Waste not: promise of clean nuclear power

THE problem with nuclear power is that it comes with an unpleasant side-effect: highly radioactive waste material that will still be dangerous in thousands, if not millions, of years. But what if we could build a reactor that ran on nuclear waste - a reactor that generated power by cleaning up the most toxic substance in existence?

Tucked among the fertile vineyards of central California is a machine that promises to do just that. Work has just finished on the National Ignition Facility, before its official opening next month. The NIF contains the most powerful laser in history, a machine in a building the size of three football fields.

The aim is to split the laser into 192 separate beams, then focus them on a speck of fuel little bigger than a pinhead. The resulting temperatures and pressures will be millions and billions times greater than those on Earth, triggering the start of a fusion reaction. This will allow researchers to peer into the hearts of planets and stars - but it also has a more practical application, which has nuclear scientists salivating.

By using hydrogen as its fuel, the NIF should become the first fusion machine that gives off more energy than is needed to trigger the reaction. But there is a problem: due to the enormous energies involved (1000 times the amount produced by America's national grid), the laser can only fire once every three hours. For a commercial fusion power station, that would need to be 10 times a second.

So far, so disappointing. But the scientists at the NIF have a crafty solution. Rather than creating a pure fusion reaction, they plan to combine their technology with a traditional nuclear fission reactor, which would require the laser to fire at a far lower frequency.

"Using the laser to trigger nuclear fusion and drive a fission reaction means we can deliver the benefits of fusion to the utility companies far sooner," says Ed Moses, director of the NIF. "We will be getting energy from both the fission reaction and the fusion reaction, so for each kilo of fuel used in a traditional fission reaction, we will get about 20 times more energy."

Nuclear fission occurs when heavy atoms such as uranium or plutonium split into smaller, lighter atoms, releasing neutrons and energy. The neutrons released trigger fission in nearby atoms, setting off a cascade: a chain reaction.

Nuclear fusion, on the other hand, occurs when two small atoms, typically forms of hydrogen, are heated and compressed together until they fuse to form a heavier atom, releasing large numbers of neutrons and vast amounts of energy.

Under the NIF's plan, the neutrons that power the fission would come from the laser-triggered fusion reaction: a "fission fusion" reactor in which the nuclear fuel is encased in a blanket around the fusion reactor. The team claim they can have a working power station running by 2020.

Jeff Wisoff, a former astronaut who is deputy principal associate director of science at the NIF, thinks this type of hybrid energy source could transform the way the world views nuclear energy.

"This is our vision for a safe, clean and sustainable energy source of the future," Dr Wisoff said. "As the source of the neutrons is separate from the nuclear fuel, it becomes highly controllable and it can never go into meltdown by accident."

But there are more advantages to the concept. In nuclear fission, the cascade of neutrons eventually becomes too weak for the fuel to be a viable energy source. The waste, however, continues to undergo radioactive decay, remaining highly dangerous.

Yet by using a separate source of neutrons - taken from the fusion reaction - the fuel can be burned up almost completely. The NIF claims that spent nuclear fuel, and even weapons-grade plutonium, could be put into the fission blanket and used up over a 50-year period in a highly controllable way.

"The beauty of it is that we can put in anything that will burn up," Dr Wisoff said. "It will essentially allow us to use the spent fuel left behind by traditional nuclear fission. It really could help us clean up the world."

Professor Mike Dunne, director of the central laser facility at the Rutherford Appleton Laboratory, near Oxford, England, is equally enthusiastic. He is one of the leaders of the High Powered Laser Research Facility, which is aiming to build a fusion reactor that can fire a laser once every couple of minutes rather than hours.

"The concept of hybrid fission fusion reactors has been around for a long time, but they have always been deemed to need significant storage or reprocessing of waste," he said. "What this does is close that fuel cycle. It is an amazing design, and should allow us to get fusion energy on to the national grid far sooner than we have previously thought possible."

Under the NIF's plan, the neutrons that power the fission would come from the laser-triggered fusion reaction: a "fission fusion" reactor in which the nuclear fuel is encased in a blanket around the fusion reactor. The team claim they can have a working power station running by 2020.

Jeff Wisoff, a former astronaut who is deputy principal associate director of science at the NIF, thinks this type of hybrid energy source could transform the way the world views nuclear energy.

"This is our vision for a safe, clean and sustainable energy source of the future," Dr Wisoff said. "As the source of the neutrons is separate from the nuclear fuel, it becomes highly controllable and it can never go into meltdown by accident."

But there are more advantages to the concept. In nuclear fission, the cascade of neutrons eventually becomes too weak for the fuel to be a viable energy source. The waste, however, continues to undergo radioactive decay, remaining highly dangerous.

Yet by using a separate source of neutrons - taken from the fusion reaction - the fuel can be burned up almost completely. The NIF claims that spent nuclear fuel, and even weapons-grade plutonium, could be put into the fission blanket and used up over a 50-year period in a highly controllable way.

"The beauty of it is that we can put in anything that will burn up," Dr Wisoff said. "It will essentially allow us to use the spent fuel left behind by traditional nuclear fission. It really could help us clean up the world."

Professor Mike Dunne, director of the central laser facility at the Rutherford Appleton Laboratory, near Oxford, England, is equally enthusiastic. He is one of the leaders of the High Powered Laser Research Facility, which is aiming to build a fusion reactor that can fire a laser once every couple of minutes rather than hours.

"The concept of hybrid fission fusion reactors has been around for a long time, but they have always been deemed to need significant storage or reprocessing of waste," he said. "What this does is close that fuel cycle. It is an amazing design, and should allow us to get fusion energy on to the national grid far sooner than we have previously thought possible."