Adonai Herrera-Martínez, Yacine Kadi, Geoff Parks, Vasilis Vlachoudis High-Level Waste Transmutation: Thorium Cycle vs Multi-Tier Strategy

Background Effective methods of dealing with high-level nuclear waste (HLW) are essential if global use of nuclear power is to expand One potentially attractive option to transmute HLW in accelerator-driven reactor systems Fast systems seem to be necessary for this purpose The uranium-plutonium fuel cycle cannot be “closed” by this strategy – minor actinide (MA) production rates are too high

HLW Transmutation Options Two main HLW transmutation approaches have been conceived: 1.Use of ADSRs loaded with thorium-based fuel, enriched with plutonium to provide initial reactivity, thus eliminating a significant amount of transuranics (TRUs) from spent fuel 2.Use of a few ADSRs heavily loaded with MAs at the end of a multi-strata fuel cycle, effectively closing the fuel cycle

Energy Amplifier An Energy Amplifier (EA) module might consist of a 1500 MW th unit coupled with a GeV proton accelerator of mA Heat extraction is based on a natural convection- driven molten lead pool This device would operate away from criticality at all times (0.95 ≤ k ≤ 0.98) The nominal external beam power is P ext ~12.5 MW, corresponding to an energy gain G = P EA /P ext ~120

Transmutation in an EA The EA can operate with a wide range of fuels, such as thorium-plutonium-based fuels, using plutonium to drive the breeding of 233 U from thorium Equilibrium concentrations for higher actinides are much lower for thorium-based fuels Long-lived fission fragments (LLFFs) can be inserted in order to transmute them into short-lived isotopes, by the means of the Transmutation by Adiabatic Resonance Crossing method The EA might initially be loaded with a mixture of actinide waste and thorium, in an approximate ratio of 0.16 to 0.84 by mass

Transmutation in an EA During operation, the actinides would be fissioned, while 233 U would be produced The bred 233 U would compensate for the reduction in reactivity due to the decreasing actinide mixture and the build-up of fission fragments (FFs) Operation without external intervention over a long burn-up (corresponding to five years operation) would be possible

EA Model Used A lead-cooled 1 GW th EA Thorium oxide fuel ‘enriched’ with ~24% TRUs from Light Water Reactor (LWR) spent fuel The isotopic composition was that of LWR spent fuel with a burn-up of ~33 GWd/tHM, after a cool-down period of ~15 years: 86.1% Pu, 5.4% Np, 8.4% Am, 0.2% Cm

Variation of k src with Burn-up Relatively constant k src from 0 to 120 GWd/t burn-up Proton current remains below 15 mA

Evolution of the Actinide Inventory 30% of the initial plutonium is transmuted 30% of the initial americium is transmuted 45% of the initial 237 Np is transmuted Uranium inventory increases (by design) Curium inventory increases by ~400%

Isotopic Evolution of the Actinide Inventory Large amount of 233 U produced (~610 kg) 236 U production three orders of magnitude less Significant reductions in 239 Pu (–752 kg), 241 Pu (–61 kg), 237 Np (–68 kg) and 241 Am (–83 kg) But increases in 238 Pu (+80 kg), 242* Am, 243 Am and all curium isotopes

Comparison of Actinide Balance per TW.h Actinide balance (kg/TW.h) PWR (UOX) 41 GWd/tHM EADF (UPuO 2 ) 23 GWd/tHM EA (ThPuO 2 ) 120 GWd/t of fuel Plutonium+11.3–8.1–27.6 Neptunium –2.55 Americium –2.65 Curium For the same energy output, one EA could eliminate the waste of ~2.5 conventional LWRs The mass balances of Pu, Np and Am are all –ve Some Cm is created and its disposal needs some thought

Double Strata Schemes Several strategies to utilise the Pu present in HLW have been suggested, some include the use of MOX in LWRs, others the use of Fast Breeder/Burner Reactors or both These reactors produce an increasing amount of MAs Therefore, a second stratum has been proposed to deal with these elements

Double Strata Schemes S1 S2

Partitioning & Transmutation Stratum The P-T stratum (S2) takes the spent fuel effluents after reprocessing and, once appropriate fuel is fabricated, eliminates them in a dedicated ADSR The mass flow from S1 to S2 is ~5% of the spent fuel mass, 90% of which is FFs and 10% TRUs (all the MAs produced in S1 plus a small fraction of Pu) 99.9% of the U in the spent fuel is reprocessed and remains in S1, together with most of the Pu, reinserted as MOX in LWRs and fast burners

MA Burner Model Used A lead-bismuth eutectic (LBE) cooled 250 MW th reactor The reactor is a stainless steel cylindrical tank ~9 m in height and 4.4 m in diameter The cylindrical fuel core is 1.9 m in height and 1.7 m in diameter The core is formed by 7.5 mm  7.5 mm fuel cells containing ~6 mm diameter cylindrical fuel pins The pure metallic MA fuel composition is similar to that coming from a first stratum combining LWR UOX and MOX use, i.e. 12.5% 237 Np, 50% 241 Am, 25% 243 Am and 12.5% 244 Cm

Variation of k src with Burn-up This very low initial k src entails an excessive proton current (~64 mA) It will be necessary to add Pu (~10%) to the MA fuel to increase the initial reactivity and reduce the reactivity swing during burn-up A maximum cycle length of 5000 days (600 GWd/tHM burn-up) seems feasible

Capture and Fission Rates with Burn-up When the burn-up reaches 600 GWd/tHM the number of captures in FFs is of the same order of magnitude as the number of captures or fissions in MAs A cool-down period then would allow the decay of the short-lived FFs poisoning the reactor and reprocessing would allow the insertion of ‘fresh’ MAs

Actinide Mass Evolution with Burn-up While the total amount of actinides is reduced at a constant rate of ~100 kg/yr, equivalent to the yearly production of 3-4 conventional 1 GW e LWRs, the actinide mixture composition changes significantly – in particular there is more Pu at EOC than BOC

Transmutation Strategies Comparison The Thorium Cycle Offers clear advantages in terms of MA production Offers the advantage of treating the HLW in the spent fuel effectively without the need for TRU separation These systems would be designed to produce a significant amount of power: this could allow a progressive change from conventional U-based nuclear power plants to reactors based on the thorium cycle The need to develop a completely new fuel cycle might be the main drawback of this approach, as it may be a significant economic burden

Transmutation Strategies Comparison Double Stratum Scheme Pu is managed by using MOX, either in LWRs or in fast burner reactors, at the expense of higher MA production The partitioning phase gains importance in this strategy, since the Pu has to be separated from the MAs at EOC This makes the fuel fabrication process more complex than for the EA (which uses TRUs directly from the spent fuel) Pu separation entails proliferation and radiological risks

Comparison of Actinide Balance per TW.h Actinide balance (kg/TW.h) EA (ThPuO 2 ) 120 GWd/t of fuel MA burner (pure MAs) 600 GWd/tHM Plutonium– Neptunium–2.55–7.7 Americium–2.65–46.3 Curium+0.86–2.3 The MA burner is very efficient in eliminating MAs, in particular Am, while producing Pu

Transmutation Strategies Comparison A 1 GW th MA burner could eliminate the MAs of ~10 LWRs (1 GW e ) using both UOX and MOX A 1.5 GW th EA would reduce the stockpile of MAs at a lower rate (1 EA to 1.5 LWRs) Conversely, the EA would eliminate both the MA inventory and waste Pu, which in the double strata approach is left for the commercial cycle with associated proliferation and radiological risks The EA Th-U transmutation strategy entails ~30% of installed power being produced by EAs, compared to the ~3% of installed power from MA burners required by the double strata scheme

Transmutation Strategies Comparison Scaled to the 363 GW e /yr produced globally, the elimination of the MA waste being produced would require the construction of ~37 MA burners, producing ~13 GW e plus the extensive use of MOX in LWRs, whereas the elimination of the HLW in EAs would involve the construction of ~242 EA units The added value of the latter scenario is the addition of ~145 GW e to the grid Although it requires the development of a new fuel cycle, the advantages in terms of availability and waste reduction of the thorium cycle are clear

Research Associate in the Design of Accelerator Driven Subcritical Reactors Research Associate in the Technology Assessment of Accelerator Driven Subcritical Reactors