Analysis of the running-in phase of a Passively Safe Thorium Breeder Pebble Bed Reactor. (July 2015)

• Record Type: Journal Article
• Authors: Wols, F.J.; Kloosterman, J.L.; Lathouwers, D.; van der Hagen, T.H.J.J.
• Is Part Of: Annals of nuclear energy. Volume 81(2015:Jul.)
• Page Start: 227
• Page End: 239
• Publication Date: 2015-07
• Subjects: Running-in phase -- Pebble Bed Reactor -- Thorium breeder -- Passive safety; Nuclear energy -- Periodicals; Nuclear engineering -- Periodicals; 621.4805
• DOI: 10.1016/j.anucene.2015.02.043
• Languages: English
• ISSNs: 0306-4549

Abstract

Highlights: 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 7 years by cleverly adjusting the feed fuel enrichment. A safety analysis shows the thorium PBR is passively safe during the running-in phase. Abstract: The present work investigates the running-in phase of a 100 MW th 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 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 °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.