NUREG/IA-0250, "Simulation of the F2.1 Experiment at PKL Facility using RELAP5/MOD3."

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 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 thermalhydraulic processes following the loss of the RHR during shutdown, transients of this kind have been simulated using best-estimate codes, comparing their results against experimental data taken from different integral test facilities. This paper focuses on the simulation, using the best estimate code RELAP5/Mod 3.3, of the experiment F2.1 conducted at the PKL facility, within the OECD/PKL project. 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 the experimental series F2.1the physical phenomena to investigate are the mechanisms of heat removal in presence of nitrogen and the deboration in critical parts of the primary system. Two experiments belonging to this experimental series have been performed. The simulations present differences in the initial plant coolant inventory and temperature in the pressurizer, F2.1RUN1 and F2.1RUN2, to asses the influence of these differences in the transient evolution.