The saga of troubled tubing at California's San Onofre nuclear power plant could prompt a review of federal rules under which equipment is routinely replaced inside the nation's reactors, the country's top nuclear regulator said Wednesday.
San Onofre, located on the coast between Los Angeles and San Diego, has been shuttered for nearly four months while investigators try to determine why tubing that carries radioactive water in relatively new steam generators eroded at an unusual rate, in some cases rapidly.
At issue is whether a series of modifications to the generators, including increasing the number of tubes, should have triggered a more thorough review by the Nuclear Regulatory Commission. It's possible that the generator changes could be the cause of the excessive tube wear, which remains under review.
Speaking in North Carolina, NRC Chairman Gregory Jaczko said the problems at San Onofre suggest the rules under which the generators were installed might need to be changed.
The agency is reviewing whether Edison operated within those guidelines when modifying and installing the generators -- two in each reactor.
"We really need to take a look at this process, one way or another," Jaczko said at a news conference.
Operator Southern California Edison initially targeted a June restart for at least one of the twin reactors, but that appears increasingly unlikely as investigators continue to review the widespread problem.
The four generators were installed in 2009 and 2010 under a widely used rule that allows nuclear plant operators to change equipment or procedures without prior NRC approval, providing the change has only a minimal impact on safety and operation.
Engineers who modified the generators set out to meet the federal test to qualify the equipment as essentially identical replacements, which allowed it to be installed without prior NRC approval.
Among the changes, the new generators used a different tube alloy believed to be more resistant to cracking, and engineers designed V-shaped tube supports to minimize the potential for tube wear and vibration.
An environmental group, Friends of the Earth, has claimed Edison misled the NRC about the design changes, an allegation the federal agency disputes.
Test results show that two types of wear have occurred at both units. Tubes are rubbing and vibrating against adjacent tubes as well as against support structures inside the generators. Company officials have yet to say if the cause of the problem is related to design, improper installation or maintenance, or other issues.
The trouble began to unfold in late January, when the Unit 3 reactor was shut down as a precaution after a tube break. Traces of radiation escaped, but officials said there was no danger to workers or neighbors.
Unit 2 had been taken offline earlier that month for routine maintenance and refueling, but investigators later found unusual wear on tubing in both units.
Gradual wear is common in such tubing, but the rate of erosion in some tubes at San Onofre alarmed officials since the equipment is relatively new. The company has said 1,300 tubes are so damaged that they will be taken out of service. the number is well within the margin to allow the generators to keep operating.
The steam generators -- each with 9,727 alloy tubes -- function something like a car radiator, which controls heat in a vehicle's engine. The generator tubes circulate hot, radioactive water from the reactors, which then heats a bath of non-radioactive water surrounding them. That makes steam, which turns turbines to make electricity.