Rajendran Raja, a physicist at Fermilab — the U.S. Department of Energy’s Fermi National Accelerator Laboratory in Batavia, Illinois — said by telephone that the benefit of adding thorium to the fuel mix would be to create much more fuel using existing abundant resources and to reduce waste.

This could be done by building a high-intensity proton accelerator with the capacity to produce fast neutrons that could convert nuclear waste, thorium-232 and uranium-238 into fuel, he said. But to accomplish this, a proton accelerator would need to be 10 times more power-intense than anything that has been produced to date.

The concept of an accelerator-driven subcritical system, known as ADS, has been around since the 1990s. It differs from conventional reactors which operate at criticality, which is the point at which a nuclear reaction can be self-sustaining. But if a chain reaction gets out of control, accidents like those at Chernobyl could occur, and high levels of radioactive material could be released into the atmosphere.

Subcritical reactors, however, would use neutrons provided by the accelerator to continue the fission. This means that criticality could be avoided by switching off the accelerator, which in turn would switch off the neutrons. This type of reactor has not been built because an accelerator with sufficient power does not exist, Dr. Raja said.

But experts in the United States and elsewhere are taking steps in that direction.

In September, a U.S. Department of Energy facility successfully completed a test of the first U.S.-built superconducting radio frequency niobium cavity intended to become part of the prototype accelerator at Fermilab. The niobium cavity, which is more efficient than the more common copper cavities, can be used to build a particle accelerator producing 10 megawatts of beam power, which in turn could convert thorium into nuclear fuel.