A procedure to optimize the timing of operator actions of accident management procedures

The analysis of beyond design basis accidents (BDBA) is an essential component of the safety concept of nuclear power plants (NPP). Goal of the analysis is to achieve a set of actions aimed to prevent the escalation into a severe accident, to mitigate consequences of a severe accident, and to achieve a long term controllable state of the NPP. This paper presents an analytical procedure to optimize the timing of operator interventions. The procedure is demonstrated based on four sets of parameters, first, parameters which define the operator actions are chosen. Second, parameters which define the system availability are chosen. Third, parameters which define in a continuous way the status of the plant are chosen. Finally, one looks for a functional dependency of the accident management (AM)-parameters and the parameters describing the plant status. Once a function could be found, this function is “optimized” in the sense that the AM-parameters are varied to find a optimal overall condition for the plant. In the first part, the paper presents the analytical procedure in a general way, in the second part, an initiating event is chosen. The procedure is applied to a station black out (SBO) transient, and as operator action secondary side bleed and feed, followed by primary side bleed and feed, is foreseen. As result, the optimal timing to initiate both actions is achieved.