The Nuclear Regulatory Commission has issued a bulletin (Bulletin 2001-01, "Circumferential Cracking of Reactor Pressure Vessel Head Penetration Nozzles," dated August 3, 2001) seeking information from several nuclear power licensees regarding the structural integrity of reactor pressure vessel head penetrations.

The bulletin has been sent to the licensees of 69 pressurized water reactors (PWRs) as the result of recent discoveries of cracked and leaking penetration nozzles in the top of reactor pressure vessels. These nozzles are fabricated from Alloy 600 material.

These discoveries have raised concerns about the structural integrity of reactor penetration nozzles in the top of reactor pressure vessels at PWRs throughout the industry. Control rod drive shafts pass through penetration nozzles which sit at the top of a reactor vessel head. Control rod drive mechanisms (CRDMs) are used to guide the movement of control rods in and out of a reactor core.

Axial cracking (along the length) of these nozzles has previously been observed but not considered a safety concern requiring immediate attention. This issue was addressed by the NRC in Generic Letter 97-01, issued in 1997. Inspections and repairs have generally been made during plant maintenance outages. However, in 2001, circumferential cracking (part way around the width of the nozzle, above the structural retaining weld) was discovered at several PWRs.

One function of the nozzles is to maintain the reactor coolant system pressure boundary. Cracking of the control rod drive mechanism nozzles represent a degradation of the primary reactor coolant system boundary, and hence, is potentially safety significant.

NRC requested information from PWR licensees to determine whether current inspection practices are adequate and whether any additional regulatory response by the agency is necessary.

The NRC bulletin requested PWR licensees to provide information related to the structural integrity of these reactor pressure vessel head penetration nozzles, "as needed to ensure the integrity of their reactor coolant system pressure boundaries and demonstrate compliance with applicable regulatory requirements." The bulletin also required that PWR licensees submit a written response indicating whether the requested information will be submitted within the required 30-day time period. NRC licensees unable to satisfy this requirement had to submit a written description of any alternative course of action they proposed to take, including the basis for its acceptability.

Hot Leg Axial Cracking at the V.C. Summer Nuclear Station

The hot leg weld axial cracking was identified at V.C. Summer on October 7, 2000, during a normally scheduled inspection of the V.C. Summer containment after entering a refueling outage. The licensee found over 200 pounds of boric acid crystals on the containment floor and protruding from the air boot around the "A" loop reactor coolant system (RCS) hot leg pipe. Examinations showed a short through-wall axial crack in the hot leg nozzle safe end weld, approximately 3 feet from the reactor vessel. Additional examinations of the other 5 nozzle safe end welds found crack indications but no through-wall cracks. A 12-inch long section of the hot leg pipe containing the leaking weld was replaced with a new section of stainless steel pipe and Alloy 52/152 welds. The remaining weld indications were analyzed and found to be safe for another cycle of operation.

In a letter dated December 14, 2000, Mr. David J. Modeen of the Nuclear Energy Institute (NEI) informed Dr. Brian W. Sheron of the NRC that the EPRI Materials Reliability Project (MRP) intended to lead the industry's actions to address the generic implications of the cracking seen at V.C. Summer. The MRP is a utility-directed oversight organization of the PWR Owners Group whose purpose is to address and resolve, on a consistent industry-wide basis, PWR material-related issues.

CRDM Nozzles Circumferential Cracking at
Oconee Nuclear Station

Cracking in the Control Rod Drive Mechanism (CRDM) nozzles and seal welds at Oconee Nuclear Station was first noted on Unit 1 when small amounts of boron residue was found on the top of the reactor pressure vessel (RPV) head in November 2000, during a refueling outage inspection. Similar problems were found on Unit 3 in February 2001, during a planned maintenance outage, and on Unit 2 in May 2001, during a refueling outage. Subsequent examinations of the CRDM nozzles with boron residue found through-wall axial cracking in these nozzles, and through-wall circumferential cracking above the weld in two of nozzles in Unit 3, and in one nozzle in Unit 2.

Since April 2001, the NRC staff has held a series of public meetings with the industry at NRC Headquarters to discuss Circumferential Cracking of CRDM Nozzles.