Abstract
About the boiling transition (BT) that determines the maximum thermal output of the BWR, it is considered that the spacers have significant effects on the occurrence of the BT. The occurrence conditions of the BT can be changed by devising the spacer shapes because it will affect to entrainment and deposition behaviors of droplets. In the light water cooled fast reactor: RBWR, thermal-hydraulics conditions are more challenging than in the current BWR. Then, the effect of the spacer on the BT should be sufficiently utilized in the RBWR. In the thermal-hydraulics design for the current BWR, large-scale tests were carried out and used to evaluate BT conditions. The RBWR is still in the design stage, and there is room to be changed many parameters. Then, it is not reasonable to determine the shape of the spacer only by large-scale tests but also by considering local effects on droplet entrainment and deposition.
On the other hand, by applying a two-phase CFD method with remarkable development in recent years, we can develop a model that can predict the effect of the spacers mechanistically. This research used the detailed two-phase flow simulation code TPFIT developed by JAEA to simulate annular dispersed flow in RBWR subchannels. In the occurrence of the BT, it is considered that the two-phase flow pattern is the annular dispersed flow, and we want to evaluate the effects of the spacer on annular dispersed flow in the RBWR subchannels. We performed numerical simulations of annular dispersed flow in the simplified subchannel of the RBWR. As a simulation parameter, we choose the existence of the spacer. The spacer in the simulation has a simplified shape and the same blockage ratio as the RBWR. In addition, we analyzed numerical simulation data and identified each droplet's occurrence and disappearance points. Based on these data, we evaluate the numbers of entrainment and deposition distribution in and around the spacer.
