VVER-440/V213 long-term containment pressurization during severe accident

Abstract The contribution deals with the performance of a new containment heat removal system that is intended to be installed in near future at Dukovany NPP in Czech Republic. This NPP is equipped with 4 units of VVER-440/V213 reactors. During the last two decades these units were systematically backfitted with new safety systems in order to cope with consequences of severe accidents. These safety systems were designed especially for the 4th level of defence in depth and thus they are – as much as possible – independent from the original plant systems. Namely, installation of a new depressurisation line is currently under preparation in order to avoid high-pressure core melting scenarios. In-vessel corium retention via external reactor vessel cooling (“reactor cavity flooding”) was adopted as a cornerstone strategy of severe accident management for all VVER-440/V213 reactors operated in central Europe. Regarding the hydrogen issue, large capacity passive autocatalytic recombiners were installed in containment. The prevention of containment pressurisation in the case of loss of ultimate heat sink represents the most important point of remaining safety issues. At first, special features of VVER-440/V213 containment equipped with bubbler condenser pressure suppression system are briefly described in presented paper. Than the mechanism of containment pressurisation due to steaming as a consequence of in-vessel retention and possible solutions for heat removal from the containment are discussed. The final design chosen at Dukovany NPP is based on the heat exchanger and turbo pump located inside the containment that are cooled and driven by external cooling water. External cooling circuit will be connected during the accident via special inlet/outlet nozzles installed at containment boundary. Main advantage of such containment cooling system is full independence on other plant systems (including essential service water), the fact that radioactive coolant is not taken beyond the containment boundary and all active components of the system (mobile pump, isolation valves) are located outside the containment and, thus, are easily accessible. The performance of this containment heat removal system during severe accidents were analysed using the ASTEC code and the most important results are presented. From the obtained results it follows that there is sufficient time margin for assembling the external cooling loop. Thus, the containment cooling can be assured long before the containment ultimate pressure is reached or before the coolant level in containment sump drops below critical level, when the external reactor cooling can be lost. After start of the heat removal system, the system is capable to prevent further containment pressurisation, increase the coolant level in containment sump and decrease the pressure to atmospheric or even sub-atmospheric level.