The general approach and philosophy used follows the approached taken by GRS to estimate frequency of LOCA initiating events at passive systems for German PSAs.

The major steps and assumptions of this approach are as follows:

— In principle, wall penetration of pipes which would result into a leak follows in their geometries either

• a slit type penetration originating from cracks caused by fatigue or corrosion or

• a bulging type penetration caused by wall thinning.

Beyond critical dimensions wall penetrating stable defects turn into a full break. This means in practice that for each pipe size there are two or more leak sizes which are of a distinct different probability of occurrence governed by the likelihood of the respective active failure mechanism and the reliability to detect leaks and to take actions to avoid aggravation of the situation e. g. isolation of the leak, stop operation.

The maximum leak size related to a wall penetrating stable defect (undercritical crack, bulge, pit) depends on the actual load specifically the relationship between membrane and bending stresses. The majority of systems being considered in the safety analysis of NPP’s fall either into the category of high pressure or low pressure systems. For reasons of simplicity UB values can be taken to describe maximum leak sizes connected to wall penetrating stable defects for each pipe size. Based on experimental evidence as well as fracture mechanics calculations the maximum leak size resulting from an undercritical crack is rather limited, expressed in terms of fractions of the pipe cross section it is only a few percent. This approach uses 2% of the cross section as a rule of the thumb for high pressure systems. Through wall corrosion pits are generally very small. Bulge-type wall penetrations caused by wall thinning have a potential for stable leaks of a considerable size.

— The frequency of leaks is estimated based on the operating experience of the national population of nuclear power plants and in addition the worldwide experience is considered as far as applicable and available. In general, the operating experience give indications of leaks in a sense of precursors for most classes of piping or give indications of zero failures statistics only.

— To estimate the probability of a break (which is connected by the diameter of the piping to the flow rate) a relationship is used to describe the frequency of breaks in relation to the frequency of leaks as the function of the diameter of piping systems being designed to the same design parameters. For the small bore piping (less than 2 inch) the relationship between leak and break is arrived from operating experience. For the largest pipe (main primary pipe) the relationship between leak and break is based on a number of technical arguments and PFM analyses. For the pipe sizes in between a linear relationship is used between the UB and LB as described before.

— For reasons of simplicity and in accordance with technical experience it is assumed that within the piping systems only so called leak relevant elements are contributing to the frequencies. These leak relevant elements are essentially welds which are adjacent to changes in geometry (nozzles, branches, reducers etc.) which in itself would introduce enhanced loads and to some extend represent more difficult areas for manufacturing and inspection.

— The whole population of pipes, nozzles and penetrations connected to the main components are divided into subpopulations taking pipe diameters as orientation values, using e. g. 5 or 6 subpopulations to represent the difference pipe sizes, materials and operating conditions. For each subpopulation the frequency of leaks is based according to the procedure described before (operating experience, zero failure statistics), the frequency of leaks is adjusted to the size of the relevant population each time and the frequency of breaks within the subpopulation is estimated using the described relationship.

— The frequency of the different subpopulation which could contribute in a different way (leak or break) to the specified LOCA classes is then summed up. In view of the limitation regarding the verification of very low values of estimated frequencies a cut-off value is used.