Theoretical calculations of pressure vessel failure frequencies for Sizewell 'B' transients

The technique of probabilistic fracture mechanics is now widely used to assess the integrity of reactor pressure vessels when subjected to transient conditions. In this report the technique is applied to estimate the failure frequency of the Sizewell 'B' reactor pressure vessel under the possible transient loadings. The results are presented for both normal operational conditions (Condition I and II events) and for faulted conditions (Condition III and IV events), for a number of regions of the vessel. Estimates of the failure frequency based on the initiation of crack extension and on allowing a limited amount of stable crack extension are given. These show that the failure frequency of 10-7 per reactor year used in the degraded core analysis for Sizewell 'B' is conservative, and a lower value of 3 x 10-9 per reactor year is calculated, assuming that the material fracture toughness is that appropriate to the 'upper shelf' during the large loss-of-coolant accident. For the pessimistic assumption that this latter condition is not valid the estimated value is 4 x 10-8 per reactor year. (author)