Sensitivity of SC-HTGR Conduction Cooldown to Reactor Cavity Cooling System Failure

The inherent passive heat removal characteristics of modular High Temperature Gas-Cooled Reactors (HTGRs) are well known. Modular HTGRs use a combination of coated-particle fuel, ceramic core materials, core geometry, and power level to maintain acceptable fuel temperatures for all credible operating and accident conditions. Heat from the reactor vessel is radiated to a passive reactor cavity cooling system (RCCS), which removes excess heat from the reactor cavity. The RCCS for Framatome’s Steam Cycle–High Temperature Gas-Cooled Reactor (SC-HTGR) is a highly reliable, redundant system. Similar to most other modular HTGR concepts, RCCS failure is not considered credible for any accident scenario. Nonetheless, reactor module performance with a compromised RCCS is still of interest. Evaluation of such beyond-design-basis scenarios supports safety assessment of extremely low probability beyond-design-basis events (BDBEs) as well as the development of RCCS design requirements and plant emergency procedures. This study evaluates the performance of the SC-HTGR during a long-term depressurized loss of forced circulation event without RCCS operation. Boundary conditions are varied to determine their effect on reactor temperatures. Safety and investment risk considerations are addressed. The results of this study indicate that the safety impact is modest since fuel temperatures remain within their limits. However, the investment risk is more significant since vessel temperatures could significantly exceed design limits for these hypothetical BDBEs.