Canopy Seal Weld Overlay Repair
SIB-96-127

The Problem: Leakage in CRDMs
Leakage has been detected in active control rod drive mechanism (CRDM) intermediate and lower canopy seal welds (CSWs) of pressurized water reactors (PWRs). Through-wall defects have also been detected in spare CRDM CSWs, between the head adapter and head adapter plug. A schematic of a typical CRDM is shown below:

The Issue: Poor Weld Quality and SCC
Major factors promoting leakage are lack of weld penetration and poor quality of original CSWs. Such crevices can concentrate off-chemistry conditions and contribute to stress corrosion cracking (SCC).

The Solution: The Weld Overlay
In 1982, weld overlays were first applied to austenitic stainless steel welds in boiling water reactor (BWR) nuclear power plant piping systems as a repair for observed intergranular stress corrosion cracking (IGSCC). The purpose of this type of repair was to provide a new pressure boundary with material resistant to the cracking mechanism.

Since 1987, the weld overlay concept has been applied to cracked lower and intermediate CRDM canopy seal welds in PWRs. The repair has been performed by applying corrosion-resistant weld metal as reinforcement, using either high ferrite Type 309 stainless steel, or Alloy 625, a nickel-based austenitic material. A schematic of the weld overlay applied to an intermediate CSW CRDM is shown on the right.

A schematic of the weld overlay applied to a lower CSW CRDM is shown on the left.

The canopy seal weld overlay repair has been performed remotely on both peripheral and active drives using automatic orbital welding equipment. The repair has been approved by the NRC as a result of licensing submittals using Code Case N-504 as the guiding document.

Since its inception, SI has been at the forefront of the development, design, and licensing activities associated with the canopy seal weld overlay repair. SI has been associated with these repairs as the design organization and has provided input to licensing submittals by the utilities. SI has designed these repairs for eleven nuclear power plants to date. In each case, no further action has been required at the repaired location during subsequent outages. For additional information on canopy seal overlay weld overlay repairs or SI's capabilities in this area, please contact SI.