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.