As a part of the Nuclear Regulatory Commission (NRC) effort to obtain a resolution to the pressurized water reactor (PWR) pressurized-thermal-shock (PTS) issue, a probabilistic approach has been applied that includes a probabilistic fracture-mechanics (PFM) analysis. The PFM analysis is performed with OCA-P, a computer code that performs thermal, stress, and fracture-mechanics analyses and estimates the conditional probability of vessel failure, P(F|E), using Monte Carlo techniques. The stress intensity factor (K₁) is calculated for two- and three-dimensional surface flaws using superposition techniques and influence coefficients. Importance-sampling techniques are used, as necessary, to limit to a reasonable value the number of vessels actually calculated.

Analyses of three PWR plants indicate that (1) the critical initial flaw depth is very small (5 to 15 mm), (2) the benefit of warm prestressing and the role of crack arrest are transient dependent, (3) crack arrest does not occur for the dominant transients, and (4) the single largest uncertainty in the overall probabilistic analysis is the number of surface flaws per vessel.