Emergent development and application of reactor vessel head penetration inspections and repairs in the United States

In the fall of 2000, a U. S. nuclear power plant discovered cracks in the Control Rod Drive Mechanism (CRDM) reactor head penetration nozzles. The discovery occurred during a routine outage by visually noting boron crystals that had accumulated on the topside outer edges of the penetration nozzle. Subsequent inspections identified nine nozzles with cracks and emergent manual repairs were performed prior to restart. In the spring of 2001, similar cracking was found and repaired at three additional plants. These events served to quickly drive the U. S. nuclear utilities, the Nuclear Regulatory Commission and nuclear service companies to develop acceptable solutions, from both a regulatory and technical standpoint, for identifying and repairing reactor head penetration defects. Unlike the problems discovered previously in France, the cracking of the Alloy 600 nozzles discovered at U. S. plants was initiated at the OD of the nozzle in the J groove weld and not the ID. Inspection guidelines were developed and each PWR was ranked relative to their susceptibility to this problem. In the fall of 2001, PWRs with outages performed at least bare head visual inspections of their head penetrations. Five more plants identified defects requiring emergent nozzle repairs. This paper will provide a summary of the events that occurred in the U. S. nuclear industry in 2001 on this issue and the implications for other PWR plants in 2002 in the U. S. and elsewhere. This will address the technical issues involved with the Alloy 600 cracking in the CRDM nozzles, inspection and repair techniques developed and deployed, and provide an overview of the inspection and repair operations performed. The paper will also touch on the industry and regulatory guidance and requirements set in place to address this issue