Analysis of the running-in phase of a Passively Safe Thorium Breeder Pebble Bed Reactor

The present work investigates the running-in phase of a 100 MWth Passively Safe Thorium Breeder Pebble Bed Reactor (PBR), a conceptual design introduced in previous equilibrium core design studies by the authors. Since U-233 is not available in nature, an alternative fuel, e.g. U-235/U-238, is required to start such a reactor. This work investigates how long it takes to converge to the equilibrium core composition and to achieve a net production of U-233, and how this can be accelerated. For this purpose, a fast and flexible calculation scheme was developed to analyze these aspects of the running-in phase. Depletion equations with an axial fuel movement term are solved in MATLAB for the most relevant actinides (Th-232, Pa-233, U-233, U-234, U-235, U-236 and U-238) and the fission products are lumped into a fission product pair. A finite difference discretization is used for the axial coordinate in combination with an implicit Euler time discretization scheme. Results show that a time dependent adjustment scheme for the enrichment (in case of U-235/U-238 start-up fuel) or U-233 weight fraction of the feed driver fuel helps to restrict excess reactivity, to improve the fuel economy and to achieve a net production of U-233 faster. After using U-235/U-238 start-up fuel for 1300 days, the system starts to work as a breeder, i.e. the U-233 (and Pa-233) extraction rate exceeds the U-233 feed rate, within 7 years after start of reactor operation. The final part of the work presents a basic safety analysis, which shows that the thorium PBR fulfills the same passive safety requirements as the equilibrium core during every stage of the running-in phase. The maximum fuel temperature during a Depressurized Loss of Forced Cooling (DLOFC) with scram remains below 1400 degrees C throughout the running-in phase, quite a bit below the TRISO failure temperature of 1600 degrees C. The uniform reactivity coefficients of cores with U-2351U-238 driver fuel are much stronger negative compared to U-233/Th driver fuel, which implies that the stronger reactivity insertion by water ingress and the reactivity addition by xenon decay during a DLOFC without scram can be compensated without fuel temperatures exceeding 1600 degrees C. (C) 2015 Elsevier Ltd. All rights reserved.