Reduction of accident occurrence frequencies by taking safety measures for defense-in-depth level 4 in a sodium-cooled fast reactor

ナトリウム冷却高速炉の深層防護レベル4の対策による事故シーケンス発生頻度の低減について

西野 裕之  ; 栗坂 健一 ; 鳴戸 健一*; 権代 陽嗣; 山本 雅也

Nishino, Hiroyuki; Kurisaka, Kenichi; Naruto, Kenichi*; Gondai, Yoji; Yamamoto, Masaya

シビアアクシデントに対する安全対策の有効性評価は日本の高速実験炉「常陽」の再稼働にも必要となる。それらの安全対策には深層防護レベル4の中の対策も含まれる。既往研究では、出力運転時のレベル1PRAを実施し、深層防護レベル1から3までの安全対策の失敗の事故シーケンスの発生頻度を計算、支配的な事故シーケンスグループの同定、支配的な事故シーケンスの同定を実施した。このような成果に基づき、本研究では深層防護レベル4の対策によって発生頻度の減少を示す出力運転時のレベル1PRAを実施した。結果、各事故シーケンスグループの発生頻度は減少し、発生頻度は約1E-6/炉年となった。既往研究で算出した発生頻度と比べて本研究で算出した発生頻度は1/1000倍になった。崩壊熱除去喪失が全ての事故カテゴリの中で最も支配的であった。

The effectiveness evaluation of safety measures against severe accident is necessary for restart of experimental sodium-cooled fast reactor Joyo in Japan. These safety measures correspond to those in defense-in-depth (DiD) level 4. In the previous study, a level-1 probabilistic risk assessment (PRA) at power was performed to calculate frequencies of the accident sequences of failure of safety measures in DiD level 1 to 3, to identify dominant accident sequence groups, and to identify dominant accident sequence for selecting important accident sequences in each accident sequence group which are needed for implementing the effectiveness evaluation of safety measures in DiD level 4. Based on this, the present study implemented level-1 PRA at power to show quantitatively reduction of those occurrence frequency by the safety measure in the DiD level 4. As the result, the frequency of each accident sequence group decreased significantly, and total frequency of the accident sequence groups decreased to about 1E-6 /reactor-year which is about 1/1000 times the one estimated in the previous study. The protected loss of heat sink was the largest contributor in all the accident groups and a dominant accident sequence in each accident group was also identified in this study.