Improvement of Core Performance by Introduction of Moderators in a Blanket Region of Fast Reactors

An application of deuteride moderator for fast reactor cores is proposed for power flattening that can mitigate thermal spikes and alleviate the decrease in breeding ratio, which sometimes occurs when hydrogen moderator is applied as a moderator. Zirconium deuteride is employed in a form of pin arrays at the inner most rows of radial blanket fuel assemblies, which works as a reflector in order to flatten the radial power distribution in the outer core region of MONJU. The power flattening can be utilized to increase core average burn-up by increasing operational time. The core characteristics have been evaluated with a continuous-energy model Monte Carlo code MVP and the JENDL-3.3 cross-section library. The result indicates that the discharged fuel burn-up can be increased by about 7% relative to that of no moderator in the blanket region due to the power flattening when the number of deuteride moderator pins is 61. The core characteristics and core safety such as void reactivity, Doppler coefficient, and reactivity insertion that occurred at dissolution of deuteron were evaluated. It was clear that the serious drawback did not appear from the viewpoints of the core characteristics and core safety. 1. Introduction In order to flatten radial power distribution in fast reactors, ordinary fast reactor cores employ two enrichment zones where outer zone has higher plutonium enrichment. Even in such design the power is dropped at the outer zone of the outer core due to the neutron leakage at the peripheral regions. Zirconium hydride has advantages of high moderation ratio as well as the stability to neutron irradiation as no gas emission occurs at neutron absorption. On the other side, it sometimes induces thermal spikes at the fuel pins adjacent to the moderator zones and reduces breeding ratios due to the large absorption cross section of hydrogen contained even in the fast reactor hard spectrum. However, such features of generating thermal spikes will be useful to increase the power at low power region such as core peripherals if the moderator is appropriately arranged, and it can provide the flattening in power distributions in fast reactors. There are many studies [1–8] for the application of moderator in fast reactors. When the moderator material is mixed with absorber or long-life fission products (LLFPs) in the fast reactor core, it will offer many advantages in core performances such as an increase in control rod worth or transmutation rate of LLFP [1–5]. The most promising moderator applicable to the fast reactor has been considered to be zirconium