Root Cause of Instability in Liquid Zone Control System of CANDU Reactor

When reactivity insertion such as refueling occurs in Canada deuterium uranium (CANDU) reactors, the power and the water level are tilted in the upper outer zone of the liquid zone control system (LZCS) and fluctuate unstably for a certain period of time (1 to 5 days). Such instability is observed in most of the CANDU reactors in service around the world, but neither its root cause has been identified nor have solutions against it been established. Therefore, this study experimentally and analytically attempted to prove that the root cause lies in the holdup of light water on the top of the tube support plate (TSP) due to the mismatch between the net volumetric flow rate of light water and helium crossing the narrowed porous TSP installed within the LZCS compartment by performing hydrodynamic simulation of the inflow/outflow of light water and helium. Two solutions against the aforementioned instability of LZCS were suggested. One is to regulate volumes of helium gas flowing into the compartment and light water flowing therefrom, and the other is to enlarge the flowing paths of helium and light water within the TSP. The former may be applicable to nuclear reactors in service and the latter to those planned to be constructed.