Analysis of the Metal Containment Vessel Wall Thickness Effect on the Coolability of the Flooding Safety System using the MELCOR Code

Abstract

To remove decay heat during an accident, flooding safety system (FSS), a passive safety system (PSS), was designed for small modular reactors (SMRs) employing a metal containment vessel (MCV). The FSS provides emergency coolant and submerges the reactor modules (RMs) installed in the reactor module cavity during an early phase of the accident. The sensible heat from primary coolant during the blowdown phase and decay heat from the core during the boiloff phase are removed by the heat capacity of the MCV wall before the sufficient flooding. Thus, the thickness of the MCV wall was expected to affect accident sequences. Thus, in this study, the effects of the wall thickness of the MCV on the decay heat removal and accident sequences were investigated through numerical analysis using MELCOR code. A natural circulation SMR whose thermal power is 330 MWt and a loss of coolant accident were selected as the reference reactor and the postulated accident. The effect of the variables such as wall thickness and delay time for cavity flooding were evaluated. The thickness and the delay time ranged from 5 cm to 10 cm and from 0 minute to 75 minutes, respectively. As the results, the thicker MCV wall absorbed more heat during time before flooding, but the MCV pressure after flooding was higher due to the higher MCV inner surface temperature. Moreover, the thicker MCV expedited the achievement of recirculation due to the smaller pressure difference between the reactor pressure vessel (RPV) and the MCV.