Seismic classification of high temperature engineering test reactor

•A new seismic classification of high temperature gas cooled reactors was proposed.•The reactor cooling behavior and exposure evaluations confirmed the validity of the new seismic classification.•The new classification was approved by the Nuclear Regulatory Authority in Japan. In the late 1980s, the seismic classification of the high temperature engineering test reactor (HTTR) was formulated during its design stage. Because of the HTTR’s lack of operation experiences to sufficiently understand the safety characteristics of high temperature gas cooled reactors (HTGRs) at that time, the seismic classification of commercial light water reactors (LWRs) was applied to HTTR. However, subsequent operation experiences and test results using HTTR made it clear that the seismic classification of commercial LWRs was somewhat too conservative for the HTGRs. The seismic classification of HTTR was reevaluated using the following approach. Class S facilities with high safety requirements ensured that they maintained their function even when the safety functions of Class B and Class C facilities were lost during earthquake-triggered accidents, thereby safely controlling the accident without exposing the members of the public in the vicinity of the reactor to excessive radiation exposure. Consequently, Class S facilities were downgraded compared with the commercial LWRs. Moreover, an evaluation was conducted to verify the validity of the new seismic classification under the conditions where the functions of Class B and Class C facilities are lost. It was confirmed that the reactor was stable and that the evaluated public dose was below the authorized limit. In June 2020, the Nuclear Regulatory Authority in Japan confirmed that the results of the seismic classification conformed to the standard rules of the reactor installation change.