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もんじゅPRA-プラント応答定量評価(II)補助冷却設備による崩壊熱除去解析

Analysis of plant thermohydraulic response for the Monju PRA study; Evaluation of decay heat removal capability of auxiliary cooling system

山口 彰*; 長谷川 俊行*

not registered; not registered

原型炉の崩壊熱除去系である補助冷却設備(ACS)の自然循環除熱性能をプラントシステムコードSSC-Lにより評価した。もんじゅPRAのシステム解析によれば,除熱失敗(PLOHS)事故の発生頻度に寄与する最大の要因として共通原因故障による全ループのACS出口止め弁開失敗事象が摘出されている。本研究では,その事象に対して,回復操作によって崩壊熱除去に成功する規準を求めている。ACS出口止め弁にはバイパス弁が設置されており,2ループ以上のバイパス弁を手動で開とすれば自然循環崩壊熱除去に成功するとの結論を得た。感度解析により,開操作のための時間余裕は十分にあること,圧損係数の不確定性を考慮してもこの結論を支持できることを示した。なお,出口止め弁開成功時には,1ループのみで自然循環崩壊熱除去が可能である。本研究により,SSC-Lを用いて自然循環時の除熱特性を定量的に評価できることが示された。例えば,自然循環時の除熱性能は,崩壊熱と除熱量の収支に加えてプラントシステムの熱容量によって大きく左右される,除熱に利用できる主冷却系ループ数が少ない場合には1ループ当りの冷却材流量や除熱量が増加する,除熱量は冷却材温度に依存するため高温条件では実効的な除熱容量は設計値よりも大きくなる等の特性が解析された。

In the Probabilistic Risk Assessment (PRA) of Monju, annual frequency of core damage was evaluated by the systems analysis for each accident category. Common mode or cause failure to open the stop valves of Auxiliary Cooling System (ACS) was found to be the most dominant contributor to the core damage frequency caused by Protected-Loss-Of-Heat-Sink (PLOHS) accidents. In this study, possibility of the recovery from the PLOHS condition by opening bypass valves has been investigated using Loop version of Super System Code (SSC-L). The bypass valve is located in parallel to the stop valve of ACS. Success criterion for the recovery has been proposed and general characteristics of the natural circulation decay heat removal have been pointed out. 0pening of the bypass valve in more than two loops is the proposed criterion for the successful decay heat removal by natural circulation. Based on a sensitivity analysis, it has been found that the grace time for the recovery action is sufficiently long. Upper bound of the pressure loss coefficient of the bypass valve has been also selected as a parameter for the sensitivity analysis to check the certainty of the conclusion. It has been found that the conclusion is supported within the uncertainty range of the pressure loss coefficient. In the situations where the stop valves are opened successfully, the decay heat can be removed by one loop natural circulation. Furthermore, the maximum coolant temperature reduces considerably by taking into account the mass heat capacity of the remaining two loops. The decay heat removal characteristics in the natural circulation mode are summarized as follows: (1)As the number of available heat transport loops lessens, the mass flow rate and heat removal capability per loop decreases. (2)The maximum coolant temperature is sensitive to the mass heat capacity of the plant system. Therefore, availability of the fluid and structural mass heat capacity should be investigated carefully ...

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