Identifying and quantifying a complete set of full-power initiating events during early design stages of high-temperature gas-cooled reactors

Identifying initiating events is the first step in building a probabilistic risk assessment model. The history of commercial nuclear power plant is primarily light-water reactors, hence, a comprehensive set of possible initiating events that covers most available light-water reactors is identified allowing for a frequentist approach to estimating their associated frequencies. However, other technologies of nuclear power plants do not share the same operating experience, hence, there are no comprehensive lists of initiating events for non-light-water reactors. This paper presents an approach that utilizes top-down deductive methodologies, master logic diagrams, and heat balance fault trees; the bottom-up inductive methodology of failure modes and effects analysis; and legacy and contemporary information to identify a complete list of initiating events for the Xe-100 high-temperature gas-cooled reactor (HTGR). Moreover, the paper presents the approach on how to estimate the frequencies using legacy sources along with deterministic analysis and fault trees. Furthermore, the paper presents an approach to estimate uncertainty parameters associated with each of these initiating events using constrained non-informative distributions to account for lack of operating experience. Finally, the paper presents the list of identified initiating events for the Xe-100 along with their frequencies and uncertainty parameters which serves, along with other contemporary sources, as another building block in having a comprehensive set of initiating events for HTGRs.