Development of a design-stage PRA for the XE-100

Next generation reactor PRAs are in a unique position to influence reactor design prior to construction. Design-stage PRAs typically require more assumptions, are less detailed, and have larger uncertainties due to the conceptual nature of many plant systems and components. However, despite the large number of assumptions on system parameters, human actions, and the internal events only scope, the Xe-100 PRA has provided several risk insights that have supported design decisions. This is one of the two critical benefits for performing a designstage PRA. The other key benefit is that the PRA will support many highly engaging risk-informed activities such as Licensing Basis Event (LBE) selection, safety classification of structures, systems, and components (SSCs), and Defense-in-Depth evaluation. In addition to these primarily safety-based applications, the PRA will also support an investment risk assessment, ensuring long outages and plant write-off events will be minimized. This paper presents key features of the rapidly evolving PRA model for the Xe-100, a Pebble Bed-type High Temperature Gas Reactor (PB-HTGR) being developed by X-Energy. Consistent with the ASME/ANS PRA Standard for Advanced non-LWRs, the scope and level of detail of the PRA is consistent with the availability and level of detail of the design, operation, and siting information for the Xe-100. A review of the key event sequences relevant to PB-HTGRs, as well as the respective safety design mitigation strategies for the Xe-100 are presented. A small helium pressure boundary break is presented as an example of the initiating event selection to event sequence analysis approach.