Risk Analysis of the Residual Heat Removal System at South Texas Project With a Special Focus on Breach of Containment

A unique feature in the design of the reactors at South Texas Project (STP) is that each unit’s Residual Heat Removal System (RHRS) is located within containment. The aim of this work is to identify the potential failure modes of the Residual Heat Removal System that could lead to a breach of containment during reactor operation and thereby may increase Core Damage Frequency (CDF). The analysis began with a Failure Modes and Effects Analysis (FMEA) of the RHRS based on a Piping and Instrumentation Diagram. The two motor operated valves that isolate the RHRS from the Reactor Coolant System (RCS) were assumed to fail with an internal leak, exposing downstream components to reactor coolant. Pathways for coolant to exit containment were identified and analyzed for severity, occurrence, and detectability of the failure modes. The analyses of these factors lead to the determination of a criticality rating, which assisted in the ultimate findings. The results of the FMEA were used to construct an event tree of the failure modes of interest and the composite probability of each failure. The highest probability failure mode of interest was a breach of containment by a tube of the heat exchanger leaking into the Component Cooling Water (CCW) with a failure probability of 2.5E−10 per reactor year. The insights gained in this analysis will be used by the South Texas Project for future risk analysis and decision-making regarding the RHRS.