Workers enter San Onofre nuclear reactor, prepare inspection

A reactor at the San Onofre nuclear plant has cooled down enough for workers to install inspection equipment Monday that may help find a leak among generator steam tubes.

Heat transfer tubes are under scrutiny at the oceanfront nuclear plant 45 miles north of San Diego nearly a week after radiation alarms prompted a shutdown of the southernmost of two reactors.

Small traces of radioactive gas from that leak may have reached the atmosphere without endangering workers or neighbors, according to plant operator Southern California Edison and officials at the Nuclear Regulatory Commission.

At the same time, concerns about unusual wear on recently installed generator steam tubes at the northern reactor have emerged as workers perform routine maintenance and refueling.

A first round of tests by the plant operator has confirmed concerns about accelerated wear on a small portion of the nearly 20,000 tubes within two generators at the northern reactor, Edison spokesman Gil Alexander said Monday. He declined to provide detailed findings on the number of tubes affected or the extent of accelerated wear.

According to the nuclear agency, more than a third of the wall had been worn away in two tubes, which will require them to be plugged and taken out of service.

In another 800 tubes, the thinning was between 10 percent and 30 percent. Without repair, such thinning could lead to leaks.

Generators in both reactors were replaced over the past two years with new equipment manufactured in Japan by Mitsubishi Heavy Industries, at an overall cost of $670 million.

Reactors can continue to work effectively even as hundreds of tubes are gradually plugged during maintenance and refueling shutdowns.

One expert and former plant worker has estimated the unplanned shutdown of the southern reactor on Jan. 31 could be costing more than $600,000 a day in replacement energy costs. Edison said it does not yet have a cost estimate.

The plant at San Onofre supplies about 20 percent of the area's energy needs and is owned by Edison, San Diego Gas & Electric and the city of Riverside.

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