Proliferation Risk Assessment of a Pebble-Bed Reactor

Abstract

An analysis of a pebble bed reactor (PBR) system has been completed to assess its proliferation resistance (PR) against multiple special nuclear material-SNM (uranium and plutonium) diversion scenarios. The PR assessment uses data generated using Monte Carlo N-particle (MCNP) code simulations on infinite lattice and full core PBR models. The PR assessment is based on multi-attribute utility analysis (MAUA) technique. A combined intrinsic and extrinsic barriers PR assessment is done using proliferation resistance analysis and evaluation tool for observed risk (PRAETOR) code using 68 attributes. In addition, an intrinsic PR assessment method is developed by utilizing only four attributes: spontaneous fission neutrons, heat load, radiation load, and Rossi-α. Compared to PRAETOR, the new method is more capable to differentiate PR of Pu diversion scenarios with respect to PBR operations. This study finds that the spent fuel of PBR is more attractive for proliferation if the fuel has a higher 235U enrichment and/or lower burnup level. A full core modeling and simulation on a one-batch refueling scheme shows that if HTR-10, a PBR-type is rescaled to 250 MWth, it would yield 1.9 significant quantity (SQ) of plutonium and 1.73 SQ of low enriched uranium (LEU) per year with a fissile plutonium quantity of 82.9% at a fuel burnup of 65.9 GWd/MTU. A once-through-then-out (OTTO) refueling scheme enables the fuel to reach a higher burnup level, resulting in less leftover 235U and fissile Pu content in the spent fuel compared to the one-batch refueling scheme. A 3-pass refueling scheme discharges fuel with 5.6% less fissile plutonium content (at 0.0414 g per pebble) and 3.1% total plutonium content (at 0.0532 g per pebble) than the OTTO, but it leaves slightly higher 235U by 0.3% (at 0.357 g per pebble). At fuel burnup of 80 GWd/MTU, the intrinsic PR of the 3-pass refueling scheme is 0.3109 ± 0.004 that is practically the same as the OTTO refueling scheme (0.3037 ± 0.0044). The PBR system has a lower intrinsic PR than the current technology of Pressurized Water Reactor (PWR). An intrinsic PR comparison on integrated PWR (iPWR) and PBMR-400 designs at a same rated output power of 500 MWth with a one-batch refueling scheme is performed using the new methodology. In terms of Pu diversion, the PR of PWR system (0.345 ± 0.002) is higher than the PBR (PBMR-500) system (0.282 ± 0.001) while both reactors have a similar PR of U diversion (PWR: 0.263 ± 0.001 vs. PBR: 0.261 ± 0.001). This study showed that the PRAETOR code formalism is comparatively less useful in differentiating between SNM diversion scenarios in PBR’ case, if a large number of intrinsic and extrinsic attributes (such as 68 used in this study) are utilized, compared to the new four-intrinsic-attribute method developed.