Impact of the moderation ratio over the performance of different BWR fuel assemblies

Fuel assembly design plays a very important role in the reactor core performance. A fuel assembly has to be designed to achieve safe and efficient performance during its active life inside the nuclear reactor core. Fuel assemblies are designed to be under-moderated to produce a negative moderator temperature coefficient under all operational circumstances. This study assesses the behavior of the infinite multiplication factor (k∞) as a function of the moderation ratio and its dependence on the burnup, for several BWR fuel assemblies. The results show that the moderation ratio at which the fuel assembly transitions from under-moderated to over-moderated changes through the life of the fuel assembly (i.e. with burnup). This study shows that the fuel assembly designs considered, operate in the over-moderated region for burnups over 30 GWd/MT. In a typical 18-month cycle BWR core, even though the fraction of fuel assemblies with burnups over 40 GWd/MT can reach about 50% at the end of cycle the core still meets safety constraints. However, if the fuel assembly designs used were to experience burnups over 45 GWd/MT, the fraction of fuel assemblies operating in the over-moderated region would be high enough to compromise the safety performance of the core.