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I-97-49

May 8, 1997

The Nuclear Regulatory Commission staff has authorized, subject to license commitments, the restart of Niagara Mohawk Power Corporation's Unit 1 of the Nine Mile Point Unit nuclear power plant, operated by Niagara Mohawk Power Corportion at Scriba, New York.

A staff safety evaluation concludes the unit is safe to operate for 10,600 hours (approximately 14 1/2 months), provided that reactor water chemistry is maintained consistent with Electric Power Research Institute guidelines to minimize the growth of vertical weld cracks in the unit's core shroud. The conditional approval also requires that, within 60 days, Niagara Mohawk must apply for a license amendment reflecting this water chemistry requirement. These conditions are legally binding and enforceable.

Niagara Mohawk found the vertical weld cracking through inspections conducted during the recent Unit 1 refueling outage. The company also found that four tie rod assemblies -- which were installed as a pre-emptive repair during the 1995 outage -- had lost some tension and that the lower clips on three rods were damaged.

The core shroud, which controls the flow of water through the reactor core, is a cylindrical stainless steel assembly that surrounds the core. Operating time, coolant chemistry, carbon content, neutron flux, residual stress from welding, and fabrication and operating stresses all contribute to intergranular stress corrosion cracking.

At a meeting between the NRC staff and Niagara Mohawk on April 14, the company presented its analysis of the cracking and also presented a description of the actions it had taken to replace the lower clips on the tie rod assemblies.

In addition to its technical analysis, the NRC staff also considered questions and comments made by the public at the April 14 meeting and in correspondence received since that meeting.

The company also has agreed to propose an inspection plan for the next scheduled outage and submit the plan to the NRC at least three months before the outage is scheduled to begin. The plan, which will specify the inspection methods to be used, will provide details on inspection of the shroud repair components and the shroud's horizontal, vertical and ring segment welds.

NOTE: The Safety Evaluation Report is available on the NRC's Web Site at http://www.nrc.gov/OPA/reports. Copies are also available from the NRC's Local Public Document Room in the Penfield Library at SUNY-Oswego.