Recriticality of the HTR-Module Power Reactor after hypothetical accidents

Abstract

One of the most important characteristics of the HTR-Module is the potential to cope with accidents by mere passive and inherent safety features that are deliberately included in its physical and technical design. As a consequence of the design of the HTR-Module fuel element, a significant release of radioactive fission products is safely prevented up to a fuel element temperature of 1620°C. This paper reports on the power and temperature response of the reactor under the following postulated severe accident conditions: Core heat-up accidents initiated by reactivity insertion due to the complete withdrawal of all shutdown absorbers combined with the long-term non-availability of all shutdown systems. Hypothetical reactivity accidents can be controlled merely by means of coolant blower coast-down that leads to reactor shutdown in a short time. Due to the loss of the xenon poisoning in the long time regime the reactor returns to criticality resulting in nuclear power production and core heat-up in order to compensate the excess reactivity released. The results demonstrate that the 1620°C fuel element temperature limit is not exceeded. The maximum fuel element temperature is calculated to be 1330°C at full system pressure and 1536°C for loss of pressure. The temperature transients in the core adjacent reactor components do not lead to excessive stresses with severe consequences. The results of the investigations point out the inherent safety potential of the HTR-Module, even under postulated accident conditions far in the hypothetical regime.