Opinion ID: 150668
Heading Depth: 3
Heading Rank: 2

Heading: NRC Bulletin 2001-01

Text: In 2001, small popcorn deposits of boric acid were found at the nozzle penetrations of the reactor at the Oconee Nuclear Station in South Carolina, a nuclear plant of similar design to Davis-Besse. Earlier cracks had been lengthwise, but the 2001 cracks were circumferential (around the nozzle), and above the J-groove weld and within the pressure boundary. This posed a risk that the nozzle would blow out of the vessel head and cause significant loss of coolant and structural threats, including possible plant safety failure. In the early 1990s, the NRC determined that nozzles were susceptible to stress corrosion cracking on the nozzles and on the welding but determined that the cracks did not pose an imminent safety threat because the NRC presumed that any leakage would be readily apparent before threatening the structural integrity of the reactor or catastrophic failure. The leakages occur when coolant escapes the containment vessel within the reactor and either exits the reactor or comes into contact with the hot vessel head. The result is that the coolant flashes to steam and the boric acid within the coolant fluid is left as a deposit on the reactor head near the leak. In 1997, the NRC advised licensees of this type of reactor to develop programs to periodically inspect the vessel head penetrations and look for cracks, but, because it was not yet aware of the problem, did not warn about the link between popcorn deposits and circumferential cracking. In light of the Oconee incident and similar experiences in the French nuclear industry, on August 3, 2001, the NRC issued NRC Bulletin 2001-01 (NRC 2001-01 or the Bulletin), entitled Circumferential Cracking of Reactor Pressure Vessel Head Penetration Nozzles. The Bulletin outlined which plants had a high susceptibility to nozzle stress cracking, and the NRC's criteria indicated that Davis-Besse was among them. The Bulletin also requested information from affected nuclear power stations such as Davis-Besse. The Bulletin stated that such plants need to use a qualified visual examination of 100% of the ... nozzles, that the inspection should be able to reliably detect and accurately characterize leakage from cracking, and that the effectiveness of the ... examination should not be compromised by the presence of insulation, existing deposits on the RPV head, or other factors that could interfere with the detection of leakage. Due to the risks, the NRC wanted all high-risk plants such as Davis-Besse to shut down and conduct a complete inspection for nozzle cracks by December 31, 2001. Because of the costs involved in an early and unscheduled shutdown, Davis-Besse wanted to continue operation until its scheduled RFO13 in April 2002. The Bulletin required plants to provide detailed information about susceptibility to cracking and previous inspections within thirty days. As part of that information, the NRC directed high-risk plants that, [i]f [the plant's] future inspection plans do not include performing inspections before December 31, 2001, [the plant must] provide [the] basis for concluding that the regulatory requirements discussed in the Applicable Regulatory Requirements section will continue to be met until the inspections are performed. Section 1.d. required all such plants to provide: [A] description of the [vessel head penetration] nozzle and RPV head inspections (type, scope, qualification requirements, and acceptance criteria) that have been performed at your plant(s) in the past 4 years, and the findings. Include a description of any limitations (insulation or other impediments) to accessibility of the bare metal of the RPV head for visual examinations.