Some of the advanced nuclear reactors employ an ex-vessel core catcher to mitigate core melt scenarios by stabilizing and cooling the corium for a prolonged period by strategically flooding it. The side indirect cooling with top flooding strategy described in this study may lead to water ingression through the melt crust, which may lead to interaction between unoxidized metal and water, leading to hydrogen production. In order to avoid this deleterious scenario, water ingression into the interior region of the melt should be avoided. The studies described in this paper show that water ingression depends on the flooding strategy, i.e., the time delay between top flooding and melt relocation. Two experiments under identical conditions of simulant temperature, melt material, and test section geometry were conducted with simulated decay heat of 1 MW/m3. Sodium borosilicate glass was used as the corium simulant. In the first experiment, water was flooded onto the top of melt pool soon after melt relocation. In the second experiment, water flooding at the top of melt pool was made after 30 min of the melt relocation. The results show that a finite time delay of introduction of water onto the top of the melt pool is paramount to engender the development of a stable crust around the melt and therefore eliminating water ingression into melt pool and ensuring controlled coolability of the melt.

An ex-vessel core catcher is generally used in advanced reactors to mitigate core melt scenarios by stabilizing and cooling the corium for prolonged period by strategically flooding it. The side indirect cooling along with delayed top flooding of water ensures that the water interacts with the oxidic components only after melt inversion. However, water ingression either through the top of melt pool or through the crust-vessel gap may lead to unoxidized metal-water interaction in the melt leading to hydrogen production. To avoid this deleterious scenario, water ingression into the bulk of the melt is to be avoided. In this study, a series of experiments using a scaled down core catcher has been conducted to study the phenomena of melt coolability and water ingression by varying the bottom vessel angle of the core catcher. Three different angles of the bottom plate were considered: 10, 20, and 30 deg. The melt used was a corium simulant in the form of CaO-B2O3.The transient temperature history of melt pool, inside and outside vessel surface temperatures along with the post-test evaluation of the test section reveals that the bottom angle has an effect on water ingression and the resultant melt eruption at different locations. The tests conducted showed that the scaled down core catcher with 10-deg inclination of the bottom plate does not exhibit water ingression, whereas the 20- and 30-deg angle-scaled down core catchers showed water ingression and subsequent melt eruption.

To minimize the potential risk of design extension conditions (DEC) with core meltdown, some advanced reactors employ ex-vessel core catchers which stabilize and cool the corium for prolonged period by strategically flooding it. This paper describes the coolability of the melt pool and ablation process in a scaled down ex-vessel core catcher employing sacrificial material which reduces the specific volumetric heat, temperature, and density of the melt pool. To understand these phenomena, a simulated experiment was carried out. The experiment was performed by melting about 500 kg of corium simulant using thermite reaction at about 2500 °C. The bricks of oxidic sacrificial material were arranged in the core catcher vessel which was surrounded by a tank filled with water up to a certain level. After the time required for melt inversion, water was introduced to flood the test section from the top. The melt pool temperatures were monitored at various locations using “K” and “C” type thermocouples to obtain ablation depth at different elevations with time. The results show that the coolability of the molten pool in the presence of water for the present geometry is achievable with outside vessel temperatures not exceeding 100 °C. A ceramic stable crust was observed at the top surface of the melt pool, which prevented water ingression into the molten corium. The ablation rate was found to be maximum at the lower corners of the brick arrangement with the maximum value being 0.75 mm/s. An average rate of about 0.18 mm/s was obtained in the brick matrix.