The rising price of petroleum and the growing worldwide demand for energy is fueling a renewed interest in nuclear power as a source of electricity. Despite their initial popularity, commercial nuclear plants fell into disfavor in the United States, due to safety concerns. Those concerns were heightened by the 1979 near-catastrophe at Pennsylvania's Three Mile Island facility, and in fact, the US government has granted no new commercial licenses since 1978.

However, new fuel and reactor technologies are being developed that could make nuclear energy more appealing.

The atomic and subatomic worlds where nuclear energy resides are pretty complex operations, but the principle underlying nuclear power generation is fairly simple. At the core of every nuclear power plant is a nuclear reactor, where a dense chemical element - usually a uranium ore - is contained within fuel rods. Tiny subatomic particles called neutrons are shot into the uranium, which breaks apart into smaller particles, releasing huge stores of energy in the form of heat. That heat is absorbed by a cooling agent, usually water, that's life.

While that process may be simple, it is one in which many things can go - and occasionally have gone - terribly wrong, putting the reactor, its operators, and nearby populations at great risk.

"One of the things we're looking at in a new generation of reactors is making them what we call 'naturally safe,' [by] relying on things like gravity," says Kathy McCarthy, the director of advanced nuclear energy systems integration at the Idaho National Laboratory, a large federal nuclear research facility.

These naturally safe reactors are known as Generation Four nuclear reactors. They incorporate significant safety improvements over older reactors that use mechanical pumps to circulate their coolant. In existing plants, the coolant stops circulating if there's a mechanical or power failure, but the nuclear reactions continue. This may cause a "meltdown," or an explosion where radioactive material could be released. The new reactors don't need mechanical pumps. They are designed to use the great heat produced by the fuel rods to create convection currents that keep the coolant water moving.

"It's all because heat rises," McCarthy explains. "So the hotter water rises and the cooler water goes down. So if you lose the mechanical pump, you actually can still get this natural circulation. But it will do that without anybody flipping a switch or turning on a mechanical pump or relying on something mechanical to start."

In the United States, nuclear power plants are a controversial source of energy. But McCarthy contends that every other energy source also has some potential adverse environmental impact with which engineers must contend. In the case of nuclear energy, the problem lies with the fuel that comes out of the reactor. "In a lot of ways," she observes, "we are very lucky, because our waste - if you want to call it waste - or the used fuel, is right there. It doesn't go into the sky. There are no particulates. There aren't emissions. So we have the opportunity to deal with that waste."

However, used nuclear fuel is highly radioactive. That is, it emits gamma rays and neutrons that can penetrate cells and change their structure, causing radiation sickness, cancer and other diseases. And, it can remain radioactive for tens of thousands of years. "We need to be able to control that," says McCarthy. "And we can do that with something called 'shielding.'"

In most cases, that means burying or storing the used fuel. In the past, the spent fuel was placed in metal canisters. But those containers were designed for short-term use and were subject to rust; sometimes they were placed in unstable locations where radioactive waste could potentially leak into the environment. Today's nuclear fuel storage technology - including a technique for encasing the fuel in solid glass - is designed for the very long term. A proposal is being considered to store used material inside a mountain in the Nevada desert, far from present-day human populations.

Another promising approach is to vastly reduce the amount of radioactivity in the fuel that is stored by recycling what comes out of the reactor more efficiently. McCarthy explains that is possible because fuel waste is not really waste at all. "It actually has a significant energy value. We use less than one percent of the energy content of the original ore. Because there is a vast amount of energy left over that hasn't been tapped into, we can take that material, put it back into a reactor, cause it to fission, to break it apart and reduce the radioactivity. So we have ways not only to get the energy value from the fuel, but to use it safely."

The American nuclear industry once used reprocessed fuel extensively. But President Jimmy Carter ended that practice back in the 1970s as a way to limit the possibility that plutonium-rich material could end up in the wrong hands. "Basically you don't want people to take material and make weapons out of it," cautions McCarthy. "We're looking at processes that never separate plutonium by itself. So it makes it much less easy to take the material and use it for non-peaceful purposes."