Canned Heat

This is part of IEEE Spectrum's special report: Nuclear Power Gets a Second Look

Illustration: Steve Stankiewicz

A metal cask can store up to 32 fuel assemblies--about six months' supply--and weigh over 100 tons when full. Click on the image for the full illustration view.

Of the many problems that brought the nuclear power industry to its knees in the 1980s and 1990s, two in particular seemed insurmountable: deep public antipathy and the lack of a way to dispose of high-level waste. At the time it was hard to say which was more onerous. Finally, the answer has become clear: it was the waste problem, by a long shot.

Fifty-nine years after the dawning of the nuclear age, not one country has managed to find anything more than a temporary resting place for its tons of nuclear detritus. In the United States, a sympathetic administration and increasingly favorable opinion polls have given the moribund nuclear industry new optimism, at least. But it's a long way from good feelings to reactor orders, and an even longer way to commercial viability.

Improvements over the past couple of decades in the design and operation of power reactors and scattered shortages of electricity this past summer have made the waste problem the biggest obstacle to any resurgence of nuclear power. Such a reemergence is needed, supporters say, to reduce acid rain and to keep more than a billion tons of greenhouse gases from fossil fuel plants out of the atmosphere over the next 10 or 20 years. It would also help reduce the dependence of the U.S., European, and Japanese economies on oil, thereby lessening their exposure to the seemingly endless cycles of instability, violence, and terrorism in parts of the Middle East.

But there is no agreement anywhere as yet on a permanent solution for dealing with the waste, which will remain radioactive enough to harm human beings for thousands of years. Still, consensus over a temporary fix has finally emerged in the United States, Europe, and Japan. With dry-cask storage, as the technique is known, workers seal spent nuclear fuel or high-level waste in metal or metal-and-concrete containers that are guarded and monitored, typically in a fenced-in compound or warehouse near the reactor where the fuel was used. The casks are roughly 5 meters tall, 2.5 meters in diameter, and weigh more than 100 metric tons when loaded.

After decades of uncertainty, utilities finally have a direction, at least, and a sorely needed one. At 30 of the 103 operating power reactors in the United States, the pools where spent fuel is cooled and stored will run out of room by 2004, according to the Nuclear Energy Institute, the Washington, D.C.-based trade organization.

Utilities all over the world are grappling with the same problem. Besides recent dry-cask installations in Canada, Czechoslovakia, Lithuania, Switzerland, Belgium, Japan, and South Africa, others are under construction in Spain, Italy, Armenia, and Hungary, according to Wolfgang Sowa, managing director of Gesellschaft für Nuklear-Behälter mbh (GNB), a cask maker in Essen, Germany. Among the many planned installations is one for the Chernobyl nuclear plant in Ukraine.

Growth industry

With or without a resurgence in the nuclear industry, the dry-cask business is poised for serious expansion over the next several years. In the United States alone over the next 20 years, the market for casks could be worth up to US $10 billion, estimated David C. Blee, group executive vice president of NAC Worldwide Consulting. The Atlanta, Ga., nuclear-fuel consulting concern is affiliated with NAC International, a cask maker. Even if reactors are decommissioned rather than built in the near future, it will simply mean that much more spent fuel must be put into casks.

"People have been waiting for this business to mature for 15 years," said Blee. "Clearly, it has now matured."

Dry-cask storage is not exactly new. It was invented almost 20 years ago by GNB's predecessor in Germany, where nearly 490 casks have been loaded. Also, in the United States, some 240 casks are storing waste at 16 power-plant sites. What is new, besides the increased demand, are casks that can be used to transport the waste as well as store it. That feature is important to utilities, which look forward to a day when they can ship the waste off to a federally run underground repository, where it will eventually be shut away for all time.

In fact, most governments [see table] are planning to build permanent repositories for the waste. But if the contentious, decades-long struggle to build a repository in the United States is any indication, dry-cask storage on a utility's site could prove less temporary than nuclear utilities hope.

Click on image for full view of chart.

In the United States, under a contract signed in 1983, nuclear utilities paid more than $15 billion into a Nuclear Waste Disposal Fund, with the understanding that a repository would be ready to start accepting their waste on 31 January 1998. The U.S. Senate decided in 1987 to narrow the possible sites for the dumping ground to one, at Yucca Mountain in the Nevada desert. Fourteen years and more than $4 billion later, the site boasts a great deal of state-of-the-art digging machinery, a series of tunnels, and a pile of geological analyses. But there is no repository.

Opposition from the state of Nevada and antinuclear activists, who insist that the repository would eventually leak its radioactive contents, has so far blocked the deal. A majority vote in both houses of Congress could override the objections of the Nevadans; such a vote is expected some time next year. But experts say it would be a decade after that, at least, before a repository could be completed and begin accepting waste at Yucca. By that time, 78 of the country's 103 reactors will have run out of room in their cooling pools, the Nuclear Energy Institute says.

The longer the delay, the better, according to antinuclear activists, who consider dry-cask storage the lesser of two evils. "It isn't a really good solution, but it's better than the transportation schemes to move the waste to a dump [repository] that we know will eventually leak," declared Wenonah Hauter, director of Public Citizen, an advocacy group in Washington, D.C.

Hot stuff

Dry-cask storage may sound simple; it is not. When an assembly of spent nuclear fuel rods exits a nuclear reactor, perhaps 18 months after it entered, it is radioactive enough to deliver within a second a lethal dose to any unfortunate who approaches without proper shielding. If the fuel were to be exposed to the air, its temperature would soar to well above 1000 °C--hot enough to melt anything it came in contact with.

To preclude that possibility, workers put the assemblies in 12-meter-deep pools, where chilled water circulates to hold down the rods' temperature. Boron atoms in the racks holding the spent-fuel assemblies and in the water itself absorb neutrons, putting a damper on nuclear reactions and limiting the amount of radiation that can escape from the pool. Gamma rays are absorbed by the water, which converts them to heat.

PHOTO: JIM MONE/AP

Twelve metal casks store spent nuclear fuel at the Prairie Island plant in southeastern Minnesota. A 1994 agreement with a local Indian tribe permits only five more casks to be added to the site.

As their pools filled up with spent fuel, some utilities contemplated building more of them. Several actually did so, at reactors in Belgium, Germany, Finland, Slovak Republic, and Bulgaria. Most utilities opted not to build another pool, however, because in comparison with dry-cask storage, pools are more difficult to license and much more costly to maintain and operate.

A dry-cask installation starts with a place to put the casks--a simple concrete pad or warehouse, usually, along with security fences, alarm systems, and other necessities. Generally, the price tag for these facilities is about $10 million to $20 million. The casks themselves cost close to $1 million apiece. Each cask can hold 10 to 15 metric tons of spent nuclear material, an amount that would have kept a 1300-MW reactor going for perhaps six or eight months.

The latest models have three main functions: containing the radioactivity, conveying the heat away, and preventing an uncontrolled nuclear chain reaction (called a "criticality") by absorbing neutrons and by holding the spent fuel assemblies securely, for example, during transport.

The containers are either full-metal casks, {see photo] which have metal walls up to 38 cm thick, or canisters, which have much thinner metal walls and are designed to be placed in thicker concrete or metal "overpacks" for storage.

Look, but don't touch

Both cask types are loaded with waste in much the same way. After fuel assemblies have been in the pool for five or six years, they are placed in baskets in the cask or canister. For safety, the operation is carried out under water in the cooling pool. Then the container is removed from the pool, vacuum-dried inside, and filled with helium at low pressure. The inert gas conducts heat away from the hot fuel much better than air would and does not corrode the cladding that covers the spent fuel. After loading, the full-metal casks are almost 100 C at their outer surface.

In the metal designs, the thick walls block the gamma rays, while in the canister models, the concrete overpack serves that function. In either, a hydrogen-containing material, such as polyethylene or a resin of some kind, slows down the fast-moving neutrons so that the metal can absorb them.

According to an official of one of the European cask manufacturers, the dose rates at the outside surface of a fully loaded cask are about 500 microsieverts per hour. An unprotected adult could stand 2 meters from a cask, where the dose rate is 100 µSv per hour, and it would take 10 hours to accumulate the annual dosage permitted by the International Commission on Radiological Protection, in Stockholm, Sweden. People who are not nuclear workers would not receive those kinds of doses, however, because fences, berms, or walls around the compound would cut their exposure to almost nothing.

Metal casks are manufactured by the German company, GNB, and by Transnuclear Inc., in Hawthorne, N.Y., partly owned by Paris-based Cogéma. Canister-and-concrete models are made by NAC International, in Atlanta, Ga.; Holtec International, in Marlton, N.J.; BNFL Fuel Solutions, a subsidiary of British Nuclear Fuels, in Risley, England; and by Transnuclear West Inc. (TN-West), Fremont, Calif., another U.S. property of Cogéma.

Both the canisters and the metal casks have failed at times, but without releasing any radioactivity. Most of the canister problems involved a single faulty design, the model VSC-24 manufactured by Sierra Nuclear Corp., which is no longer on the market. (Sierra Nuclear has since been acquired by BNFL Fuel Solutions.) In the mid-1990s, cracks developed in the welded lid seals of at least five of these units. Engineers at three of the utilities that had purchased the containers--Consumers Energy in Michigan, Wisconsin Electric Power, and Arkansas Power & Light--solved the problem by devising new welding procedures.

In May 1996, a Wisconsin Electric technician lit an arc to begin welding a lid onto a VSC-24 canister and triggered an explosion powerful enough to tilt the 2900-kg lid. The welder was not hurt. Investigators later found that the coating used by Sierra on the baskets and the canister's interior had reacted with the acidic water from the cooling pool, producing explosive hydrogen gas that collected inside the canister.

Inspectors from the U.S. Nuclear Regulatory Commission, Rockville, Md., also found unspecified deficiencies in another canister model, produced by Vectra Technologies (now TN-West).

As for the metal types, over the past couple of years leaks have been detected in the secondary O-ring seals of three metal casks at the Surry Nuclear Power Plant in Virginia. Two of the casks were Transnuclear models and the other one was from GNB. According to an engineer at Dominion Virginia Power, which operates the plant, the O-ring seals on the casks had been corroded by salty air from the neighboring estuarine river. Yet another GNB cask was hit by lightning, which knocked out a pressure sensor but left the cask otherwise undamaged. Engineers solved the O-ring problem by replacing the aluminum-clad O-rings with more corrosion-resistant silver-clad ones, the engineer said.

The long haul

Currently the Nuclear Regulatory Commission issues licenses for dry casks that are good for 20 years, but is likely to allow utilities to re-license them for another 20. In fact, a consensus is emerging that the casks could safely contain the fuel for much longer. "At this point we haven't identified anything that would keep them from going to 100 years," said M. Wayne Hodges, deputy director of the spent fuel project office at the commission. Presumably, long before then, the casks' contents will be transferred to some other kind of container for storage in an underground repository.

Given the recent history of Yucca Mountain, however, the casks' apparent longevity both reassures and scares utilities, an indication of the strangeness of repository politics. It's good news that the casks are fundamentally sound, but it also raises the possibility, almost unthinkable to utilities, that the casks could languish for decades, like tombstones for long-gone reactors--an enduring responsibility and a small but persistent financial drain.

There are rationales other than economic ones for putting the waste in the ground. "You don't want to rely on institutions and governments for the long-term security of nuclear waste," said Thomas Cochran, a nuclear physicist in the Washington, D.C., office of the Natural Resources Defense Council.

Before the end of this year, Secretary of Energy Spencer Abraham will likely recommend to President George W. Bush that a repository be built at Yucca, insiders told IEEE Spectrum. Bush will probably accept the recommendation; the Nevada Congressional delegation will object; but some time next year, Congress will probably try to override the objection. Even if it does, though, it is not clear that the Nevadans will have then exhausted their legal options for blocking the repository at Yucca.

No one really expects spent fuel to sit on those pads, monitored and guarded and with periodic changes of cask, until it is twice as old as the Sphinx in Egypt is now. But it is probably just as well that the casks can apparently hold the waste for a century or more.