The safety behaviors of a NPP after an external hazard-initiated event, as well as after a small break LOCA are already well known as part of the analyses done for standard licensing. In this paper, a combination of both event categories is investigated by means of the thermohydraulic system code ATHLET (analysis of the thermohydraulics of leaks and transients) developed by the Gesellschaft für Anlagen- und Reaktorsicherheit (GRS, Germany). The scenario assumes an external event with a loss of coolant accident caused by induced vibrations on a small pipe attached to the primary circuit, although all pipes are designed to withstand the loads created by such an external event. The coincidence of both events leads to a beyond-design basis consideration. Furthermore, in the context of both robustness and enveloping analyses, the unavailability of both the internal power supply via the external grid connection and the emergency diesel power supply due to the external impact is postulated. For emergency cooling, the NPP in the scenario considered has only access to the passive accumulators and to systems supplied via the safeguard emergency diesel engines (2nd quartet of emergency diesel engines) — a design feature of the Siemens KWU type PWRs —, which are housed in the bunkered emergency feed building and operate with lower voltage levels.
The scenario is exemplarily modeled and simulated with ATHLET. The manner of the external event itself is not in focus, but rather the thermohydraulic behavior of the NPP is considered in the reported analysis. Beside the model assumptions and the boundary conditions, the accident sequence is explained in detail. It turns out that the remaining systems for emergency cooling are able to handle the LOCA under such demanding boundary conditions. Most importantly, the primary pressure can be lowered below the zero flow pump head of the pool cooling pumps. The long-term cooling can thereby be ensured. Furthermore, the heat removal out of the core is always sufficient.
Overall it can be concluded that all safety protection objectives have been complied for this beyond-design basis scenario.
