Safety analysis of reactivity initiated accidents in a HPLWR reactor by the coupled ATHLET-KIKO3D code

The dynamic core behavior of the HPLWR (High Performance Light Water Reactor) concept (Schulenberg, 2006), which is the European version of the GEN4 Supercritical Water Cooled Reactors (SCWR) was investigated in case of RIA (Reactivity Initiated Accidents) events. Before the analyses, the heat transfer correlations and property table of the supercritical water, built into the ATHLET code, were verified by using the Kirillov single channel thermal hydraulic measurements. According to the comparison of the calculations and the measurements, the Watt's correlation proved to be an appropriate one for average HPLWR conditions. The presented results of the performed analyses are reporting on significant perturbations of the power distribution. Furthermore, in comparison to PWRs, a more effective moderator feedback was obtained. The acceptance criteria are fulfilled, however, in spite of the relatively strong feedback, in some cases the hot channel temperatures are not far from the limits, which points out the importance of the RIA analyses. The observations can be explained by the sensitivity of the neutronic characteristics on the moderator density, changing in a wide range and, as a consequence, the significant perturbation of power peaking factors during the transients resulting in some cases larger local power increase than that originating from the average reactor power rise.