Numerical investigation on the formation of focusing effect in the IVR strategy

The "focusing effect" is the main challenging issue to the success of the IVR strategy, since the heat flux to the RPV wall could be significantly larger in the thin metal layer region than that in the oxide layer region. This paper numerically investigates the formation of focusing effect using validated CFD approach. The influences of the top cooling condition, layer height and material properties on the formation of focusing effect are investigated. Results indicate that, enhancing the top cooling mitigates the focusing effect. For the insufficiently-cooled top radiation situation, reducing the pool height significantly increases the focusing effect. For the sufficientlycooled top surface (e.g., with top water cooling), the focusing effect is not formed for all the cases regardless of the pool height. It demonstrates/supports the benefit of adding in-vessel flooding to IVR strategy as a supplementary measurement. It means that once the in-vessel flooding could be established in engineering to allow for a sufficient top cooling, the focusing effect would not likely be formed regardless of the pool height. It also confirms enhancing top cooling condition an efficient way to reduce focusing effect. As two main influential material properties, the effects of thermal conductivity and viscosity are also investigated. Either decreasing the thermal conductivity or increasing the viscosity (e.g., by addition of other materials) may reduce the focusing effect. Since IVR is a widely adopted severe accident mitigation strategy, this study could provide some insights in the formation of focusing effect and help inspiring or supporting possible new engineering features for a safety IVR design.