A Second Generation Scheduler for Dynamic Event Trees

摘要

Several licensing programs have focused on the evaluation of the importance of operating events occurred in Nuclear Power Plants (NPPs). Some have studied the dynamic aspects of the sequence of events involved, reproducing the incidents, while others are based on PSA applications to incident analysis. A method integrating both approaches derives from the Integrated Sequence Analysis methodology (ISA). The dynamics of the event is followed by transient simulation in tree form, building a Setpoint or Deterministic Dynamic Event Tree (DDET). This implies opening branches whenever a new setpoint for systems intervention is exceeded and simultaneously simulating the dynamics of every branch and calculating the corresponding Boolean product of the fault trees involved up to that point. Crucial-within the additional transient simulation capabilities required by ISA is the development of simulation drivers (schedulers) able to generate the branching. The aim of the work described here is to develop an event scheduler-with enough flexibility and generality. The tree-will be composed of branches corresponding to the simulation of the sequences of events, i.e. transitions among the different states of the status vector that characterize the status of preselected facility components. The transitions are triggered after a decision making process, conditioned by specific relations between the process variables, and executed by decision modules. One of the branches is the nominal sequence of the simulation that does not consider failures neither in the equipment nor in the human actions during the transient evolution. The failures considered in the nominal sequence are only those conditions existing before the initiating event. The Scheduler manages messages coming from several calculation modules with different purposes (simulation of the behavior of a physical system or decision processes), that indicate the paths to follow (in parallel and in tree form) to guide the simulation. In other terms, the Scheduler acknowledges events coming from the processes in execution and takes actions in response to those "requests", acting as a discrete event processor.