Development of accident sequence analysis method and operation procedure for success of feed-and-bleed operation under combined accident

Abstract

A feed-and-bleed (F&B) operation in typical pressurized water reactor is very important since it provides residual heat removal when the secondary cooling system is not available. An F&B operation can cover from single event to combined accident which are related to failure of secondary side. However, operators may hesitate to initiate an F&B operation if a clear cue is not provided because its initiation will result in the release of radioactive coolant into the containment structure. An analysis of the necessity and effects of an F&B operation was performed systematically. Combined accidents are considered as very rare events and therefore are not usually considered in deterministic or probabilistic safety analyses. Yet, despite being rare, it is necessary to examine combined accidents as their effects could become very large following poor treatment from a lack of information. In a combined accident, the most important safety actions are the functions for heat removal, as initiating and maintaining proper safety actions are critical to prevent core damage. In order to analyze the plant conditions requiring safety action to prevent core damage and the success conditions of the safety actions under a combined accident, sequence tree modeling is suggested. A sequence tree is a branch model to classify the plant condition considering plant dynamics. Since a sequence tree model can reflect the plant dynamics arising from the interaction of different accident timings and plant conditions, and also from the relations be-tween operator action, mitigation systems, and the indicators for operation, it can be used to develop a dynamic event tree model. To develop the sequence tree model, indicators are identified which inform about the availability of heat removal mechanisms and the plant condition. This study develops a sequence tree model to core damage requiring an F&B operation under combined accident (a total loss of feedwater accident with a loss of coolant accident). Based on the conventional PSA model and indicators, the sequence tree model for a TLOFW accident was developed. For a TLOFW accident with LOCA, second accident timings were categorized as 5 types according to plant condition. Indicators were selected as branch point using the flow chart and tables, and a corresponding sequence tree model was developed. After LOCA occurs, the plant condition which needs an F&B operation can be also categorized. Sub group of sequence represents the plant condition after LOCA occurs. In each sub group of sequence, the indicators are identified. After identification of indicators in all sub-sequence groups, the permutation of indicators is performed to obtain all sequences. After elimination of impossible, all possible sequences which are necessary to an F&B operation to prevent core damage are identified as sequence tree model. Sequence tree model for another combined accident can be developed to identify plant condition which is necessary the specific safety function easily. Moreover, if new safety components for heat removal is applied, the proper condition to use new safety component can be identified using sequence tree model. An F&B operation should be initiated when an F&B operation is necessary. However, conventional operation procedure in emergency operating procedures (EOPs) does not cover all possible conditions to initiate an F&B operation based on results of sampling analysis. If the EOP informs on the urgency of an F&B operation, operators will be able to more clearly make decisions regarding an F&B operation initiation. In order to cover all possible scenarios for an F&B operation and systematically inform its urgency, an advanced operating strategy using a decision tree is developed in this study. The plant condition can be classified according to failure of secondary side, RCS pressure condition, injectable inventory to RCS, and remaining core inventory. RCS pressure, core level, and RCS temperature are representative indicators which provide information regarding the initiation of an F&B operation. Indicators can be selected based on their detectability and quantification, and a decision tree is developed according to combinations of indicators. To estimate the effects of the advanced operation procedure, human error probability of an F&B operation is re-estimated based on a thermohydraulic analysis. The available time for operators to initiate an F&B operation is also re-estimated to obtain more realistic data. This study is expected to provide a systematic operation procedure to initiate an F&B operation under various plant situations. An OPR1000 is used in this study as an example plant, with the resulting advanced operating strategy able to be applied to most PWRs which have an F&B operation capability. To obtain the realistic success criteria, a safety margin is necessary to estimate considering the availability of safety components. The conditional core damage probability of combined accident is 1 from deterministic safety analysis since the design does not covered the combined accident. To estimate the effects of success F&B operation under combined accident, the conditional core damage probability is estimated considering component availability. The maximum peak cladding temperature distribution in consideration of components availability were obtained based on results of sampling analysis. According to components availability, the conditional core damage probability is from 0.00322 to 0.0263. When the advanced operation procedure is used, the conditional core damage probability decreases 6 ~ 64% compared with conditional core damage probability when the conventional EOP is used. Even though the plant condition is very complex due to combined accident, the plant will be safe with very high probability based on the advanced operation procedure.