Probabilistic risk assessment method development for high temperature gas-cooled reactor, 5; Accident progression analysis

Honda, Yuki; Sato, Hiroyuki  ; Ohashi, Hirofumi 

We have been conducting a source term evaluation method development for high temperature gas-cooled reactors considering structural failures in the major components. This paper present the results of transient analysis for depressurized loss-of-forced cooling accident with ruptures of the cross cut ducts and standpipe, which may be initiated by earthquake. The sequences accounts failures of mitigation systems such as core heat removal by Vessel Cooling System (VCS) and reactor shut down by control rod systems. We will show the effect of mitigation system failure to depressurized loss-of-forced cooling accident in the view point of fuel temperature and natural circulation flow rate which is important for source term evaluation. The major findings obtained in this study showed that multiple failures in mitigation systems for a representative HTGR plant do not aggravate the accident. The result demonstrated that a simplification of event sequence analysis and source term analysis can be achieved with a design fully utilizing the safety characteristics of HTGR.