Analytical verification of the cool-down procedure in the WWER-440/213 reactor vessel head under natural circulation

The paper shows some results obtained from the application of the RELAP5/MOD3 code to the analysis of the procedure of cooling down the WWER-440/213 reactor vessel head under natural circulation. The calculations were performed according to the specification supplied by Mochovce NPP. Two scenarios, with and without creation of a steam bubble in the reactor pressure vessel head are considered. The transient starts when all of the 6 reactor coolant pumps are tripped. In the analyses loss of off-site power is not assumed. The phenomena which are analyzed in more detail are: the cool-down trend of the water volume in the reactor pressure vessel (RPV) head, formation/condensation of a bubble in the RPV head, action to reduce or condense s steam bubble in RPV head including repressuring the primary system, opening RPV head venting line and restarting main circulating pumps (MCPs).(author)