The internals will eventually go wherever the fuel—uranium pellets encased in pencil-thin rods—goes. In theory, that will be Yucca Mountain, in Nevada, where the Department of Energy hopes to build a nuclear waste repository. In any case, the internals will wait in four giant steel-and-concrete casks, alongside 60 other casks filled with spent fuel.
These, on a six-acre plot, form the new Independent Spent Fuel Storage Installation. The ISFSI, one of the newer acronyms to enter the nuclear lexicon, is similar to those springing up at plants around the country. Maine Yankee’s has earthen berms around the 18-foot-high canisters, an electrified fence, closed-circuit cameras and a solid-looking guard building. If the Energy Department sticks to its latest schedule for finishing Yucca Mountain and accepting waste, which would be remarkable, the plot here will be in use for about 20 years. But it is expected to be far longer.
In fact, although the NRC refuses to certify the casks indefinitely, it is not clear what would make them unsafe to use over the next 100 years or more, except global sea-level rise or, perhaps, terrorism. Critics say the casks are vulnerable to attack. Some have suggested sheltering the canisters in the dome, but the owners counter that it is too small. Nuclear experts argue that breaking the canisters would be difficult and that the material inside, already at a low-enough temperature that it does not require mechanical cooling, is not prone to aerosolizing and spreading over large distances. The NRC says it believes the casks are safe, but in September 2002 the agency imposed new security rules on them; the rules are secret.
How Clean Is “Clean”? The fuel is an obvious problem. Much of the rest of the plant presents a more subtle one. Technicians made 14,300 measurements, a little more than half in areas where they did not expect to find contamination. On the other hand, certain parts were barely tested, such as the reactor cooling system, the emergency core cooling system, and the chemical volume and control system; these were presumed to be dirty. Some sampling was done by running a vehicle over the land at speeds lower than five miles an hour. Many samples were sent to off-site labs for more sensitive analysis than was possible using Geiger-Mueller detectors.
The residual radiation permitted by state and federal regulations was so low that plant managers concluded that they would have to determine what normal background was, lest they end up removing radionuclides that would have been present had the plant never been built. (For instance, one major source of background radiation is fallout from atmospheric nuclear tests, mostly cesium 137.) So they went to the headquarters of one of Maine Yankee’s owners, the Central Maine Power Company in Augusta, and sampled for beta activity on painted and unpainted concrete, ceramic tile, and asphalt.
While trying to discount natural background sources, managers also looked for the unnatural ones. As part of an agreement with a local environmental group, Friends of the Coast, they invited former workers back to Maine Yankee to discuss locations where materials had been dumped or spilled. The General Accounting Office (GAO), the investigative arm of Congress, lists this opportunity as a factor favoring prompt decommissioning.
Pressurized water reactors like Maine Yankee have multiple layers to hold in radioactive materials, but they always escape and turn up in odd places. In Maine Yankee’s case, that included cobalt 60 on the employees’ baseball field. (Decommissioning managers think it was brought there with snow plowed from the area immediately around the plant.)
A power reactor makes two kinds of radioactive materials. The dominant type is fission products. As nuclear plants run, they split uranium, which emits so little radiation that technicians handle raw fuel in nothing more than cotton gloves. But uranium splits into a dozen major kinds of fragments, which in turn decay into others. The fragments, and many of the decay products, are highly unstable. They readily give off energy—in the form of a gamma ray, an alpha or beta particle, or sometimes a gamma ray and a particle—to return to equilibrium. The fuel begins as a ceramic pellet wrapped in a metal tube and bathed in ordinary water. But in operation the ceramic fractures; at several plants, including Maine Yankee, the tubing leaked, allowing fission products to enter the cooling water. Many of these radioactive particles “plate out” on the interior of the vessel or on the piping.
In the pressurized-water design, the water that circulates past the fuel runs through giant heat exchangers, called steam generators, streaming inside thin-walled metal pipes, while clean water on the outside is boiled into steam, which then flows to the turbine. At Maine Yankee, those tubes leaked, too. And as is common at industrial plants, contaminated water was sometimes spilled into drains.
To cope with these fission products, plant technicians washed the piping with chemicals, lowering the radiation in the primary coolant loops fivefold. For surface-contaminated concrete, workers turned to “scabbling,” or blasting away the first quarter- to half-inch; dust was vacuumed out and went through a high-efficiency particulate air, or HEPA, filtration system.
Even if the tubes or the fuel had never leaked, there is a second kind of contamination: activation products, atoms that are struck by neutrons from the fissioning uranium, absorb the neutron and become unstable, or radioactive, instead of splitting. Technicians found evidence of activation products up to two feet deep into concrete. Over the years of operation, the reactor internals are generally so transformed by neutron irradiation that they must be treated as high-level waste.
According to the NRC, one of the dominant activation products and a major source of radioactivity aside from the fuel is cobalt 60. It is produced by the interaction of neutrons and cobalt 59 or nickel, both components of various metal alloys. There is a saving grace to cobalt 60: its half-life, or the period that it takes half the material to give off its particles and gamma rays and transmute itself to nonradioactive nickel 60, is just 5.27 years. In theory, workers could simply wait it out; in 21 years, 15⁄ 16 of the cobalt 60 would be gone.
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1 Comments
Add CommentIt would be scientifically necessary to see a fully independant report by an international commission on this subject before the results could be accepted as refereed and sound.
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