26 December 2005Climate ArkMatthew L. Wald
Decades ago, scientists and engineers thought it would be easy enough to deal with the radioactive waste from nuclear power plants: sort out and save the small portion that was reusable, and put the rest in a hole in the ground.
It did not work out that way. Reprocessing the waste proved to be both expensive and risky: the main material being scavenged, plutonium, is a nuclear bomb fuel. And that hole in the ground - the proposed Yucca Mountain repository in Nevada - is years behind schedule, bogged down in politics and environmental disputes. Even if it opens, it will be far too small for the amount of waste that is being generated.So last month, Congress voted $50 million for the Energy Department to explore a new kind of reprocessing, one that would reuse a much larger fraction of the waste. The idea is extremely ambitious. It would require perfecting not only a new method of reprocessing, but also a new class of reactors to burn the salvaged material. Still, proponents said it would have two great advantages: It would mean that Yucca Mountain would be big enough to accommodate the waste that could not be recycled. And it would make Yucca easier to open, because the material still to be buried would generate less heat in the centuries to come. "Reprocessing, or processing spent fuel before it's put in the repository, is a very good way to buy time," said Roger W. Gale, a former Energy Department official who is now an electricity consultant. "It's a fail-safe in case we continue to have problems with Yucca Mountain." Many experts are skeptical that the new strategy, which would involve separating the components of spent fuel and putting the salvaged material in reactors using higher-energy neutrons, will work. Another former Energy Department official, Robert Alvarez, noted that the idea of reprocessing had been around for at least 40 years, each time with a different rationale. "Once, it was part of breeder program," Mr. Alvarez said, referring to a scheme to use reactors to make more nuclear fuel than the reactor consumed. "Then it became a proliferation thing," with supporters reasoning that such a system would safely consume materials that could be used for a bomb."And now it's a waste-management thing," he said. "But the whole problem is they're pouring money into something that's cutting-edge for the late 1960's."Some scientists argue that recycling is essential. At a recent Washington forum on nuclear waste and its possible uses, Phillip J. Finck, deputy associate director of the Argonne National Laboratory, an Energy Department complex, said that by 2010, long before Yucca Mountain can open (if, indeed, it ever does), the United States would have more than the 70,000 metric tons of fuel that will fit there. Moreover, Mr. Finck argued, without recycled fuel, the world will have to rely on finite reserves of uranium. At the forum, sponsored by the Foundation for Nuclear Studies of Washington, Ernest J. Moniz, a physics professor at the Massachusetts Institute of Technology and a former under secretary of energy, said that if the world built enough reactors to provide energy without contributing to global warming, a new Yucca Mountain would be needed every three and a half years. But Professor Moniz and others expressed caution about reprocessing. Frank N. von Hippel, a physicist at Princeton, said that a new generation of reactors would cost tens of billions of dollars and that it would be a long time before it was clear that reprocessed fuel was needed. The fuel to be reprocessed would be too radioactive to move very far; hence the idea was that the reprocessing plant would be adjacent to the reactor. Ivan Oelrich, vice president of the Federation of American Scientists, said that building scores of new reactors, with a reprocessing plant adjacent, was unlikely, and that while opening Yucca would be hard, switching to this kind of reprocessing was "trading one difficult political problem for an impossible problem." Still, concern over global warming and the increase in natural gas prices have given hope to nuclear advocates, who want new waste techniques as well as new reactors. The reprocessing strategy is subtle - to extract more use out of used fuel and to reduce the heat created by waste that cannot be recycled and still has to be buried.The heat is not a problem in the first few decades, when a repository could be left open for ventilation. The harder time is the next 1,500 years, when heat would be given off by longer-lived radioactive materials, mostly a category called actinides, and also the isotopes that are created as those actinides go through radioactive decay.
Heat, not volume or weight, determines the physical capacity of Yucca or any other underground repository, because designers want to keep the repository below the boiling point of water. Above the boiling point, the resulting steam could damage the containers and possibly the rock as well.Reprocessing means chopping up nuclear fuel and separating the ingredients, uranium that was not used in the reactor and other elements that were created in the reactor and could be used as fuel, including plutonium and neptunium. Gulf Oil tried to do that in the early 60's in West Valley, in upstate New York, but dropped it as uneconomical, leaving the taxpayers with a cleanup bill of more than $1 billion.At that plant, and at plants still operated in Britain and France, the plutonium is recovered by chemical separation. The new plan is for "electrometallurgical" reprocessing, in which giant electrodes are inserted in a mix of waste components, somewhat like electroplating.The salvaged materials include uranium 235, the isotope used in bombs, which splits easily, and uranium 238, which makes up more than 99 percent of uranium in nature but is harder to split.One use of uranium 238 in a reactor is as a "fertile" material that can absorb stray neutrons and become plutonium 239, which can be used in reactors and bombs. But existing reactors split the uranium using "thermal" neutrons. The new ones would use "fast" neutrons, which travel thousands of times as fast. The current generation of American power reactors uses water to slow the neutrons to the speed optimal for splitting uranium 235. The water also carries off the heat, which is used to make electricity. Fast neutrons, in contrast, have enough energy to split uranium 238. But to make use of them, reactors would need a heat transfer fluid that does not slow down the neutrons, probably molten sodium. The water-based reactors are kept under high pressure to keep the water from boiling. A sodium reactor could run with the sodium at atmospheric pressure. At some point, the sodium has to be run through a heat exchanger, a cluster of thin-walled metal tubes, to give off its energy to ordinary water, which turns to steam and spins a turbine for electricity. And if there is a leak and the sodium and water come into contact, the sodium burns. There are other problems. Plutonium and neptunium are potential bomb fuels; the risk that they might be illicitly diverted, or that other countries might follow the United States' example and build their own reprocessing centers, led two presidents, Gerald R. Ford and Jimmy Carter, to block General Electric from opening a reprocessing center in Morris, Ill.Further, the companies that run reactors are showing no interest in new kinds of reactors and little interest in plutonium. When the Energy Department decided to get rid of some surplus weapons-type plutonium by turning it into nuclear fuel, no utilities would take it, even at no charge.The Tennessee Valley Authority finally agreed to take the fuel. It described the transaction as selling the government "irradiation services."
