Status of numerical tool development for the initiating phase in severe accidents of SFR in Japan

日本におけるナトリウム冷却高速炉のシビアアクシデント起因過程評価ツール開発の現状

深野 義隆  ; 石田 真也  ; 久保 重信 

Fukano, Yoshitaka; Ishida, Shinya; Kubo, Shigenobu

The SAS4A code has been developed and used for the analyses of initiating phase including those for licensing of sodium-cooled fast reactors (SFRs) in Japan. The SAS4A code had been originally developed in the Argonne National Laboratory (ANL) in U.S. for analysis of initiating phase during severe accident. It was further developed and improved for oxide fuels in cooperation with the Japan Atomic Energy Agency (JAEA) and European research institutes, including the addition of new models based on the findings from the in-pile CABRI test. In recent years, the phenomena identification and ranking table (PIRT) method has been applied to select important phenomena to be verified for unprotected loss-of-flow (ULOF) and unprotected transient overpower (UTOP) in a medium-sized SFR. Through these studies, 16 and 14 physical phenomena were extracted for ULOF and UTOP respectively. Then 10 and 8 key important phenomena respectively were identified through the ranking of extracted phenomena. Furthermore, validation test matrix was created for the extracted important phenomena, and validation was conducted according to the validation test matrix. These studies further improved the validity and reliability of the SAS4A code. The SAS4A code has been used in a lot of severe accident analyses and licensing of SFRs. Examples of the evaluation of ULOF event for a medium-sized SFR showed the prompt criticality was avoided in the initiating phase and total reactivity remained negative at the time of the wrapper tube melting that was the application limit of SAS4A. Compared to previous evaluations, the present SAS4A calculations show that the elimination of mechanical energy release due to prompt criticality during initiating phase is possible even under conservative conditions due to the reduction of excess uncertainty. The paper describes the status of development and application of SAS4A code in JAEA and the future research plan.