Investigation into the effects of a coastdown flow on the characteristics of early stage cooling of the reactor pool in KALIMER-600

A numerical investigation into the effect of a coastdown flow on the early stage cooling of the reactor pool in Korea Advanced Liquid Metal Reactor (KALIMER)-600 during a loss of normal heat sink accident has been carried out. Based on the design values of KALIMER-600, thermal-hydraulic calculations for steady and transient states have been done using the COMMIX-1AR/P code. Coastdown flow effect was evaluated based on a transient analysis of reactors employing various flywheels, which had coastdown flow time (CDT) values ranging from 0 (without a flywheel) to 300 s. The transient analysis has been done from a reactor trip to the onset of an overflow into the DHX support barrel. It was found that the coastdown flow range could be divided into three zones, based on its effect. Among them an excessive core coolant peak temperature and a reversed flow at the core region were observed for a medium coastdown flow range. The medium ranged coastdown flow induces the development of a high density layer near the core exit. This layer contributes to the development of an adverse effect in the core coolant flow, and finally results in increasing the core peak temperature. It was also found that the initiation of heat removal by DHX could be accelerated by the increase of the CDT, although it needs a large flywheel. From this analysis the best CDT is determined to be 25 s.