Pool boiling heat transfer of downward-facing oxidized metal heaters in atmospheric saturated water

Reactor Pressure Vessel (RPV) is designed for lifetime use in the Nuclear Power Plant (NPP) and it is subject to experiencing natural corrosion. As a consequence, unless special treatment is employed, the outer surface of the RPV is likely to be oxidized constantly during reactor operation. The oxidized surface may affect boiling heat transfer during the execution of In-Vessel Retention through External Reactor Vessel Cooling (IVR-ERVC) strategy when severe accident occurs. Especially, Critical Heat Flux (CHF) is a key variable assessing the thermal margin of RPV outer surface and this could be affected as the surface changes significantly from its original surface finish. Therefore, in order to evaluate the thermal margin and performance of the RPV outer surface, pool boiling heat transfer experiment was performed for downward-facing oxidized heater surface in atmospheric saturated water. Heater specimens made of stainless steel grade 316 (SS316) were prepared and oxidized at several distinctive periods. In general, it was observed that oxidized surface became more wettable and rougher than the bare surface. However, no systematic trend was observed during the oxidation period. Short-term oxidized surface showed locally pitted. On the other hand, long-term oxidized surface became fully flat with specific oxide layer thickness. CHF experiment results showed that, CHF of the short-term oxidized surface was similar to the bare surface although the good wettability was measured with the oxidized surface. On the contrast, CHF of the long-term oxidized surface was enhanced significantly, which may be attributed to the formation of the continuous porous channel. In the non-porous surface, the effect of wettability and roughness on the CHF was not appeared. As a result, it was confirmed that long-term natural corrosion of the RPV outer vessel was helpful for conducting the IVR-ERVC strategy. (author)