MELCOR integrated severe accident code application to safety assessment of high-temperature gas-cooled reactors

MELCOR is an integrated thermal hydraulics, accident progression and source term code for reactor safety analysis that has been developed at Sandia National Laboratories for the United States Nuclear Regulatory Commission (U.S. NRC) since the early 1980 s. Though MELCOR originated as a light water reactor (LWR) code, development and modernization efforts over the past decades have expanded its application space to non-LWR reactor concepts. Current MELCOR development efforts have been focused on providing the U.S. NRC with the analytical capabilities to support regulatory readiness for licensing non-LWR technologies under Strategy 2 of the NRC's near-term Implementation Action Plans. Beginning with the Next Generation Nuclear Project (NGNP), MELCOR has undergone a range of enhancements to provide analytical capabilities for modeling the spectrum of advanced non-LWR concepts. In this paper, we describe the generic plant model developed to demonstrate MELCOR capabilities to perform high-temperature gas reactor (HTGR) safety evaluations. This model represents a TRi-structural ISOtropic particle fuel (TRISO) pebble bed HTGR with a primary system rejecting heat to a recuperative heat exchanger. Surrounding the reactor vessel is a reactor cavity contained within a confinement volume cooled by the Reactor Cavity Cooling System (RCCS). A range of demonstration calculations are performed to evaluate the plant response and MELCOR capabilities to characterize a range of accident conditions. The accidents selected for evaluation consider a range of degraded and failed modes of operation for key safety functions providing reactivity control, primary system heat removal and reactor vessel decay heat removal, and confinement cooling.