Fuels • The Triso Coated Fuel Particles can withstand very high burn-ups. } Pyrolytic Carbon Silicon Carbide Porous Carbon Buffer TRU - Fuel Kernel TRISO coating 747 MW-days/kg >95% 239 Pu & >65% all Pu Transmuted Thermal Spectrum 9/29/2020 Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 3

Description of Spent Fuel • Description of Actinides composition for transmutation. Element Mass % Np 5. 61% Pu 86. 24% Am 7. 86% Cm 0. 29% Pu Isotopes Mass % Pu 238 Pu 239 Pu 240 Pu 241 Pu 242 Pu 244 2, 27% 59, 04% 25, 90% 6, 81% 5, 98% 0, 00% Isotopic Composition of Actinides in the LWR Discharged Fuel, after 40 MWd/kg burn-up and 15 y cooling. 9/29/2020 Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 4

Description of Spent Fuel • The fuel cycle for a LWR park is defined: Mining 1 ton Nat U Depleted Uranium. 0. 755 tons. 9/29/2020 Fabrication Enriched (3. 64%) Fuel 0. 245 tons. LWR burn-up 10. 7 MWe. 40 MWd/kg. 15 y Storage. Depleted Uranium 0. 8 % Enrichment: Fission Products: Pu isotopes: Minor Actinides: TRU (Pu+MA) Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 232. 00 Kg 9. 92 Kg 2. 44 Kg (86, 24%) 0. 39 Kg (13, 76%) 2. 83 kg (100%) 5

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 210 Po 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. 9/29/2020 Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 6

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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: Pu 238 c Pu 239 f 9/29/2020 c Pu 240 f c Pu 241 f c Pu 242 f Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 c f { Am Cm 8

Once Through Strategy • The Once Through Strategy limit the maximum burnup to 700 MWd/kg, approximately. This is the nominal burn-up taken for calculations. • The amount of TRUs mass transmuted to obtain 700 MWd/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. 9/29/2020 Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 9

Once Through Strategy • As it has been demostrated in previous calculations, the Am and Cm isotopes have a high radiotoxicity level. • Cm isotopes (Cm 242 to Cm 248) decay by ‘ ’ disintegration to Pu isotopes, so the final radiotoxicity evolution with time is high (specially for Cm 244. ) • Am isotopes (Am 243 to Am 241) behaves differently than Cm isotopes. Am 243 and Am 241 decay by ‘ ’ to Np. Np 239 decays to Pu 239 by ‘ -’, and Np 237 decays by ‘ ’. Am 242 decays primarily (83%) to Cm 242 by ‘ -’ disintegration, and the rest to Pu 242. • If the 28. 27% 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. 9/29/2020 Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 10

Once Through Strategy • One of the isotopes with a lower radiotoxicity level is Pu 242. The half life of this isotope is T 1/2 =3. 7 E 5 s, and by ‘ ’ disintegration, it decays to U 238, the isotope that starts the nuclear fuel cycle. • If the Pu capture chain during transmutation (86. 24% Actinides mass) can be broken in Pu 242 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 Pu 242 mass accumulation. Pu 238 c Pu 239 f 9/29/2020 c Pu 240 f c Pu 241 f c Pu 242 f Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 c f { Am Cm 11

• Theoretical Analysis: 9/29/2020 Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 12

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Once Through Strategy • To obtain a final accumulation of Pu 242 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 Pu 239 fission cross section, for a given transmutator power density. • The spectral index to be studied is then the ratio of Pu 242 capture cross section and the Pu 239 fission cross section. The smaller the spectral index, the higher the Pu 242 accumulation in the spent fuel. 9/29/2020 Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 14

Once Through Strategy • The minimum of the spectral index is at 0. 3 e. V neutron energy. 0. 3 e. V 9/29/2020 Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 15

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 9/29/2020 Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 16

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 242 Pu capture, different masses charged per pebble have been analyzed (MCNPX. ) 9/29/2020 Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 17

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. 9/29/2020 Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 18

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. ) 9/29/2020 Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 19

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. 9/29/2020 Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 20

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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. 9/29/2020 Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 22

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. 9/29/2020 Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 23

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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. 9/29/2020 Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 25

• Proton Beam • Pebbles • Pb-Bi 9/29/2020 Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 26

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. 9/29/2020 Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 27

Pebble Bed Transmutator • The axial and radial power distribution in the reactor have been calculated, with MCNPX. 9/29/2020 Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 28

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H 2 Prod 9/29/2020 Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 32

Conclusions • Pebble Bed Reactors present good characteristics to be used in a “Once-Trough Strategy” transmutation. • The Pu 242 accumulation in the final Actinides mass not transmutted after maximum BUP (28. 27% for 700 MWd/kg) minimize final radiotoxicity for a oncethrough 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 nonproliferation strategy (Pu 239 BUP. ) • High temperature operation can permit H 2 production during transmutation. 9/29/2020 Actinides and Fis. Product P&T. Las Vegas, EEUU, 2004 33