Residual life assessment of French PWR vessel head penetrations through metallurgical analysis

In September 1991, a vessel head penetration was found leaking at Bugey 3 plant during the hydrotest included in the framework of decennial in Service Inspections. Non destructive examinations performed afterwards on several other plants have shown some cracked penetrations. Destructive expertises confirmed quickly that again this new problem is related to stress corrosion cracking of Alloy 600 used as base material. During the last 15 years, similar cracking have been met in steam generator tubes and secondly in pressurizer instrumentation tubes. In spite of all the work performed since that time an extension appear to be necessary for explaining the features of this new event ; however material sensitivity, stress and temperature still remain the key parameters governing the behavior of Alloy 600 in PWR environment. In this paper, only the material sensitivity of vessel head penetrations is examined through metallurgical analysis in relation with SCC tests. On the basis of vessel head field experience in combination with thermomechanical process used for fabrication of original bars criteria for a sensitivity ranking of penetrations are proposed. Metallurgical investigations and SCC tests were carried out to support this sensitivity ranking. The final aim is to use such information among those quoted above for assessment of vessel heads residual life. This document is an overview of the work performed in France concerning the material sensitivity of forged Alloy 600. It represents an important part of the assessments and investigations undertaken in France on the stress corrosion cracking phenomenon affecting the reactor vessel head penetrations in PWR's. (authors). 3 refs., 2 figs., 5 tabs