Modelling of supercritical conditions in the safety analysis codes

Thermal hydraulic modelling is needed for describing processes that use water for cooling. Simulator codes solve conservation equations of coolant mass, momentum and energy using numerical methods. Additionally, correlation based models are needed for describing, among others, heat transfer and friction between the structures and the coolant. In two-phase flow the correlations for interfacial heat transfer and friction are used for exchanging of mass, momentum and energy between liquid and gas phases. Typical applications are various boiler processes and nuclear reactors. Some modern boilers operate in supercritical water conditions as well as a new nuclear reactor concept SCWR (Super Critical Water Cooled Reactor). Water properties change fast around the critical point (220.64 bar, 374.15 C) and heat transfer in supercritical state is not fully understood, which makes modelling challenging and is typically beyond normal operation regime of the analysis codes. At the moment VTT is modifying two of its code packages, APROS and TRAB-SMABRE to adapt for supercritical conditions. APROS is a general purpose simulation code both for nuclear and non-nuclear applications, while TRAB-SMABRE is used solely for nuclear reactor analysis. The models for water properties, such as density, specific heat capacity and heat conductivity, have already been developed. The homogeneous water model of APROS (3 conservation equations) has already been tested in supercritical boiler conditions and operates in principle. The more sophisticated operation mode of APROS (6 conservation equations) operates in supercritical state but the transition from supercritical to subcritical two-phase state has not yet been tested. The correlation package of heat transfer and friction has to be tested and possibly modified in supercritical conditions. The testing of the TRAB-SMABRE code (5 conservation equations) is to be started soon. TRAB-SMABRE and possibly also APROS are to be applied for reactivity transient analysis of the European development version of the supercritical water reactor in the Euratom 6th Framework Programme project High Performance Light Water Reactor, HPLWR Phase 2.