Thorium and the dream of clean nuclear power

HONG KONG (Reuters) - China isn’t alone in turning to thorium as a potential source of power. Enthusiasm for exploiting this alternative to uranium is on the rise across the world, even as the cleanup continues from the Fukushima nuclear accident in Japan.

A new generation of scientists and nuclear engineers argue that thorium could be the key to realizing a dream of safe, cheap and plentiful nuclear power for an energy hungry world.

Thorium deposits, estimated to be about four times more abundant than uranium, are widely distributed: Substantial reserves have been found in China, Australia, the United States, Turkey, India, and Norway. About 6,600 metric tons (7.75.3 tons) of thorium used to power the most efficient proposed reactors would provide enough energy to replace all of the fossil fuels and nuclear energy consumed globally each year, proponents say.

Uranium-poor India has a long-term research effort under way and has decided thorium will become the mainstay of its nuclear energy industry later this century. The French government has a research program. Companies in the United States, Australia, Norway and the Czech Republic are working on reactor designs or thorium fuel technology.

Energy from thorium is not just scientific theory. On April 25, Thor Energy, a private Norwegian company, began producing power from thorium - named after the Norse god of thunder - at the Halden test reactor in Norway.

“It is the fundamental first step in the thorium evolution,” says company CEO Oystein Asphjell. The tests are aimed at showing the fuel could be a valuable alternative to uranium for existing reactor operators. Nuclear giant Westinghouse, a unit of Toshiba Corp, is part of an international consortium that Thor Energy established to fund and manage the experiments.

A Westinghouse spokesman said the company was “providing viewpoints” on the research.

Asphjell says burning thorium in current pressurized water reactors could boost safety and provide greater fuel security, especially for countries with limited access to uranium. Eventually, proponents want to pair thorium with a new kind of reactor, cooled not by water but by molten salt. That, booster say, would realize thorium’s full potential as a fuel.

Thorium is a shiny, slightly radioactive metal. In its natural form, thorium isn’t fissile - meaning that, in contrast to uranium, it can’t split to sustain a nuclear chain reaction.

But if thorium is bombarded with neutrons from a small amount of fissile nuclear fuel acting as a starter, either uranium-235 or plutonium-239, it is converted to uranium-233 - a form of uranium that is a first-rate nuclear fuel. Once started in a reactor, the process is self-sustaining, with subsequent fissions of uranium-233 in turn converting more thorium to nuclear fuel.

In the kind of molten-salt cooled reactor favored by many thorium proponents, the uranium-233 fuel would be dissolved in a coolant of liquid fluoride salts contained in a graphite core. Surrounding the core would be a blanket of thorium, also dissolved in liquid fluoride salts.

When the fuel in the core fissions, it produces heat and a barrage of neutrons that pass through the graphite and convert some of the thorium in the blanket to uranium-233. This is then removed from the blanket and fed into the core, while fresh thorium is supplied to the blanket. The coolant and fuel mixture from the reactor core is circulated through a heat exchanger, so that the energy can be extracted to power a turbine and generate electricity.

One advantage of this system is that the fluoride salt coolant has an extremely high boiling point of 1,400 degrees Celsius, far higher than the reactor’s operating temperature of about 750 degrees Celsius. That means the whole system can operate at close to normal atmospheric pressure.

In a conventional water-cooled reactor, the cooling system must be designed to withstand high pressure. That means reactors also must have massive, heavily engineered and expensive containment structures to minimize the danger from leaks or pressure explosions.

Because the core in a thorium molten-salt reactor is already liquid, it can’t melt down. The design calls for a plug of frozen salt at the bottom of the system. If the reactor overheats, the plug would melt and the fuel and coolant would drain into a containment vessel below, where it would rapidly solidify and could be recovered for future use, proponents say.

These reactors could be much more efficient than most current nuclear plants, which extract between three and five percent of the energy in uranium fuel rods. In a molten salt reactor, almost all the fuel is consumed.

One metric ton of thorium fuel would deliver the same amount of energy as 250 metric tons of uranium in a pressurized water reactor, according to a briefing paper published by the United Kingdom All Party Parliamentary Group on Thorium, a group of UK lawmakers who advocate adoption of the alternative fuel.

Also, because most of the fuel is consumed, thorium yields little waste and is much less radioactive, proponents say. Most of the residue will become inert within 30 years, with about 17 per cent needing secure storage for about 300 years.

The most dangerous waste from current generation reactors requires storage for 10,000 years.

The molten-salt reactor may have one further benefit. Some advocates believe they can be used to burn off existing nuclear waste.

A privately owned U.S. start-up, Transatomic Power of Cambridge, Massachusetts, says it plans to build molten salt cooled reactors to burn some of the 270,000 metric tons of nuclear waste accumulated worldwide. “There is enough waste just in the U.S. to power the country for a century,” says Russ Wilcox, company CEO and co-founder.

Reporting By David Lague; edited by Bill Tarrant