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

• Published in: Annals of nuclear energy Vol. 81; pp. 227 - 239
• Main Authors: Wols, F.J., Kloosterman, J.L., Lathouwers, D., van der Hagen, T.H.J.J.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.07.2015
• Subjects: Fuels; Mathematical analysis; Nuclear engineering; Nuclear power generation; Nuclear reactor components; Nuclear reactors; Passive safety; Pebble Bed Reactor; Running-in phase; Thorium; Thorium breeder; Uranium; Running-in phase; Passive safety; Pebble Bed Reactor; Thorium breeder
• DOI: 10.1016/j.anucene.2015.02.043
• ISSN: 0306-4549

•This work analyzes important trends of the running-in phase of a thorium breeder PBR.•Depletion equations are solved for important actinides and a fission product pair.•Breeding U-233 is achieved in 7years by cleverly adjusting the feed fuel enrichment.•A safety analysis shows the thorium PBR is passively safe during the running-in phase. The present work investigates the running-in phase of a 100MWth 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 startup fuel for 1300days, 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 7years 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°C throughout the running-in phase, quite a bit below the TRISO failure temperature of 1600°C. The uniform reactivity coefficients of cores with U-235/U-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°C.