Qualification of multiphase CFD for nuclear reactor safety – status and perspective

The thermal-hydraulic part of safety analyses for design based accidents mainly relies on 1D system codes. This enables a safe design and operation of existing nuclear reactors. However due to the 3D characteristics of fluid flows some additional conservatism is required. Especially for large vessels, as the Reactor Pressure Vessel, Computational Fluid Dynamics (CFD) may provide more realistic information on relevant flow and heat transfer phenomena. Single phase CFD is widely used for industrial applications, e.g. in automotive and aviation industries. However CFD is not mature for two-phase flows, which may be relevant e.g. in Loss Of Coolant Accident scenarios leading to steam-water flows in the primary circuit of a Pressurized Water Reactor. This paper discusses the present shortcomings and strategies to improve the situation.