Safety evaluation of the HTTR

Abstract JAERI conducted the safety evaluation of the HTTR considering various characteristics of the HTGR in order to confirm the adequacy of safety in all operational states. In the depressurization accident caused by a coaxial double primary pipe rupture, air ingress from the ruptured pipe oxidizes graphite blocks and fuel in the core. It is a unique accident only happened in HTGRs. JAERI established an evaluation procedure and developed analytical tools to evaluate the transient behavior during the depressurization accident, especially, oxidation of the core and fission product release into the environment. In a regulatory review, the safety evaluation procedure and results of the depressurization accident were thoroughly discussed including the necessity of the containment vessel to prevent air ingress. The regulatory authorized that reactor safety was ensured in all off-normal states including the depressurization accident. The evaluation procedure and evaluation tools will be applicable to the safety evaluation of future Japanese HTGRs. This paper describes the procedure and results of the safety evaluation especially focusing on the depressurization accident together with brief description of their analytical tools. Also, it presents topics in the regulatory review and R&D needs for the safety evaluation of future HTGRs.