Analysis of the reactivity coefficients and the stability of a BWR loaded with MOX fuel

In the past few years, the use of MOX fuel in commercial nuclear power stations has been considered in the western countries. France has already developed an extensive reprocessing program based on the use of uranium-plutonium mixed oxide fuel in PWRs. Whereas most of the electricity in France is provided by PWRs, the BWR type is predominantly used in USA, Japan, Germany, Sweden, and some of these countries plan to load MOX bundles in BWRs. The aim of this study is to model the impact of a core-loading pattern containing MOX bundles upon the main characteristics of a BWR (reactivity coefficients, stability). For this purpose, the Studsvik Scandpower code package (CASMO-4/TABLES-3/SIMULATE-3) is used since it has been shown that these codes are able to accurately represent and model MOX bundles. This study is thus devoted to the modelling of BWR cores (of the General Electric BWR/6 type) loaded partially (approximately 2/3 of UOX bundles and 1/3 of MOX bundles) or totally with MOX bundles. The plutonium quality used is the Pu type 2016 (mostly Pu-239, 56%, and Pu-240, 26%), but a variation of the plutonium isotopic vector is also investigated, in case of a partial MOX loading. One notices that the reactivity coefficients do not show significant changes in comparison with a full UOX loading. Nevertheless, two main problems arise: the shutdown margin at BOC is lower than 1% and the stability to in-phase oscillations is decreased