Direct Containment Heating Investigations for European Pressurized Water Reactors

While the issue on Direct Containment Heating (DCH) was resolved for US reactor plants in the 1990s it was found that the consequences of DCH processes are strongly dependent on the reactor cavity configuration. Therefore, an experimental and analytical program was started in 1998 to investigate melt ejection scenarios for typical German and European reactor designs. Six experiments have been performed in a 1:18 scaled reactor geometry, characterized by a narrow pit without exit other than through the annular space between pressure vessel and cavity wall, leading either directly to the upper containment or into the pump and steam generator rooms along the flow path around the main cooling lines. The corium was modeled by an iron-alumina melt that was driven by steam, and a prototypic atmosphere in the containment was applied. Since the system pressure at core melt accidents will be low due to compulsory system depressurization, the vessel failure pressures were kept between 0.8 and 2.2 MPa in these experiments. The experiments showed the importance of the direct path from the reactor pit to the containment. Without that path, there will be a considerable ejection of melt into the pump and steam generator rooms, but almost nothing into the open space of the containment. The hydrogen production and combustion will also be lower. Consequently, the pressure increase will stay moderate and well below the design pressure of most containments. The experiments are analyzed by a CFD code with the objective to transform the experimental results to the reactor case.