Development of a systematic sequence tree model for feed-and-bleed operation under a combined accident

• A sequence tree is a type of branch model that categorizes the plant condition under a combined accident.
• A sequence tree model to core damage requiring F&B operation under the combination of a TLOFW with a LOCA is developed.
• The sequence group can be categorized according to second accident timing.
• With a sampling analysis, the practical accident cases are obtained.

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 between 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 F&B operation under a combined accident, designated here as the combination of a total loss of feedwater accident with a loss of coolant accident. Sequences of the sequence tree model can be categorized according to second accident timing. With a sampling analysis, the practical accident cases are obtained. The sequence tree model can translate into a dynamic event tree model if the initiating event frequency under a combined accident can be quantified.