House events matrix for shutdown probabilistic safety assessment

Probabilistic safety assessment is one of the standardized ways of assessing safety of nuclear power plants. The objective of the method is to present an extension of the fault tree in order to reduce the size of the shutdown probabilistic safety assessment model. The shutdown probabilistic safety assessment method is developed. The modelling for all plant operating states include consideration of 15 states, which were determined as appropriate representations of much more plant configurations in addition to the plant full power operation. For dealing with the complexity of the models and manageable size of the models for the sensitivity studies it is essential that the models optimizations are performed. House events matrix plays an important role as it reduces the number of the fault trees significantly. The results include the time dependent representation of the core damage frequency contributions weighted for their plant operating state durations over total duration of the shutdown timely through the whole shutdown. The risk of the plant during shutdown is smaller in general than in full power operation, however for certain specific plants and their specific plant operating states the risk for a short duration of time may increase beyond the risk of full steady state power operation.