The role of crack arrest in the evaluation of PWR pressure vessel integrity during PTS transients

Abstract

The PWR pressurized thermal-shock (PTS) issue, which is concerned with the integrity of the reactor pressure vessel during postulated overcooling transients, is under intensive investigation by the USNRC. The USNRC-sponsored Integrated Pressurized Thermal-Shock (IPTS) and the Heavy-Section Steel Technology (HSST) Programs are dedicated to a better understanding and a timely resolution of the problem. The HSST program is investigating flaw behavior in large cylinders and is also obtaining fracture-mechanics-related material properties, while the IPTS program is primarily concerned with an estimation of the overall frequency of vessel failure and identification of dominant transients and design and operating features contributing thereto for specific nuclear plants. One important component of the IPTS study is a probabilistic fracture-mechanics analysis of the reactor vessel, and a point of particular interest therein is the role of crack arrest in mitigating the consequences of the postulated PTS transients. The HSST program has provided crack-arrest data from small specimens and large thermal-and pressure-loaded cylinders that tend to establish the validity of the crack-arrest concept for application to the PTS problem. Unfortunately, recent results of the IPTS studies indicate that the inclusion of crack arrest in the probabilistic fracture-mechanics model does not substantially influence the calculated frequency of vessel failure. However, there are still significant questions regarding flaw behavior at upper-shelf temperatures, and the HSST program is continuing to pursue this area of uncertainty.