Experimental Investigation of Melt Coolability Under Bottom Injection: Effects of Melt Volume, Melt Composition, Nozzle Diameter, and Inlet Pressure

Abstract

Melt pool coolability is one of the concerns when addressing severe accident scenarios. Long-term cooling and stabilizing of highly radioactive and reactive molten corium are still issues to be addressed and understood in order to devise a successful handling strategy. Core catchers have been envisaged in present and future advanced reactors to manage this concern. In the ex-vessel core catcher scenario, corium relocates to form a molten pool. Injecting water from the bottom of this melt pool to achieve coolability is found to be an efficient technique so far. However, the numbers of tests with prototypic materials and conditions are very limited and difficult to perform. In view of this, most of the earlier studies have been conducted with simulated melts. There are concerns with regard to scalability of experiments, effects of melt composition, and geometric parameters on melt coolability. To address these issues, series of experiments have been conducted and are presented in this paper. The experiments are performed with borosilicate glass, with two melt volumes, i.e., 3 and 20 L, and are compared. They show that the melt quenching time was more or less the same and suggest that the results can be extrapolated to higher scales. Two different simulants were used in the experiments, i.e., sodium borosilicate glass and CaO-B₂O₃, to study the effect of melt composition, and it was observed that the coolability behavior remains the same but the melt quenching time varies. The effect of nozzle diameters was studied by conducting experiments with three different nozzle diameters of 8, 12, and 18 mm keeping the same inlet pressure. It was found that the quenching time was higher for the 8-mm-diameter nozzle experiment due to smaller flow rates compared to the others. The experiments were repeated at two inlet water pressures of 0.35 and 0.75 bar(g) for the same nozzle diameter to study their effects on melt coolability. As expected, the quenching time was found to be less for the case of higher inlet pressure. The experimental measurements suggest that the overall average melt pool coolability behavior under bottom flooding was almost the same in all cases. However, each physical parameter affects the melt quenching time required to cool the melt. This average melt quenching time can be optimized using suitable combinations of geometric and physical parameters. The debris sizes and porosities formed during the melt eruption were also measured. The measured porosities ranged between 50% and 60% in all experiments.

Keywords:

• Severe accidents
• melt coolability
• quenching time