Use of Class 1 Analysis to Aid Inservice Inspection

This paper describes an experience of utilization of ASME Section III Code Class 1 analysis to aid in inservice inspection (ISI). The pressurizer surge line weld to the pressurizer nozzle was targeted to be inspected under the ASME Section XI program and also to address Alloy 600 issues. The pipe close to the nozzle was encased by a source shield to protect the pressurizer and the pressurizer support from the effect of postulated longitudinal rupture of the pressurizer surge line. Consequently, the shield did not permit the access needed to perform ISI. The encasement was segmented with bolted joints for removal and installation. With the passage of time, the environmental condition around the encasement made the removal and replacement complex and dose intensive. A modification to the shield by permanent removal of a portion was considered as an attractive option to help provide access for the current inspection, as well as anticipated future inspections. This modification exposed a portion of the pipe which may have longitudinal rupture. ASME Class 1 analysis was performed for the portion of the exposed pipe. Analysis was performed in detail to establish that stress and cumulative usage factor is below the threshold level at which a break has to be postulated. The paper concludes that detailed use of stress analysis results in elimination of the postulated break. As a result, permanent access could be created for current and future inservice inspections of the pressurizer nozzle.