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Development of margin assessment methodology of decay heat removal function against external hazards, 3; Forest fire hazard assessment methodology

外部事象に対する崩壊熱除去系のマージン評価手法の開発,3;森林火災ハザード評価手法

岡野 靖; 山野 秀将   

Okano, Yasushi; Yamano, Hidemasa

本研究は、ナトリウム冷却高速炉を対象に、森林火災に対しPRAを用いハザード評価手法を構築するものである。森林火災によるインパクトの発生頻度と影響を定量的に評価し、最終的に炉心損傷頻度を求める。新評価手法はハザードカーブの導出とレベル1PRAからなり、本論文は1つ目に含まれる森林火災進展シミュレーションを扱ったものである。森林火災によるチャレンジ要因を定量的に求めるとともに、天候条件に対する感度分析を行った。森林火災シミュレーションはFARSITEコードを用いた。感度解析の結果、風の有無と湿度の高低に対する感度が高い一方、気温に対する感度は低いことが判明した。すなわち、火災がサイトに到達する時間を比較した場合、風が観測最大値と無しの場合で5倍程度、湿度の高低により3.4倍程度の差が見られた。火災に伴う随伴事象として、火災が高圧送電線網とサイトへ到達する時間差を定量的に求め、外部電源喪失後の火災到達までの時間猶予を評価した。

The forest fire hazard assessment methodology, which is subject to an external hazard probabilistic risk assessment, is being developed for Sodium cooled Fast Reactor in order to quantitatively evaluate frequency and consequence of a forest fire that has a potential impact on a NPP and to obtain the core damage frequency. The new methodology consists of two parts; the first one is hazard frequency-consequence domain and the second one is Level 1 PRA. This paper focuses on forest fire propagation simulations in the first part of the methodology. The simulation is utilized to evaluate intensities of the challenges by a forest fire, and sensibility studies were performed on weather conditions. Forest fire propagation simulations were performed using FARSITE code, and the results show that the key outcome parameters depend much on wind speed and humidity but less on temperature. The fire arrival time to the site is shortened around 1/5 with changing wind speed condition from the recorded highest to the condition without wind. The time is prolonged around 3.4 times with the most humid to the recorded lowest conditions, although it is changed little when varying ambient temperature from recorded-highest to the lowest. A loss of offsite power due to fireline passage across through external power lines is a potential subsequent event, and the lag time of the fire arrival to the site after the loss of offsite power was evaluated.

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