Simulation of DLOFC Accident in a Generic Pebble Bed High-Temperature Gas-Cooled Reactor using MELCOR

Abstract

To support the Canadian deployment of small modular reactors (SMRs), Canadian Nuclear Laboratories (CNL) has been investigating high-temperature gas-cooled reactor (HTGR) safety- related topics in a number of projects. One of these projects led to the development of a MELCOR model of a generic 200 MW_th pebble bed HTGR, which features detailed nodalization of the reactor th pressure vessel to assist the prediction of air ingress and onset of natural circulation (ONC). This model is intended to be used in integrated severe accident analyses, with the results used to assist in understanding the reactor behavior in postulated accidents of similar grid-scale HTGR designs, and to inform emergency response and planning with estimates of the source term. This paper includes detailed descriptions of the model, and the simulation results for three depressurized loss of forced circulation (DLOFC) transients. All three cases are assumed to have a simultaneous break of the co- axial inlet and outlet pipes but differ in the availability of the negative pressure venting system (NPVS), the reactor cavity cooling system (RCCS), and the nitrogen injection system. The results highlight the role of natural circulation (after ONC) in transporting fission products, and the role of NPVS in limiting fission product release into the environment.