Are new reactor designs the answer?

Driven by the notion that splitting atoms will remain a vital way to generate electricity, the United States nuclear industry is taking a close look at new reactor technologies for the 1990s and beyond. Given public attitudes toward nuclear energy, especially after last April's reactor accident at Chernobyl in the Soviet Union, the quest for next-generation reactors may seem a bit curious.

``The climate is not particularly hospitable for major new initiatives in the nuclear area,'' says Richard K. Lester, associate professor of nuclear engineering at the Massachusetts Institute of Technology. ``But there is an argument to be made for pursuing nuclear research at a politically feasible level. We shouldn't assume that the current public view is immutable.''

Broadly speaking, current research is aimed at simplifying designs and improving the safety of large reactors. In addition, companies are exploring designs for small reactors that make extensive use of ``hands off'' safety techniques to cope with potentially serious accidents. The goal is to offer nuclear power plants that are safer and less expensive to build and operate, and that take less time to clear regulatory hurdles.

How fast any of these designs come on line depends on a host of related factors: demand for electricity, regulatory changes, the willingness of utilities to buy into new technologies, and the thorny problem of where to put high-level radioactive waste. Perhaps most of all, flipping the switch on new nuclear power plants hinges on convincing a skeptical public that they are safe and economical.

At present, the most widely used reactors rely on so-called light water as a coolant and moderator to ensure that the nuclear chain reaction is sustained.

In boiling-water reactors, the water in the reactor vessel is converted directly into steam, which is piped through turbines that run the plant's generators. The steam is then condensed, and the resulting liquid returns to the reactor vessel to begin its trip again.

Pressurized-water reactors, on the other hand, don't directly drive the turbines. Instead, the water in the reactor vessel is put under extreme pressure - about 2,000 pounds per square inch - to keep it from boiling away. The water is run through a steam generator, where it transfers its heat to a second ``loop.'' The secondary loop's water is converted to steam, which runs the turbines. Water in the primary loop does not leave the reactor containment building, but runs back through the reactor after leaving the steam generator.

If a reactor loses its coolant, it shuts down. But emergency cooling must still be provided to carry off the heat generated by radioactive decay in the core. Without the ability to remove decay heat, the core could overheat and the fuel melt, releasing radioactivity.

Currently, the safety systems designed to provide coolant to reactors depend on complex networks of pumps, valves, and diesel generators, as well as coolant sources outside the containment building. These systems may be technically sound, but they don't give operators a lot of time to deal with a serious accident, such as loss of coolant, before the core is threatened.

Enter the alternatives. Each in its way tries to make better use of the laws of physics to buy time for the operators who must deal with breakdowns in the cooling system or a sudden loss of coolant in the reactor itself. These alternatives include:

Advanced light-water reactors. These represent evolutionary changes from current technology. In general, they try to put more water over the reactor core by deepening the reactor vessel or by building into the vessel devices that were once outside it and represented a potential source of coolant leaks. Some designs increase the physical size of the reactor core to ensure that if coolant is lost, the core takes longer to heat up. These changes, combined with simplified designs in related systems, make advanced light-water reactors (LWR) some 10 to 100 times safer than current LWR designs, says Alvin Weinberg, a nuclear specialist with the Institute for Energy Analysis.

High-temperature, gas-cooled reactors. These represent a more radical departure from water-cooled designs. They use graphite as a moderator, the fuel consists of uranium pellets encased in ceramic, and the coolant is high-temperature helium. The graphite core and the ceramic-clad fuel can withstand very high temperatures, giving reactor operators hours instead of minutes to correct conditions leading to an accident. The helium coolant does not become radioactive, and it doesn't corrode metal parts at the reactor's operating temperature. And because the reactor, steam generator, and related components are encased in solid concrete, a pipe break in the primary cooling loop won't result in a sudden loss of coolant. The thick concrete reactor vessel also serves as a good shield against the release of radioactivity.

Because this type of reactor operates at high temperatures, a power station using such a design could generate steam for industrial use at the same time it was producing electricity, says Scott Penfield, director of planning and control at the Gas-Cooled Reactor Association, made up of 32 interested utilities.

Liquid-metal, fast-breeder reactors. These reactors use liquid sodium as coolants. With a boiling point of 1,600 degrees F., the liquid sodium can easily absorb the heat from the reactor without having to be pressurized. Because it's a metal, it also conducts heat efficiently. And if the pumps circulating the sodium through the reactor should fail, convection would keep the metal flowing through the reactor. Natural air circulation would also draw the heat away from the outside walls of the reactor.

Each design has its advantages and disadvantages. But industry analysts say that the various passive safety elements can be used to best advantage when reactor output is limited to less than 600 megawatts. The reason, according to Karl E. Stahlkopf of the Electric Power Research Institute's nuclear power division, is because passive safety design means working with larger physical dimensions - a much less daunting prospect when the reactors are small. By contrast, many of the reactors in use today generate power on the order of 1,000 megawatts.

General Electric and Westinghouse are exploring small light-water reactors that would rely on gravity, instead of pumps, to flood the reactor if it lost its primary coolant. In addition, their designs incorporate various passive schemes to get rid of decay heat once the core has been flooded.

Proponents of small high-temperature, gas-cooled reactors say units can be designed so that if the reactor loses its coolant, the temperature of the core would never exceed safe limits. Indeed, the concept has earned a compliment of sorts from Robert Pollard, a nuclear safety engineer with the Union of Concerned Scientists. He says that if it's desirable for the US to build new nuclear power plants, a point he is not ready to concede, modular gas-cooled reactors would be the best technology to pursue.

As for liquid-sodium-cooled reactors, Yoon Chang, general manager of the integral fast-reactor program at Argonne National Laboratory, recalls an experiment conducted last year at a US Department of Energy liquid-sodium reactor in Idaho. ``We shut down the cooling pumps at full power,'' he says. ``The reactor shut itself down without any operator intervention'' and natural circulation kept the core temperature within safe limits. ``Any other reactor would have had a meltdown.''

Next: Alternatives - feasible but not funded.

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