By Thomas W. Overton, JD
San Diego, 16 March 2012 — Yesterday, the NRC announced that it is sending a special team to San Onofre after several steam generator tubes in one of the reactors failed an inspection. This isn’t good, but probably sounds worse than it is.
First, full disclosure and some background: I am a former Navy nuclear reactor mechanic, and I helped perform an underway steam generator inspection while I served on the USS Enterprise.
There are two operating reactors at San Onofre. Both are pressurized water reactors, which means there are two loops operating when the plant is up: one high-pressure loop that transfers heat from the reactor to a pair of steam generators, and a second that runs the steam from the generators through turbines to generate electricity. The steam generators operate like huge submerged radiators. Hot water passes through a series of tubes, and the heat boils water on the other side of the tubes. All four steam generators at San Onofre were replaced a couple of years ago, so they’re essentially brand new.
Unit 2 has been shut down since January for scheduled maintenance. During a routine inspection of the steam generators, some unusual wear was detected in about 800 of the nearly 20,000 tubes, meaning that the tube walls were showing some thin spots.
Then, last month, one of the tubes in Unit 3, the other reactor, sprang a leak (Unit 1, the plant’s first reactor, was retired in 1992). The leak resulted in a very small release of radioactivity (barely detectable, according to the NRC), and the reactor shut down automatically as it should have. Subsequent inspection of the steam generator tubes showed 129 had unusual wear. Those tubes are being subjected to additional pressure testing for structural integrity, and three of them have failed. That’s what brought in the NRC yesterday.
Not good, but not Fukushima either. The thing to understand is that tube wear is a fact of life in running a steam generator. You inspect them regularly, and plug the ones that show too much wear. The generators are designed so that they can still function with a substantial number of tubes taken out of service; in the case of San Onofre, there is about an 8% reserve that can be plugged and still reach 100% power.
Tube ruptures are unusual, but also not unheard of. Typically, though, they happen as a steam generator gets close to the end of its design life. The ones at San Onofre are new, and that’s what’s got the NRC concerned. The reactors are going to remain shut down until the cause of the unusual wear can be determined.
And that is the real problem here. San Onofre supplies about 20% of the region’s electricity at full power. Right now, after a fairly mild winter, that’s not an issue. It will be an issue as the weather starts to heat up. Rolling blackouts are not out of the question, though planning has already begun to find ways to take up the slack this summer.
What’s wrong with the steam generators? My guess is that either there are some manufacturing defects in the new units (and the manufacturer, Mitsubishi Heavy Industries, is involved in the inspection right now), or swapping out the generators changed the flow characteristics through the primary loop in such a way as to generate unanticipated water erosion. Let’s hope the NRC can figure things out quickly.
Thomas W. Overton, JD is GAS POWER’s editor.