Analysis of a loss of residual heat removal system during mid-loop conditions at PKL facility using RELAP5/Mod3.3

When a nuclear power plant is in shutdown conditions for refuelling, the reactor coolant system water level is reduced. This situation is known as mid-loop operation, and the residual heat removal (RHR) system is used in this situation to remove the decay power heat generated in the reactor core.In mid-loop conditions, some accidental situations may occur with not a negligible contribution to the plant risk, and all involve the loss of the RHR system. Thus, to better understand the thermal–hydraulic processes following the loss of the RHR during shutdown, transients of this kind have been simulated using best-estimate codes in different integral test facilities. This paper focuses on the simulation, using the best estimate code RELAP5/Mod3.3, of the experiment E3.1 conducted at the PKL facility. This experiment consists of the loss of the RHR system when the plant is in mid-loop conditions for refuelling and with the primary circuit closed. In particular, in this experiment the physical phenomena to investigate are the mechanisms of heat removal in presence of nitrogen and the deboration in critical parts of the primary system.