Experimental study on CHF using a full scale 2-D curved test section with additives and SA508 heater for IVR-ERVC strategy

In vessel retention (IVR) by passive external reactor vessel cooling (ERVC) under severe accidents is a feasible approach to retain radioactive core melt within the reactor vessel. The effectiveness of IVR by external reactor vessel cooling strongly depends on the critical heat flux (CHF). As long as the local heat flux does not exceed the local CHF in the vessel, the lower head can be cooled sufficiently to prevent its failure. In this study, the FIRM subcooled flow boiling facility conducted by State Nuclear Power Technology Research & Development Center (SNPTRD) was built to simulate the IVR-ERVC condition for Chinese Advanced Passive 1400 MWe PWR, as well as to verify and validate the safety margin of CHF during severe accidents. To investigate the effects of the heater surface material, coolant additives and other thermal hydraulic parameters such as flow rate and subcooling on CHF, flow boiling CHF experiments using a full scale 2-D curved test section under atmospheric pressure with SA508 Gr3 carbon steel heater as well as coolant additives of trisodium phosphate (TSP, Na3PO4) and boric acid (BA, H3BO5) with different concentrations were performed. The results showed that SA508 material displayed quite different CHF behavior in comparison with other materials such as stainless steel, copper and aluminum. It showed that higher CHF value and the test heater surfaces were changed significantly. The test heater surface change was due to the corrosion of SA508, and the rate of corrosion increased with boiling time. CHF values showed a little reduction with increasing the concentrations of BA. In case of TSP, CHF values were enhanced with lower concentrations of 500 ppm and 1000 ppm due to the increased wettability of coolant, while it was reduced with 3500 ppm probably due to the instability of two phase flow and the preventing effect on SA508 corrosion. CHF values for the mixed solutions of BA and TSP showed the similarity with the case of TSP, which indicated that TSP played more important effect on CHF behavior.