Safety Assessment of a Passive Decay Heat Removal System Using a Model-Based Systems Engineering Approach

An innovative gas-cooled fast reactor concept being investigated in the United States is the General Atomics Fast Modular Reactor (FMR). The FMR includes a reactor vessel cooling system (RVCS) that is designed to remove decay heat from the reactor pressure vessel via natural circulation in the event that primary cooling is unavailable. Because of its potential role in maintaining the reactor within design limit temperatures, a series of complementary safety assessments were performed to elicit potential failure modes of the system; these included a component failure modes and effects analysis (FMEA), a functional FMEA (FFMEA), and development of a master logic diagram (MLD). The assessments were performed on an analog of the RVCS with design insights derived from the reactor cavity cooling system of the steam cycle–high temperature gas-cooled reactor concept. A model-based systems engineering approach was used to conduct the safety assessments on the analog RVCS via development of a SysML model that captured design features, interfaces, and primary system functions to inform the results of the assessments. Using the SysML model, potential component-level failures and functional failure modes were identified from the component FMEA and FFMEA, respectively. The SysML-facilitated MLD provided complementary results to the FFMEA. The results of the assessments indicate that the RVCS performance is dependent on system parameters that influence natural driving heads. The risk-significance of individual failure modes on RVCS performance can be further evaluated by probabilistic approaches in the future.