The CMMR Program: BWR Core Degradation in the CMMR-1 and the CMMR-2 Tests

For decommissioning the Fukushima Dai-ichi nuclear power plant (1F), understanding the final distribution of core materials and their characteristics is important. These characteristics obviously depend on the accident progression in each of the units. However, a large uncertainty is present in understanding of the accident progression behavior. For the boiling water reactor (BWR) design in particular, because the existing experimental data base is quite limited, various scenarios are possible for the core-material relocation (CMR), making it difficult to single out a reference scenario. This uncertainty, which was clarified by the MAAP-MELCOR Crosswalk, cannot be resolved with existing experimental data and knowledge. Once coolant is lost from the BWR core for some time, the following scenario can be divided symbolically into 'TMI-2 Like Path' and 'Continuous Drainage Path'. In addition to the precise boundary conditions such as water level and pressure, the actual scenario depends on uncertainties in BWR-specific CMR. Main uncertainties for this branching point are summarized as two questions: How is gas permeability of high-temperature degraded core approaching fuel melting? (Q1) How is downward relocation of hot core materials before fuel melting and its effect on structure heating? (Q2) To address these questions, the core-material melting and relocation (CMMR) experimental program using non-transfer-type plasma heating has been implemented by the JAEA (a member organization of IRID). The CMMR experimental program aims to provide experimental data that address questions Q1 and Q2, which will help us to comprehend core degradation in BWRs like those in 1F. Based on preparatory tests, two tests (CMMR-1 and 2) were conducted using a bundle-scale test piece comprising 48 simulated fuel rods (ZrO2 pellets clad in Zr alloy), a control blade (B4C particles contained in a stainless steel tube and a sheath) sandwiched by channel box walls (Zr alloy) and lower support structures. The simulated fuel assemblies of the CMMR-1 and 2 tests were 500-mm long. The test conditions simulated the oxygen concentration before the core in Unit 2 of 1F slumped (relocated to lower plenum). The CMMR-1 test adopted a 40-min heating period, whereas the heating period was reduced to 25 min in the CMMR-2 test to study the effects of a difference in the axial temperature distribution. X-ray computed tomography was used to analyze the overall material distribution in the test pieces after CMR. Melting and relocation of metals (from the control blade, channel box, and cladding) were widely observed inside the test pieces. There were selective draining paths near the control blade. The fuel columns remained standing even when exposed to an estimated peak temperature near 2500 Degree of C. These results suggest that until the ceramic fuel melting, the core remains permeable for gas and the fuel keeps column geometry as far as strong force is not encountered.(author).