Development of margin assessment methodology of decay heat removal function against external hazards, 2; Tornado PRA methodology

外部ハザードに対する崩壊熱除去機能のマージン評価手法の研究開発,2;竜巻PRA評価手法

西野 裕之  ; 栗坂 健一 ; 山野 秀将   

Nishino, Hiroyuki; Kurisaka, Kenichi; Yamano, Hidemasa

外部事象PRAは福島第一原子力発電所事故後、重要な安全評価手法として認識されてきている。PRAを日本で特に重要な地震と津波に対して実施するだけではなく、他の外部ハザード(たとえば竜巻)にも実施・発展されるべきである。本研究では、SFRに対してPRAの評価手法を発展させた。まず、日本のデータを用いて竜巻ハザード曲線を評価した。次に、崩壊熱除去に対する重要機器及び設備を同定し、炉心損傷に至るイベントツリーを風圧や飛来物に着目して構築した。重要機器及び設備に対する主なダメージの要因は、飛来物であり、そしてそれは、高所に配置されている重要機器の高さ以上に浮き上がる場合に驚異的な飛来物として同定される。この飛来物に対する重要機器の失敗確率は、飛来物が重要機器に関する開口部へ侵入する確率と、飛来物が対象の重要機器に衝突した際の機能喪失確率との積によって計算した。最後に、イベントツリーを定量化した。結果、炉心損傷頻度は10/年よりも十分に小さくなった。

Probabilistic Risk Assessment (PRA) for external events has been recognized as an important safety assessment method after the TEPCO's Fukushima Daiichi Nuclear Power Station accident. The PRA should be performed not only for earthquake and tsunami which are especially key events in Japan, but also the PRA methodology should be developed for the other external hazards (e.g. tornado). In this study, the methodology was developed for Sodium-cooled Fast Reactors paying attention to that the ambient air is their final heat sink for removing decay heat under accident conditions. First, tornado hazard curve was estimated by using data recorded in Japan. Second, important structures and components for decay heat removal were identified and an event tree resulting in core damage was developed in terms of wind load and missiles (i.e. steel pipes, boards and cars) caused by a tornado. Main damage cause for important structures and components in tornado is the missiles and the tornado missiles that can reach those components and structures placed on high elevations were identified, and the failure probabilities of the components and structures against the tornado missiles were calculated as a product of two probabilities: i.e., a probability for the missiles to enter the intake or outtake in the decay heat removal system, and a probability of failure caused by the missile impacts. Finally, the event tree was quantified. As a result, the core damage frequency was enough lower than 1E-10/ry.