Update of HTGR risk assessment study

The HTGR Accident Initiation and Progression Analysis (AIPA) study team has completed its Phase II. The latest volume of the study (GA-A15000) presents the Phase II risk assessment including implementation of R and D recommendations. This paper is based on material presented in that report and, hence, it updates the public literature on the status of the HTGR risk assessment study. Event trees for core heatup have been improved to contain about 20 additional initiating events, resulting in 22 separate heatup scenarios, and to assess containment damage caused by gases generated during concrete decomposition. Risk assessments, presented in the form of complementary cumulative distribution function of accident frequency versus health effects dose and health effects tables at meaningful frequency levels (for example, greater than 10-7 year-1), continue to show the strong effect of inherent safety features and a high degree of relative safety