Design of passive decay heat removal system for the lead cooled flexible conversion ratio fast reactor

Abstract

The lead-cooled flexible conversion ratio fast reactor shows many benefits over other fast-reactor designs; however, the higher power rating and denser primary coolant present difficulties for the design of a passive decay heat removal system. In order to achieve passive cooling, enhancements are needed over current designs, such as the S-PRISM and ABR, which utilize passive cooling through the reactor vessel to atmospheric air. Enhancements such as axial fins, a perforated plate, and round indentations, or dimples, were considered as additions to the hot air riser to increase heat transfer. Other enhancements include a liquid metal bond between the reactor and guard vessels, and a dual-level design which introduces ambient temperature air halfway up the vessel wall. A code was written in Java to simulate these conditions, leading to a promising case using dimples on the guard vessel wall as the primary mode of heat transfer enhancement, and including the dual-level design. A conservative estimate of dimple performance indicates that during a passive decay heat removal shutdown, bulk primary coolant temperature will peak at 713 'C, giving a 12 OC margin to clad failure. Attempts were made to refine the uncertainty within the calculations using a computational fluid dynamics code, Fluent, but these ultimately were unsuccessful. Additional studies were conducted on the static stress imparted on the vessel, and the dynamic stress caused by a seismic event. The static stress was found to be within ASME code limits. Seismic analysis determined that a seismic isolation scheme would be necessary in order to prevent damage to the vessel during an earthquake.