Simulation of Transient Heat Transfer in an SCWR Fuel Assembly Test at Near-Critical Pressure

While supercritical water is a perfect coolant with excellent heat transfer, a temporary decrease of the system pressure to sub-critical conditions, either during intended transients or by accident, can easily cause a boiling crisis with significantly higher cladding temperatures of the fuel assemblies. Such situation is planned to be tested with a small fuel assembly of 4 rods, to be operated in a critical arrangement with supercritical water inside a research reactor in the Czech Republic. First out-of-pile tests of this experiment have recently been performed in the SWAMUP facility at SJTU in China. Some of the transient tests have now been simulated at KIT with a one-dimensional MATLAB code, assuming quasi-steady state flow conditions, but time dependent temperatures in the fuel rods. Heat transfer at supercritical and at near-critical conditions was modelled with a recent look-up table of Zahlan (2015), and sub-critical film boiling was modelled with the look-up table of Groenveld et al. (2003), giving the best accuracy at minimum run time of the code. Moreover, a conduction controlled rewetting process was included in the analyses, which is based on an analytical solution of Schulenberg and Raqué (2014). This fast method should be applicable later to any system code for nuclear application. The new method could well reproduce the boiling crisis during depressurization from supercritical to subcritical pressure, including rewetting of the hot zone within some minutes, as shown, but the peak temperature was somewhat under-predicted. Tests with a lower heat flux, which did not cause such phenomena, could be predicted as well. In another test with increasing pressure, however, a boiling crisis was also observed at a heat flux, which was significantly lower than the critical heat flux predicted by the CHF look-up table of Groeneveld et al. (2006). The presentation is summarizing the physical models and the numerical approach. Comparison with experimental data is used to discuss the applicability of these look-up tables for the design of supercritical water-cooled reactors.