Development of an evaluation method for in-place cooling of a degraded core in severe accidents of sodium-cooled fast reactors

Imaizumi, Yuya ; Aoyagi, Mitsuhiro ; Kamiyama, Kenji  ; Matsuba, Kenichi  ; Akaev, A.*; Mikisha, A.*; Baklanov, V.*; Vurim, A.*

The cooling of the residual degraded core materials, which is called "in-place cooling", is one of the important factors for the in-vessel retention (IVR). For the evaluation of the in-place cooling, behavior in a SFR core which is cooled down by the sodium inflow through CRGT was simulated by a safety analysis code, SIMMER-III. As a result of the analysis, the core materials which were initially 2500 C were cooled down in several minutes. In order to analyze the in-place cooling, the method of phenomena identification and ranking table (PIRT) was applied, and several factors of thermal-hydraulic dynamics were extracted. Out-of-pile experiments which focus on the extracted factors were conducted, and continuous oscillations of sodium level were observed in the experiments. Analyses by SIMMER-III for the experiments were conducted, but the sodium level oscillation was not fully simulated in the analysis of IPCO-1. By investigation of the analysis result, it was revealed that the difference was due to partial occupation of non-condensable gas. In order to pre-vent the unrealistic occupation, the analysis model of inter-cell gas mixing was newly developed, and the agreement between experiment and analysis was significantly improved by that.