Repair for PWR CRDM Part-Length Housing Cracking
SIM-98-005

Cracking in Pressurized Water Reactor (PWR) CRDM canopy seals has occurred at a number of plants, and is not a new issue. Structural Integrity (SI) has designed and implemented a number of weld overlay repairs to re-establish the leak-tight boundary at the seal location with corrosion-resistant material (see figure below).

Recently, during installation of such a repair at a U.S. PWR, a defect in the motor tube base of this same "part-length housing" was observed. The defect was above and remote from the canopy seal, and was discovered during visual inspection of the welding area. Based on preliminary metallurgical evaluation, the cracking is believed to be associated with fabrication defects caused by original welding. No indication of crack growth (fatigue or stress corrosion cracking) was observed.

To resolve this issue, the utility removed the part-length CRDM housing by unthreading at the lower canopy seal location, and threaded on an adapter plug over the existing penetration. The adapter plug is seal-welded to the outside diameter of the penetration with a fillet weld (as shown below). This design is equivalent to existing spare CRDM locations, only a fillet weld is used in lieu of a canopy seal from the adapter plug to the penetration.

SI has provided services to several utilities regarding this repair issue, working with Welding Services Inc. (WSI) to implement the repair based on the SI design and analysis. The design of the weld includes the size, geometry and weld pass sequencing, and changes to the adapter plug. A design stress report is prepared documenting that the plug and fillet weld meet all applicable ASME Code Section III require-ments, considering differential expansion between the plug and penetration, in addition to other design basis loads.

If you would like more information on Structural Integrity's capabilities in this area, please contact SI: