Analysis of the Xenon Feedback on the Core Dynamics of High-Temperature Reactors During Heat Removal Transients Without Reactor Shutdown

The core dynamic analysis of an anticipated heat removal transient without scram in a high-temperature gas-cooled reactor has indicated that in case of a failure of core cooling, the reactor undergoes a selfshutdown after 1 min because of its negative temperature coefficients of reactivity. If the decay heat removal system operates according to plant specification, recriticality, and thus nuclear power generation, occurs. However, the maximum rise in fuel elements temperature is limited to 50°C due to the high heat capacity of the core.Without taking into consideration the effect of xenon feedback on the neutron kinetics, a new steady core state is established after 2 h in which the fuel temperature and gas outlet temperature at the lower core edge are 195°C higher than in normal operation. Due to transient xenon poisoning, a rise in gas outlet temperature only occurs during the first 70 min and amounts to 70°C. For this reason undesirable transient strains on the components connected behind the core are not expected. A slow xenon buildup during the first hour ensures a long-term subcriticality of the reactor. Without any contribution from the shutdown system, this leads to a decrease in nuclear power and thus to core cooling with functioning decay heat removal.