ICONE15-10776 COOLABILITY OF OXIDIZED PARTICULATE DEBRIS BED ACCUMULATED IN HORIZONTAL NARROW GAPS

Abstract

When LOCA occurs in a nuclear reactor system, the coolability of the core would be kept as reported at a series of presentations in ICONE14. Therefore the probability of the core meltdown is negligible small. However, from the view point of defense in depth, it is necessary to be sure that the coolability of the bottom of reactor pressure vessel (RPV) is maintained even if a part of the core should melt and a substantial amount of debris should be deposited on the lower plenum. We carried out an experimental study in order to observe the coolability of particulate core-metal debris bed with 12mm thickness accompanied with rapid heat generation because of oxidization, which was reported at ICONE14. The coolability was assured by a small amount of coolant supply because of high capillary force of oxidized fine particulate debris produced. In the present study, we examined the coolability of particulate debris bed deposited in narrower gap of 1mm or 5mm that coolant supply is hard. The particulate debris beds were piled up on the stainless steel sheet with 0.1mm thickness, which was used to measure the bottom temperatures of particulate debris bed by using a thermo-video camera. We set up a heat supply section with heat input of 2.1kW, which simulates the hard debris bed deposited on the particulate debris bed as reported for the TMI-2 accident. We measured the temperatures of the bottom surface of the heat supply section and the heat fluxes released into debris bed as well as the temperatures at the bottom of debris bed on the stainless steel sheet. It is found that when only the upper surface of particulate debris bed is in the film boiling, capillary force causes coolant supply to the particulate debris bed. Therefore, in the condition of thicker gap with small particulate debris, coolability of debris bed is improved. We find out that smaller particulate debris is moved by vapor movement. As a result, the area that high capillary force is caused because of smaller debris particulates and the area that vapor is easily movable because of coarse debris particulates occurred in the gap. We observed appearance and disappearance of the hotspot with metal particulate debris bed with a MMD of 0.6mm and a gap of 1mm. We found that oxidization with heat generation contributes the appearance of hotspot and after oxidization completes hotspot disappears because of high capillary force in fine particulates produced through oxidization process.