US nuclear regulators are considering a proposal from Southern California Edison (SCE) for a trial restart of unit 2 at the San Onofre Generating Station (SONGS) at 70% power. The two unit plant has been off line since the discovery of unexpected wear in steam generator tubes.
|San Onofre (Image: SCE)
After more than 170,000 inspections and expert consultations on proposed corrective actions, SCE has concluded that unit 2 at the plant can be operated "safely and within industry norms," according to company president Ron Litzinger. The US Nuclear Regulatory Commission (NRC) issued the company with a formal Confirmatory Action Letter (CAL) on 27 March, to formalize the actions that SCE had undertaken to address problems in steam generators at the units, and SCE has now submitted its response to that letter, and its restart plan for unit 2.
The steam generator problems came to light in January, when a leak was discovered in a steam generator at San Onofre unit 3. Unit 2 was already in outage at the time, but as both SONGS units had undergone steam generator replacement in 2009-2010, the operator and the regulator became concerned that the cause of the failure might be common to both units. Unit 2 therefore remained offline.
Problems with excessive vibration and tube-to-tube fretting were subsequently identified as the major cause of the unusual wear in unit 3's steam generators. The same conditions of high steam velocity and low moisture conditions which triggered the problems in unit 3 are also present in unit 2, making it susceptible to similar vibrations. However, all but two of the 20,000 tubes in unit 2 are known to have been effectively supported during the 21 months it has operated, SCE says.
SCE has plugged six of unit 2's steam generator tubes showing wear greater than 35% and plugged a further 500 tubes as a preventative measure, and is now proposing to restart unit 2 for a five month operating period at 70% power. The reduction in power will decrease steam velocity and increase moisture content in the steam generators, preventing the establishment of the vibration-causing environment, the company says.
After five months of operation, the unit would be shut down for inspections to confirm the continued structural integrity of the tubes, measure tube wear and to confirm the efficacy of the solutions.
SCE also proposes implementing additional monitoring, detection and response activities including installing early warning monitors to detect extremely small tube leaks faster, more sensitive vibration monitors, additional monitoring and analysis systems and enhanced operator training.
The plant cannot restart without approval from the NRC, a process which NRC chairman Allison Macfarlane has warned could take months to complete. "Our primary focus now must be on analyzing SCE’s response to the CAL before addressing the restart question. The agency will not permit a restart unless and until we can conclude the reactor can be operated safely," said Macfarlane, promising that the inspection and review process, which will include public meetings, would be "painstaking, thorough and will not be rushed."
Researched and written
by World Nuclear News