Conditional Probabilities of Pressure Tube Rupture During Limiting Design Basis Transients at Reactor Early Life and Late Life

Abstract

The CSA Standard N285.8 requires evaluation of the service conditions for protection against pressure tube fracture using either a deterministic or probabilistic evaluation. Based on the results of a full core deterministic fracture protection evaluation, it is possible to identify limiting locations in the reactor core, i.e., the location at which it is most challenging to demonstrate fracture protection. The conditional probability of failure for a particular design basis transient, given a through-wall flaw, can be calculated at the limiting location. The ratio of the probabilities of rupture between reactor early life and late life could be used to investigate the effect of plant ageing on the stability of a postulated pressure tube crack. The results of an example calculation for a representative heatup/cooldown transient and a representative limiting Level C design basis transient are discussed in this paper.