Final Report - Inspection Limit Confirmation for Upper Head Penetration Nozzle Cracking

Abstract

The ASME Code Case N-729-1 defines alternative examination requirements for the Control Rod Drive Mechanism (CRDM) upper head penetration nozzle welds. The basis for these examination requirements was developed as part of an Industry program conducted by the Materials Reliability Program (MRP) through the Electric Power Research Institute (EPRI). The results of this program were published in MRP-95 Rev. 1 and document a set of finite element weld residual stress analyses conducted on a variety of upper head penetration nozzles. The inspection zone selected by the industry was based on the stress where it was assumed that primary water stress corrosion cracking (PWSCC) would not initiate. As explained in MRP-95 Rev. 1, it has been illustrated that PWSCC does not occur in the Alloy 600 tube when the stresses are below the yield strength of that tube. Typical yield strengths at operating conditions for Alloy 600 range from 35 ksi to 65 ksi. A stress less than 20-ksi tension was chosen as a conservative range where PWSCC would not initiate. Over the last several years, Engineering Mechanics Corporation of Columbus (Emc2) has conducted welding residual stress analyses on upper head penetration J-welds made from Alloy 182 weld metal for the U.S. Nuclear Regulatory Commission (NRC) staff. These efforts were performed as a confirmatory evaluation of the industry’s analyses conducted as part of their MRP-95 Rev. 1 effort. To this point, the analyses conducted by Emc2 have not been compared to the MRP-95 Rev. 1 results or the examination zones defined in the Code Case. Therefore, this report summarizes the past Emc2 CRDM welding analyses and investigates the regions where the welding stresses may be sufficiently high to promote stress corrosion cracking (SCC). In all, 90 welding residual stress analyses were conducted by Emc2 and the largest distance below the weld where the stress drops below 20 ksi was 5 inches for the uphill weld of the 53-degree nozzle case. For the largest distance above the weld where stress drops below 20 ksi, the worst case was 1.5 inches above the downhill side of the 25-degree nozzle case. The inspection zones described in both MRP-95 Rev. 1 and Code Case N-729-1 were set at 1.0 inch for nozzle angles greater than 30 degrees or 1.5 inches for nozzle angles less than 30 degrees, above the highest or below the lowest point on the weld. In all cases analyzed by Emc2 in this effort, there was only one case where the stress was above 20 ksi outside of this inspection zone. For that case, the stresses were very close to 20 ksi at the inspection zone limit and were considered acceptable.