PRAISE analyses are performed on a piping location-by-location basis to provide the cumulative failure probability as a function of time, whereas the desired end result is the system failure frequency within various time frames. The time increments of interest are now (0-25 years), the near future (25-40 years) and the more distant future (40-60 years). If the reactor transients analyzed are per reactor-year (rather than calendar year), then the times considered are also per reactor-year. As an average, one calendar year corresponds to 0.8 reactor years, but no adjustments for this are made in this appendix. The system failure frequency is obtained from the failure frequency for individual locations by analyzing the most highly stressed location and multiplying by the number of such highly stressed locations in the system. The failure frequency for a flow rate exceeding q for a given time increment from tj to t2 is obtained from the following relation

The values of P& (t) are the output from PRAISE for the dominant location(s) in the system. The failure

frequency for the system is then obtained by multiplying by the number of locations in the system that have the high stresses of the dominant location.

F. 3.1 Material Properties

Numerous material properties enter into a PRAISE analysis, many of which are described in the references cited above. A Ramberg-Osgood stress-strain curve is used in the computation of the applied value of the /-integral, as represented by the following relation

Crack instability is governed by exceedance of a critical net section stress and/or a /-integral based tearing instability using a bilinear tearing resistance curve.

A set of default material tensile and fracture properties are provided in WinPRAISE [F.4], which are summarized in Table F.3. Unless otherwise stated, these properties are used in the base case analyses.

The properties of Table F.3 are generally somewhat conservative and are representative of undegraded materials. In some instances, degraded material properties are considered, as discussed at the particular component involved.

In the case of fatigue of initial cracks, the distributions of the initial crack depth and aspect ratio are probably the most important random variables. Unless otherwise stated, the depth distribution is taken from Reference F.10, which is the default in WinPRAISE and is also included in Reference F.9.