Estimation of leakage frequencies from different systems at Nordic nuclear power plants

Within the project External Events (1994-97) it was decided to initialise the work on a handbook-, called X-Book, in order to provide the PSA analyst guidance in the selection of models and frequencies of 'external events'. Thus, the aim of the X-Book is to be complementary to the I-Book, a handbook on Initiating Events at Nordic nuclear power plants. The X-Book will, among other things, encompass the frequencies of so called system external events, i.e. initiating events that are caused by events occurring outside the system boundaries. In Version 1 of the X-Book the attention is mainly focussed on the estimation of initiating fire frequencies, per plant and per building. This frequency estimation was mainly based on data, specifically collected about fires occurred at Nordic power plants. Within the project mentioned above efforts were also started with the aim to estimate the frequencies of water/steam leakage, frequencies that can be used in PSA of risks related to flooding and steam rupture. The models and frequencies of events resulting in loss of cooling water (LOCA) that were used already in WASH-1400 have 'survived' in many later studies. According to the aim and ambitions of modern PSAs the need for improved treatment of water/steam leakages is more and more emphasised, both with respect to the modelling of the course of events and the estimation of frequencies of such events. Leakage from water-bearing systems may occur more or less spontaneously through cracks, pipe ruptures, damaged packing-boxes of pumps and valves etc. Thus this type of leakage may derive from pipe failures or leaking active or passive components. External leakage may also occur if a valve is spuriously opened between low and high pressure systems or towards a temporarily opened part of a system (Interfacing V-LOCA). This report describes the methods used in the selection of leakage data from the Nordic NPP component data base TUD as well as the models and methods applied in the estimation of water/steam leakage frequencies. The report also presents the resulting frequencies of 'large' leakages both per plant and system and per system and component type