Comparison of margins of sodium bulk boiling and reactor vessel creep under RVACs operation by micro-integral effect test

Abstract

Sodium boiling and creep failure of the RV are important safety criteria for severe accident in SFR. Unlike other decay heat removal systems, RVACS employs RV in the thermal circuit to the ultimate heat sink, air. Because the RV is in the second hottest position in the thermal circuit of the RVACS, the safety margin for the creep temperature of the RV and the safety margin for sodium boiling can be comparable. To compare the margin between the RV creep and sodium boiling and focus on the more important one, temperature between the hot spot in the sodium pool and RV was experimentally studied using simulant. Water was selected as simulant based on the similarity law with modified Boussinesq number. Experimental facility SINCRO-2D was designed for integral boundary effect test, as 2-d slab model, which was scaled-down to 1/25. Considering the heat removal capacity of the RVACS, test was conducted at the 0.2, 0.4, 0.6, 0.8, and 1.0 % of the total power. Integrated boundary was given as temperature gradient along the RV wall at the corresponding power. For the bulk sodium temperature, all the cases showed smaller temperature difference between hot spot and RV wall, than temperature difference between RV creep criterion and sodium boiling, except for 1.0%. It indicates that RV creep failure has smaller margin than bulk sodium boiling.