ICONE23-1625 CORE TEMPERATURE COEFFICIENT OF SMALL LONG-LIFE HTGR FOR PASSIVE DECAY-HEAT REMOVAL

Abstract

The accident at the Fukushima Daiichi Nuclear Power Plant in 2011 has been influencing design concepts for nuclear reactors with inherent safety features for decay-heat removal. By considering these issues, we obtained design parameter conditions for prismatic high-temperature gas-cooled reactors (HTGRs) with passive safety features for decay-heat removal for both underground and aboveground reactors in our previous works. To determine those conditions, we performed a parametric survey analysis using fundamental equations for residual-heat transfer mechanisms such as conduction, convection and thermal radiation. Using the obtained parametric condition, we proposed the appropriate reactor core size for a 100-MW t reactor operating at an initial core temperature of 1123 K. Subsequently, neutronic analysis was performed for the proposed reactor core using uniformly distributed fuel to validate the possibility of designing a long-lived core with dimensions decided by the conditions. The proper optimizations for the non-uniformly distributed fuel and burnable poison particles, as well as the insertion of control rods to suppress the excess reactivity and flatten the change in power peaking factor during operation, were then successfully accomplished. The calculations show that with proper optimizations, the proposed core life is about 20 years, while the maximum power-peaking factor is less than 2.0 during operation. Therefore, calculations to find the core temperature coefficient for the proposed reactor were performed; it was always negative during reactor operation, with an average of -3.92 pcm/K. This paper shows that the excess reactivity of a long-life small HTGR whose core design satisfies the conditions for passive decay-heat removal was small, and its temperature coefficient was always negative during the operation. Small excess reactivity is a significant advantage from the viewpoint of safety in the case of a reactivity accident.