Flow blockage accident analysis in a multi purposes research reactor using Relap5 system code

Abstract

Simulation analysis using computer codes like Relap5 aids in understanding the thermal-hydraulic phenomena that occur in a nuclear installation under normal and accidental conditions. The main goal of this work is to model and simulate the overall behavior of a multi purposes research reactor during an unprotected (without scram) partial blockage transient in a single fuel assembly using the Relap5 system code. The transient is performed under several obstruction ratios of the nominal flow area, i.e. 0 %, 20 %, 40 %, 60 %, 80 % and 90 % of the hot channel. A Relap5 model was created to simulate the reactor's core, and it was rigorously validated against steady-state reactor data as well as available operating data for a normal transient such as start-up at the same power level. Good agreement was obtained between the simulation results and the commissioning data. Certain parameters were selected to demonstrate the steady-state establishment and, depending on the evolution of these parameters, the steady-state is well established after ∼200 s. The transient results were carefully analyzed to examine the evolution of the core's main thermal-hydraulic parameters. The results analysis indicates that blocking the hot channel causes an increase in temperatures due to the decrease in mass flow rate, which reduces the cooling capacity. In addition, the reactor core achieves a new stationary-state condition after ∼20 s with new values of the thermal-hydraulic parameters for partial blockages (20 %–90 %), where removal of the residual heat is maintained. Furthermore, we can conclude that the accident consequences are highly dependent on the blockage percentage; all parameters were maintained at moderate values and remained well within their safety limits. Until a blockage rate of approximately 90%, no boiling is occurred. In this situation, the reactor is considered safe.