Evaluation of core bowing reactivity of Joyo MK-III performance tests core

Takamatsu, Misao ; Kuroha, Takaya*; Yoshida, Akihiro 

A study of passive safety test using Joyo has been carried out to demonstrate the inherent safety of sodium cooled fast reactor. In this study, emphasis was placed on the improvement on feedback reactivity calculation accuracy. Especially, the core bowing reactivity has been one of the main targets of this study because it can be designed to provide negative reactivity in the event of power excursion. The core bowing reactivity was calculated by "MERBA" (MEchanical behavior and Reactivity shift caused by core Bowing Analysis code system for Joyo)" which had been developed for Joyo MK-II core. Through the operation of MK-II core, measured power reactivity coefficient shows a power dependency on reactor thermal power, which is considered to be caused by core bowing effect. In order to investigate the mechanism of this phenomenon, the measured power dependency on power reactivity coefficient were compared with the calculation by "MERBA". It was confirmed that the power dependency on the power reactivity coefficient of the MK-II core could be explained by a core bowing reactivity. In the upgraded MK-III core, neutron flux, the coolant temperature difference between reactor inlet and outlet, etc. will be changed and these may effect on the core bowing reactivity. Therefore, the core bowing reactivity of MK-III performance tests core was evaluated using "MERBA" which was modified to analyze MK-III core. As a result of "MERBA" calculation, the core bowing behavior of MK-III performance tests core was almost similar to MK-II core; the fuel subassembly was initially bent toward the core center at zero power, because the middle spacer pad of fuel subassembly was suppressed by reflectors which have rather large permanent distortion. During the thermal power increase, the fuel subassembly was bent to outward and the negative reactivity was inserted by the fuel movement to outward.