Designing Efficient Reactors for Long-Lived Nuclear Waste Recycling Division of Reactor Physics, The Royal Institute of Technology ABSTRACT Accelerator-driven systems (ADS) are being investigated and designed for recycling long-lived nuclear waste. Operation in subcritical mode enhances safety of fast reactor, and therefore allows application of advanced nuclear fuels with large fraction of minor actinides, while the fraction in conventional critical fast reactors is limited to a few percent due to the safety constraints. Additional costs, imposed by the need of a strong neutron source – linear proton accelerator – to drive the fission reaction, can be decreased by improving the ADS efficiency, particularly the external source efficiency. This research shows that the proton source efficiency of ADS can be significantly increased by utilization of advanced nitride fuels permitting compact core design and small target. On the other hand, the source efficiency of a large ADS core can be improved by applying proton beam with annular shape. High source efficiency of ADS leads to more simple accelerator design and higher reliability. CONTACT INFORMATION Andrei Fokau PhD student, Lic Eng KTH Royal Institute of Technology Department of Physics Reactor Physics Roslagstullsbacken 21 SE-106 91 Stockholm Office: 08-55 37 8204 Mobile: 073-571 03 56 [email protected] http://neutron.kth.se energy protons and the target nuclei. Leaving apart oxygen and steel, typical spent nuclear fuel consists of uranium (94.6%), plutonium (1.1%), minor actinides (0.1%) and fission products. Direct disposal of such spent fuel requires large underground storage facilities and a very long isolation time (105 years). The volume and time can be significantly reduced (in 6 and 100 times resp.) if the actinides are separated from fission products and recycled in dedicated reactors. While decisions in Europe are made in favour of the direct disposal, the recycling, or partitioning and transmutation of high-level waste (HLW), is considered as an alternative solution, being the subject of numerous research projects. How to recycle HLW efficiently? Within the framework of the European research program on partitioning and transmutation of nuclear waste two designs of an ADS are developed: experimental facility, XT-ADS (85 MWth, see Fig. 2) and industrial-scale demonstrator, EFIT (400 MWth). While construction of XT-ADS starts in 2016, the design of EFIT is still conceptual and is the subject of this work. Making ADS designs more source efficient The power of the proton beam required to reach certain fission power depends on how well the target is coupled to the sub-critical core. This can be described by the proton source efficiency: ψ*=-ρ ν fp, Fission probability of even neutron number nuclides (e.g. 242Pu, 241Am, etc.) is too low for thermal energy neutrons (Fig.1), making transmutation of the minor actinides (MA) in convectional lightwater reactors (LWR) inefficient. Moreover, the presence of americium in LWR fuel significantly deteriorates safety of the reactor and is therefore limited to 1%, leading to a large part of the nuclear reactor park dealing with Am contaminated fuel. In order to transmute MA efficiently, fast neutron reactors must be employed. Operation of such fast reactors in sufficiently sub-critical mode ensures core safety and allows much larger fractions of MA in its fuel than in critical reactors. This permits lowering the required number of dedicated transmuters involved in recycling of HLW. Accelerator-driven systems Because sub-critical reactors do not have self-sustained chain reactions, they need an external continuous neutron source to drive the fission reaction in their core. Accelerator-driven systems (ADS) employ a linear proton accelerator and a heavy-metal target to create such intensive neutron source via the spallation reaction between the high- where ρ is reactivity, ν is the total number of neutrons per fission, fp is the number of fissions per proton. The source efficiency is higher when each source proton produces more source neutrons, and when each source neutron induces more fission. By selecting parameters of the proton beam, spallation target and core, one can optimize the source efficiency. Advanced nitride fuel In our work, we studied source efficiency of the EFIT design with three fuel types (see Fig. 3) – ceramic-ceramic (AnO1.88+MgO), ceramic-metallic (AnO1.88+Mo) and nitride (AnN+ ZrN) fuels. We have shown that, thanks to the good neutron economy of the nitride fuel, EFIT core size can be more compact. Because the core power of the nitride fuelled version of EFIT is lower (200 MWth) than those of the oxide versions, it requires lower proton current, which permits reducing the size of the spallation target. The reduction of core and target sizes resulted in 45% higher proton source efficiency and therefore lowers the demand for the proton current even further. Additionally, an ADS with nitride fuel features much lower coolant void worth improving core safety. Annular proton beam In the versions of the EFIT design with oxide fuel, the target module has a large diameter of 80 cm in order to provide sufficient heat removal for the proton beam power up to 20 MW. The large target size decreases the energy of the source neutrons and consequently lowers source efficiency. We found that the application of annular (tube-shape) proton beam can compensate the drawbacks of the large target diameter and can provide high source efficiency. Selecting the beam radius close to the target border increases the energy of source neutrons and therefore results in a higher fission probability. The proton source efficiency increases by 15% for the nitride core and up to 50% for the oxide ones (Fig. 4). In addition, the annular shape of the proton beam reduces the possibility of the target material overheating, since the heat is deposited on a larger area and more easily removed. CONTACT INFORMATION ï ï ï ï ï ï Fig. 5. Optimized power distribution in the XT-ADS core with three fuel zones. Power of each fuel pin was calculated by high-energy neutron and proton transport code MCNPX. The total power peaking factor is less than 1.3. The values along the axes are in cm. ./" $,0 ./" $%! ./" $%( ./" $%$ -+" $%( .12" 342" -+" $%, *+" $%% *+" $%) !" !#$" !#%" !#&" !#'" (" Fig. 1. Fission probabilities of some actinide nuclides in thermal (PWR) and fast (SFR) neutron spectrum. Fig. 2. Primary system of the experimental facility XT-ADS (MYRRHA). Core power: 85 MW, proton energy: 600 MeV, beam power : 5 MW, coolant material: PbBi, fuel material: MOX Conclusions We have shown that the source efficiency of ADS designs can be significantly improved by optimizing parameters of the sub-critical core and the proton beam. We found that smaller ADS core and smaller spallation target features higher source efficiency. For ADS designs with big target units, the source efficiency can be improved by shaping the proton beam and redesigning the target cooling system. The higher source efficiency reduces the proton current (or proton energy) required to drive the reactor and therefore leads to higher energy gain and simpler accelerator components with higher reliability. Publications Fokau, A., et. al., 2010. A source efficient ADS for minor actinides burning, Annals of Nuclear Energy 37 (4), pp. 540-545 Zhang, Y., Wallenius, J., Fokau, A., 2010. Transmutation of americium in a medium size sodium cooled fast reactor design, Annals of Nuclear Energy 37 (5), pp. 629-638 Reactor core modelling and optimization ./" $,' Designing a fast reactor core (critical or subcritical) is a multi-stage iterative process, which starts with specifying the basic parameters such as the total core power and the maximum linear power rating, choosing suitable materials, and selecting corresponding temperature ranges. Then a number of simulations are done for defining the geometry of fuel elements and optimizing the composition of the fuel material to meet the design objectives. Pin lattice parameters The fission probability of minor actinides is high for dense fuel pin packing due to the high neutron energy. However, the coolant volume fraction must be sufficient for heat removal under accidental conditions. Fig. 3. Core loading schemes of EFIT reactor with three fuel candidates: ceramic-ceramic (AnO1.88+MgO, 180 FA), ceramic-metallic (AnO1.88+Mo, 216 FA), and nitride (AnN+ZrN, 78 FA). 220 Number of fissions per proton Andrei Fokau and Janne Wallenius Direct disposal or recycling? EFIT nitride EFIT cermet 200 180 160 140 120 0 5 10 15 20 25 30 35 40 Proton beam radius, cm Fig. 4. Source efficiency of EFIT as a function of the beam radius in case of annular proton beam. Radial power profile The external neutron source and subcriticality result in a high power peaking compared to critical reactors. One can reduce it by dividing the core into several fuel zones and tuning their fuel-to-matrix ratios (see Fig. 5). Fuel burn-up During reactor operation, more-fissionable nuclides are burning out and the core reactivity drops. The reactivity swing can be compensated by allowing fertile or lessfissionable nuclides to convert by neutron capture, so that the reactivity is kept constant during the fuel life-time. Reactor safety The main objective of the reactor design is to ensure safety of the core in all possible accident scenarios. In order to confirm this, safety parameters are evaluated by core simulation and then the reactor behaviour is studied using transient analysis codes. All operation limits and requirements must be satisfied. Target design Depending on the power of proton beam required to reach certain core power, the target design is adjusted to make sure that the heat deposited in the target material can be removed. Iterative process Since all core parameters and characteristics are interconnected, correcting one of them requires recalculation of all steps. It takes many iterations before the reactor core design satisfies all performance and safety requirements. Acknowledgements The authors acknowledge financial support of SKB AB (the Swedish Nuclear Fuel and Waste Management Company), and the European Commission within the framework of the Integrated Project EUROTRANS.