Validation of analysis models on relocation behavior of molten core materials in sodium-cooled fast reactors based on the melt discharge experiment

In order to improve the safety of nuclear power plants, it is necessary to make sure measures against their severe accidents. Especially, in the case of a sodium-cooled fast reactor, there is a possibility that molten core material would be discharged through control rod guide tubes into the inlet coolant plenums beneath the rector cores in the event of a core disruptive accident (CDA). It is important to ensure in-vessel retention that keeps and confines damaged core material in the reactor vessel even if the CDA occurs. In this study, effective cooling of the melt in coolant was confirmed by comparing the experiment and analysis. CDA scenario initiated by a unprotected loss of flow condition , which is a typical cause of core damage, is generally categorized into four phases according to the progression of core-disruptive status, which are the initiating, early-discharge, material-relocation and heat-removal phases for the latest design in Japan. During the material-relocation phase, the molten core material flows down mainly through the control rod guide tube and is discharged into the inlet coolant plenum below the bottom of the core. The discharged molten core material collides with the bottom plate of the inlet plenum. Clarification of the accumulation behavior of molten core material with such a collision on the bottom plate is important to reduce uncertainties in the safety assessment of CDA. In present study, in order to make clear behavior of core melt materials during the CDAs of sodium-cooled fast reactors, analysis was conducted using the SIMMER-III code for melt discharge simulation experiments by Imaizumi et al. in which low-melting-point alloy was discharged into a shallow water pool. As the result, temperature and pressure behaviors during the discharge almost coincided between the analysis and the experiment. Therefore, it can be concluded that the validity of the analysis cell system model was confirmed. (author)