Burnable Poisons Alternative Configurations in AT-FCM PWR Fuel Assembly

Abstract

The Fukushima Daiichi Nuclear Power Plant accident in Japan was one of the severest accidents in history of nuclear power plants. This accident changed the prospect of the Nuclear Engineers all around the globe, regarding safety enhancement of nuclear reactors. Since then many aspects of nuclear reactors regarding the improvements of safety features are under investigation and R&D efforts are underway around the world. Use of innovative fuels in present as well as future reactor designs is one of the major potential areas of these ongoing efforts. Fully Ceramic Micro-Encapsulated (FCM) fuel originally developed for use in high temperature gas cooled nuclear reactors, has proven worth for operating in high temperature environment with high burn-up. Due to its additional fission product barrier in the form of strong SiC layer, it is worth using for application as PWR fuel, thus providing potential benefits related to safety and operational aspects of power plant. However, use of FCM fuel in a PWR also has some operational constraints such as the moderator temperature coefficient (MTC) of reactivity has less negative value and even becomes positive when higher concentration of soluble boron is used for the reactivity control. Thus, use of burnable poison material becomes more important to control the access reactivity throughout the cycle length in such a way the quantity of soluble boron to be used is much lower to prevent the positive MTC value or even soluble boron free operation is possible. In present studies a new candidate designs of PWR fuel assembly of 12×12 square array configuration has been used to study the BP material impact on cycle length. Monte Carlo code MVP-BURN is utilized for the analysis to accurately model the double heterogeneity arising due to TRISO type fuel. Two standard materials i.e. Erbia and Gadolinia are used and different configuration including mixing of BP in matrix material, fuel kernel and in QUADRISO form are analyzed and compared with each other. Impact of the residual poison are also analyzed and additional enrichment required to overcome the impact of residual BP material reactivity are calculated. Different configurations support different BP materials. However, it has been found that with an appropriate combination of both materials and configuration, it is possible to minimize the use of soluble boron. Finally, the recommended assembly configuration is analyzed for MTC value during the entire cycle length, showing sustainability of negative values of MTC for the region of interest. With this kind of arrangements, it is possible to use FCM type fuel for present as well as future generations of the PWRs.