Accelerator route to nuclear energy.

There's more than one way to generate nuclear energy. Researchers have already explored various ways of harnessing both nuclear fission and nuclear fusion using atomic nuclei ranging from hydrogen to uranium and plutonium. Now, physicist Carlo Rubbia, director of the European Laboratory for Particle Physics (CERN) in Geneva, Switzerland, has announced an alternative scheme. The technique requires the use of particle accelerators to supply neutrons to drive fission reactions involving thorium-232, an isotope of a plentiful, radioactive metal.

In essence, a high-energy beam of protons shot at a target made up of heavy atomic nuclei knocks out neutrons. Collisions between these neutrons and thorium nuclei transform thorium-232 into uranium-233. Hit by additional neutrons, uranium-233 nuclei rapidly split in two, releasing energy and additional neutrons. These neutrons include further production and fission of uranium-233, but at a level insufficient to keep the reaction going on its own. The proton accelerator simultaneously supplies extra neutrons to help sustain the nuclear reaction.

Rubbia concedes that his idea is highly speculative. But he argues that his proposal -- still very much at the theoretical stage and based on sophisticated computer simulations -- would reduce the risk of serious accidents, produce no long-lived radioactive waste, and eliminate unwanted, lethal byproducts such as plutonium. He believes that a working reactor could be developed quickly using existing technology. In one possible design, the thorium fuel itself serves as the target of the proton beam, and water used to cool the device also slows neutrons to increase their interactions with nuclei.

However, critics have questioned Rubbia's claims, and many nuulear experts are skeptical that such reactors would be economically feasible and competitive with other nuclear reactor designs. At the same time, Charles D. Bowman of the Los Alamos (N.M.) National Laboratory has disputed the novelty of Rubbia's scheme. Bowman and his group recently filed a patent on a similar accelerator-based technique, which would be used not only for generating power but also for transmuting radioactive waste into elements safer to handle.

Undaunted, Rubbia plans to continue developing his concept after he retires as CERN director at the end of this year.