CHF of a Downward Curved SA508 Surface with Boric Acid and TSP

In this study, CHF experiments were conducted in the two-dimensional slice test sections to clarify the combined effects of the heater material of SA508 and additives of boric acid and TSP. In particular, the top of the lower head is the main focus due to the importance of the focusing effect of decay heat through metal layer after corium relocation during severe accident. Through this study, flow boiling CHF experiments were conducted using two types of test sections to investigate the combined effect of heater material and additives. For the case of different coolant additives, the CHF using boric acid was higher than that when using DI water, while the CHF was reduced by a TSP solution. The CHF tendencies were attributed to differences in steel oxidation according to the characteristics of the working fluid. The CHF enhancement and reduction mechanisms for the combined effects of heater material and additives were explained as aspects of carbon steel oxidation according to the pH of the working fluids. Under the boric acid conditions, the effect of steel oxidation and the consequent CHF enhancement were shown to be minimized in terms of local condition. For severe accident mitigation, a number of nuclear power plants use the in-vessel retention through external reactor vessel cooling (IVR-ERVC) strategy which removes the decay heat of the molten corium. The critical heat flux (CHF) is one of the most important criteria by which to judge the success of the IVR-ERVC strategy. The CHF is affected by the properties of cooling water and the conditions of heated surface. In APR1400, the cooling water of the IRWST, which contains boric acid (H3BO3), is injected into the reactor cavity by an external reactor vessel cooling system (ERVC, an active feature) and a cavity flooding system (CFS, a passive feature) to manage severe accidents. As the CFS begins to operate, IRWST water flows through the hold-up volume tank (HVT), which contains tri-sodium phosphate (TSP, Na3PO4.12H2O), and the TSP is dissolved into the cooling water. The reactor vessel is made of SA508 (low alloy carbon steel)