determine the stability of the sampled condition. The parameters which are sampled to provide the material toughness include the initial properties (RT NDT(U), the chemical composition (copper and nickel contents) and the neutron fluency. The flaw characteristic parameters include the flaw size distribution and the flaw density. These parameters have a key role in affecting the results of the PFM evaluation, but they also are associated with considerable uncertainty.

This study uses the results from recent PFM calculations for boiling water reactors (BWR). These evaluations assessed a variety of flaw size distribution and density parameters, including, in some cases, the effect of in-service inspection (ISI) on these parameters. These results are examined from the following perspectives :

• Effect of different flaw size distributions and densities

• Impact of ISI

• Differences between axial and circumferential welds

• Effect of different transients (i.e. temperature and pressure time histories)

The different characteristics of the vessel conditional probabilities in terms of T - RT NDT for three groups of vessels.