LOS ANGELES (AP) -- The utility that runs California's idled San Onofre nuclear power plant backpedaled Tuesday from an earlier claim that a retooled vibration-detection system for the plant's ailing steam generators would be an important safety advance that could help open the way for a possible restart.
Southern California Edison said in its October proposal to restart the Unit 2 reactor that the redesigned system, which relies on monitors to detect unusual vibration inside the huge generators, could help detect a break in a tube that carries radioactive water, according to federal documents.
Excessive tube wear has been at the heart of problems at San Onofre, which hasn't produced electricity since January, after the plant was abruptly shut down after a tube break released a trace of radiation. Investigators later found excessive wear on hundreds of tubes in the virtually new generators.
Edison officials came under sharp questioning about the monitors at a Nuclear Regulatory Commission panel meeting in Maryland, where an NRC official argued that the equipment could not do the job described by the company or provide additional safety if the plant is restarted.
"The instrumentation that you're proposing ... does not appear to be capable of detecting the conditions that would lead to actual tube wear," said Richard Stattel of the agency's instrumentation branch.
The company depicted the equipment in its restart plan as an important safety measure "but it doesn't appear to do that," Stattel said. The NRC staff "doesn't understand where that adds an additional safety margin" as proposed by the company.
Mike Short, an Edison consultant, told regulators that the company "had not intended" to characterize the system as an important safeguard, technically known as "defense-in-depth," or multiple layers of systems designed to prevent accidents or the release of radiation from a nuclear power plant.
Short said the data collected by the system could be used in future research examining vibrations picked up by the monitors. "It's our plan ... to make sure that's clear," he said.
The agency is in the midst of reviewing Edison's plan to rekindle one of two damaged reactors. The company wants to run Unit 2 at reduced power, a change that company engineers believe will reduce vibration that damaged tubes in the Japanese-manufactured generators.
A decision could come as soon as March. Critics of the nuclear industry have depicted twin-domed San Onofre, located between San Diego and Los Angeles, as a catastrophe-in-the-making.
The original monitoring system was at issue in a federal investigation after the plant was shut down in January.
NRC officials sifted through months of data to determine if Edison properly analyzed a series of mysterious vibrations detected inside the now-crippled Unit 3 reactor. Thirty times over 11 months, monitors positioned in the reactor's two steam generators triggered alarms after sensing unusual movements, according to documents and Nuclear Regulatory Commission officials involved in the probe.
At the time, it raised the questions about whether Edison missed possible clues that something was terribly wrong inside the generators. However, the agency later determined that the vibrations were not connected to tube-to-tube wear, according to NRC officials.
According to an analysis by an outside contractor that reviewed some of the data, the signals picked up by the Unit 3 monitors were similar in nature to what would occur with steep temperature changes when a reactor is starting up or shutting down. But, strangely, the vibrations were detected when the Unit 3 reactor was running at a steady clip.
The monitors, technically known as accelerometers and designed to detect loose or broken parts, were positioned near the bottom of the 65-foot high generators. In that location, federal officials say, it would be difficult or even unlikely to pick up vibration and friction among tubes at the other end, where damage was concentrated. And while 30 alarms were recorded in Unit 3, none was detected at its sister, Unit 2, which is the same Mitsubishi Heavy Industries design.
The redesigned system is expected to be more sensitive.
Earlier this year, federal officials blamed a botched computer analysis for design flaws that are largely to blame for unprecedented wear in tubes at the plant. They say a Mitsubishi analysis vastly misjudged how water and steam would flow in the reactors.
Gradual wear is common in steam generator tubing, but the rate of erosion at San Onofre stunned officials because the equipment is relatively new. The generators were replaced in a $670 million overhaul and began operating in April 2010 in Unit 2 and February 2011 in Unit 3.
Overall, records show investigators found wear from friction and vibration in 15,000 places, in varying degrees, in 3,401 tubes inside the plant's four generators, two in each reactor.
The plant is owned by SCE, San Diego Gas & Electric and the city of Riverside.
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