Inservice Requirements for Containments in the United States

Abstract

Assuring the lifetime integrity of containment structures for nuclear power plants in the United States is becoming increasingly important as today's operating plants grow older, as corrosion and other degradation mechanisms become of increasing concern, and as efforts continue to extend the operating lifetime beyond the present 40 year licensed lifetime. Title 10, Part 50 (10CFR50) of the U.S. Code of Federal Regulations contains general criteria for the safety of nuclear plants and systems. This document mandates compliance with specific rules in Section XI of the ASME Boiler and Pressure Vessel Code for the inservice inspection of nuclear power plant components. Section XI, Subsection IWE, for containment in service inspection was first published in 1981. Subsection IWE contains requirements for preservice examination, inservice inspection, leakage testing, repair and replacement for metal (Class MC) containments and metallic liners for concrete (Class CC) containments. Rules for concrete containments were published in Section XI, Subsection IWL, in 1988. Subsection IWL contains requirements for examination of the concrete surfaces and tendon anchorages as well as for examination of tendons for post-tensioned containments. Rules in Subsection IWL for leakage tests and for concrete repair and replacement . are in course of preparation. As originally published, the preservice examination and inservice inspection rules in Subsection IWE focused on welds as a basis for the required inspections. Recent experience has shown, however, that the condition of containment base metal and liners is more important to continued containment integrity. The rules in Subsection IWE are therefore being rewritten to incorporate requirements for examination of the base metal together with a reduction of weld inspections to provide better assurance of continued containment safety. Neither Subsection IWE nor Subsection IWL have been endorsed in 10CFR50 to date. Consequently , containment inservice inspection to the rules in the ASME Code is. not mandated for nuclear plants in the United States at this time. This paper presents the basis for and content of the rules in Section XI of the ASME Code for both metal and concrete containments. The revisions that are currently being formulated to redirect examinations from welds to base metal are defined. An overview of containment aging and degradation mechanisms is also presented. The paper addresses the status of current actions by the U.S. Nuclear Regulatory Commission to mandate full compliance with both Subsection IWE and Subsection IWL for nuclear plants in the United States in 10CFR50. When mandated in 1 0CFR50 and implemented by nuclear plant owners, the ASME Code inservice requirements for Class MC and Class CC containments together with the revisions to address current experience, will provide a sound basis for improved lifetime integrity for containments in the United States.