Safety of thermal energy supplying nuclear plant AST-500

The neutron physics and hydraulic instability in reactors, the stability analysis including the analytical model of dynamic behavior and the criteria for stability judgement, and the impacts of steam quality and throttling condition at the inlet of channel boxes of AST-500 are described. The design concept characteristics of AST-500 are basically natural circulation cooling of the core and heat absorption by a heat exchanger contained in a reactor vessel. As for the instability of this reactor, local hydraulic instability between channels, simultaneous vibration behavior of total core flow rate, channel box flow rate, and the orientation mode of neutron flux resonance instability of neutron flux and hydraulic gross instability in the core were theoretically analysed with the combined frequency characteristic methodology. The analytical results were compared to the experimental results obtained in the BWR VK-50. The relation of steam quality and the stability of AST-500 operation was studied, using Nikist's criterion, and the stable zone of plant operation and nuclear behavior are presented with the many kinds of relating evaluations in detail. Optimization was conducted for steam quality during plant start-up and power increase. (Nakai, Y.)