Jumper cables and shrink-wrap

The imperfect science of tuning up an old nuclear power plant

NUCLEAR REACTORS FACE the highest risk for catastrophe when they are new, and again at the end of their expected lifetime, when they begin wearing out, the Union of Concerned Scientists says.

Most of today's aging reactors are crossing into the geriatric phase. If they were people, they'd be getting tested for glaucoma and osteoporosis. But instead, many reactors that were designed to last 40 years are being given license renewals to run for another 20 or more.

What conditions are these nuclear reactors likely to face in the coming years? What procedures keep them running once the parts begin to fail?

We asked Dave Lochbaum, a nuclear safety scientist with the UCS. Last year Lochbaum penned a report titled "U.S. Nuclear Plants in the 21st Century: The Risk of a Lifetime."

He explained how neutron particles expelled from the reactor core collide with the steel and concrete that hold a nuclear reactor together. When a neutron passes through steel and concrete, it alters the structure the same way it would if shot through the body of a human being. Over time this process weakens these materials, causing them to become brittle and frail.

This becomes problematic when something (like a broken pipe) causes the reactor to overheat and the reactor cooling system puts 40-degree water in contact with metal that's been cooking at 500-degree temperatures or higher.

"Early in life that stress is no problem," Lochbaum told us. "As it becomes more and more embrittled, you get to the point where the metal can crack, kind of like hot glass under a cold running tap."

A reactor vessel in such disrepair can be fixed through a process called thermal annealing, or it can be replaced. Lochbaum said neither has actually been tried on a commercial reactor in the United States.

But here's how it would work:

To repair a reactor vessel using thermal annealing, workers would string electrical cables around the giant reactor vessel and pump it with high-voltage current until it gets hotter than during nuclear fission. They'd keep the current running for about a week, which, in theory, would restore the metal to its pre-irradiated toughness.

Thermal annealing has been tried before in Germany and Japan, but Lochbaum said there were major problems. For starters, it's superexpensive: It takes a tremendous amount of energy to heat a reactor vessel (and with California's creaky electric-power grid, it's not even clear that there would be enough left to power houses and businesses). Another hitch: If the vessel doesn't get hot enough and doesn't stay hot long enough, the process doesn't work.

Replacing a nuclear vessel might be even nuttier. A nuclear vessel is too big to fit through the door of a power plant like Diablo Canyon, so workers would have to cut a chunk out of the building's concrete wall and lift it out with cranes. First, though, they would wrap the vessel with blue, nuclear-grade shrink-wrap (made of the same material we use to wrap old cheese, only thicker, treated, and more expensive), which is supposed to keep radioisotopes from escaping while the thing is in transit. Then they'd bury it – someplace where it wouldn't come into contact with any living creature for many, many thousands of years. So far, nobody knows where that place might be.

MH