Preliminary Analysis of the Afterheat Removal in Pebble Bed Fluoride Salt Cooled High Temperature Reactors under Accident Conditions

摘要

The pebble bed fluoride salt cooled high temperature reactor (PB-FHR) is an advanced reactor program that adopts coated particle fuel embedded within graphite sphere and molten salt coolant. It benefits from the advantages of the existing HTRs and those of the molten salt reactors (MSRs). The use of passive afterheat removal during an accident to enhance full safety is one vital element of the FHR design. The afterheat transfer mechanism in stagnant pebble bed core with molten salt plenum plays a dominate role in afterheat transport from the core to the ultimate heat sink. This complex multiple heat transfer mechanisms in the core can be described by an integrated feature parameter called effective thermal conductivity. It is similar to that of HTRs, but uses a liquid salt as coolant. It is not known whether the liquid salt is a radiation transparent material like helium, as a result, it must keep in mind that the effective thermal conductivity correlation validated by experiments in HTRs could not be directly employed to conduct FHRs' safety analysis. A PB-FHR design is now being planned for construction by the Center for Thorium Molten Salt Reactor System (TMSR) of Chinese Academy of Sciences, in Shanghai Institute of Applied Physics (SINAP), foreseeing the first criticality in 2020. In this study, much emphasis is put on the effect of the effective thermal conductivity on the afterheat removal in stagnant pebble bed core with molten salt plenum. The afterheat removal is based on the capacity of the TRISO coated fuel particles to accept temperatures up to 1600°Cwithout damage, the temperature 1430°C at which the salt coolant starts to boil, and the too high temperature resulting from heat transport from the core without exceeding the limits on the surrounding metallic structures, which is the most limiting constraint of the PB-FHR design.