Experiment and new analysis model simulating in-place cooling of a degraded core in severe accidents of sodium-cooled fast reactors

•Behavior of in-place cooling of residual core fuel in SFRs was analyzed by SIMMER-III.•Phenomena in the in-place cooling were identified by PIRT.•Out-of-pile experiment simulating the in-place cooling was conducted.•Out-of-pile experiment was analyzed by SIMMER-III.•New model of gas mixing was developed and improved the numerical analysis. In severe accidents of SFRs, the cooling of the residual core materials, which is called “in-place cooling”, is one of the important factors for In-Vessel Retention (IVR). For the evaluation, behavior of the in-place cooling was analyzed by the SIMMER-III code. In order to understand the in-place cooling, method of Phenomena Identification and Ranking Table (PIRT) was applied. Based on the result, an out-of-pile experiment which focused on the extracted factors was conducted. In the experiment, continuous oscillation of sodium level was observed by sodium vaporization and condensation. Analysis for the out-of-pile experiment was conducted by SIMMER-III, but the results were different between the experiment and the analysis. By investigation of the analysis result, it was revealed that the difference was due to occupation of non-condensable gas. Therefore, an analysis model of inter-cell gas mixing was newly developed, and the agreement was significantly improved by the new model.