HTR Risk Metrics: Are CDF and LERF Appropriate?

The risk metrics of core damage frequency (CDF) and large early release frequency (LERF) have been extensively used in risk informed applications of probabilistic risk assessments (PRAs) for currently licensed and advanced light water reactors. These definitions of the reactor plant state of core damage are based on light water reactor characteristics such as liquid coolant levels in the reactor vessel, coolant outlet temperatures indicative of inadequate core cooling, and achievement of core temperatures at which damage to the Zircalloy clad fuel elements and fuel melting are expected to occur. As PRAs are being developed to support the design and licensing of High Temperature gas-cooled Reactors (HTRs), an issue has been raised about whether CDF and LERF are appropriate risk metrics for such types of reactors. The purpose of this paper is to present the author's point of view on this and related technical issues in the design and licensing of HTRs, such as the ANTARES HTR. That point of view is that CDF and LERF are not appropriate risk metrics for any HTR. Alternative and superior risk metrics are available to support HTR PRAs. (author)