Credit: Justin Randall

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THIS IS WHERE THORIUM steps in. Thorium itself is a metal in the actinide series, which is a run of 15 heavy radioactive elements that occupy their own period in the periodic table between actinium and lawrencium. Thorium sits on the periodic table two spots to the left (making it lighter) of the only other naturally occurring actinide, uranium (which is two spots to the left of synthetic plutonium). This means thorium and uranium share several characteristics.

According to Reza Hashemi-Nezhad, a nuclear physicist at the University of Sydney who has been studying the thorium fuel cycle, the most important point is that they both can absorb neutrons and transmute into fissile elements. "From the neutron-absorption point of view, U-238 is very similar to Th-232", he said.

It's these similarities that make thorium a potential alternative fuel for nuclear reactors. But it's the unique differences between thorium and uranium that make it a potentially superior fuel. First of all, unlike U-235 and Pu-239, thorium is not fissile, so no matter how much thorium you pack together, it will not start splitting atoms and blow up. This is because it cannot undergo nuclear fission by itself and it cannot sustain a nuclear chain reaction once one starts. It's a wannabe atom splitter incapable of taking the grand title.

What makes thorium suitable as a nuclear fuel is that it is fertile, much like U-238.

Natural thorium (Th-232) absorbs a neutron and quickly transmutes into unstable Th-233 and then into protactinium Pa-233, before quickly decaying into U-233, says Hashemi- Nezhad. The beauty of this complicated process is that the U-233 that's produced at the end of this breeding process is similar to U-235 and is fissile, making it suitable as a nuclear fuel. In this way, it talks like uranium and walks like uranium, but it ain't your common-or-garden variety uranium.

And this is where it gets interesting: thorium has a very different fuel cycle to uranium. The most significant benefit of thorium's journey comes from the fact that it is a lighter element than uranium. While it's fertile, it doesn't produce as many heavy and as many highly radioactive by-products. The absence of U-238 in the process also means that no plutonium is bred in the reactor.

As a result, the waste produced from burning thorium in a reactor is dramatically less radioactive than conventional nuclear waste. Where a uranium-fuelled reactor like many of those operating today might generate a tonne of high-level waste that stays toxic for tens of thousands of years, a reactor fuelled only by thorium will generate a fraction of this amount. And it would stay radioactive for only 500 years - after which it would be as manageable as coal ash.

So not only would there be less waste, the waste generated would need to be locked up for only five per cent of the time compared to most nuclear waste. Not surprisingly, the technical challenges in storing a smaller amount for 500 years are much lower than engineering something to be solid, secure and discreet for 10,000 years.

But wait, there's more: thorium has another remarkable property. Add plutonium to the mix - or any other radioactive actinide - and the thorium fuel process will actually incinerate these elements. That's right: it will chew up old nuclear waste as part of the power-generation process. It could not only generate power, but also act as a waste disposal plant for some of humanity's most heinous toxic waste.

This is especially significant when it comes to plutonium, which has proven very hard to dispose of using conventional means.

Current programs used for the disposal of plutonium reactor by-products and weapons-grade material using the MOX process are both expensive and complex. Furthermore, thorium proponents say that in conventional reactors, MOX fuel doesn't use plutonium as efficiently nor in the same volumes as thorium fuel would at lower cost.

So thorium might just be able to kill two birds with one stone. Not only does a thorium-fuelled reactor produce significantly less high-level waste, but it can also dispose of the decommissioned nuclear weapons and highly radioactive waste from nuclear reactors using more conventional fuels. Oh yes, it can also generate electricity.

SO WHY ISN'T EVERYONE using thorium reactors? The main drawback to thorium is that it's not vigorously fissile, and it needs a source of neutrons to kick off the reaction.

Unlike enriched uranium, which can be left to its own devices to start producing power, thorium needs a bit of coaxing.

Thorium also cannot maintain criticality on its own; that is, it can't sustain a nuclear reaction once it has been started. This means the U-233 produced at the end of the thorium fuel cycle doesn't pump out enough neutrons when it splits to keep the reaction self-sustaining: eventually the reaction fizzles out. It's why a reactor using thorium fuel is often called a 'sub-critical' reactor.

The main stumbling block until now has been how to provide thorium fuel with enough neutrons to keep the reaction going, and do so in an efficient and economical way.

In recent years two new technologies have been developed to do just this.

One company that has already begun developing thorium-fuelled nuclear power is the aptly named Thorium Power, based just outside Washington DC. The way Thorium Power gets around the sub-criticality of thorium is to create mixed fuels using a combination of enriched uranium, plutonium and thorium.

At the centre of the fuel rod is the 'seed' for the reaction, which contains plutonium.

Wrapped around the core is the 'blanket', which is made from a mixture of uranium and thorium. The seed then provides the necessary neutrons to the blanket to kick-start the thorium fuel cycle. Meanwhile, the plutonium and uranium are also undergoing fission.

The primary benefit of Thorium Power's system is that it can be used in existing nuclear plants with slight modification, such as Russian VVER-1000 reactors. Seth Grae, president and chief executive of Thorium Power, and his team are actively working with the Russians to develop a commercial product by the end of this decade. They already have thorium fuel running in the IR-8 research reactor at the Kurchatov Institute in Moscow.

"In the first quarter of 2008, we expect to have lead test assemblies in a full-size commercial nuclear power plant in Russia," said Grae.

He believes mixed thorium fuels can not only dispose of weapons-grade plutonium, but also be developed into a fuel for many conventional reactors to prevent production of any further plutonium as a by-product.

Thorium Power believes there is a market for about four thorium-powered reactors each in Russia and United States just for plutonium disposal. It's also aiming for reactors dealing with commercial plutonium by-products in Europe, Japan, Russia and the USA.

Grae is also enthusiastic about the benefits thorium fuels offer the environment. "All nuclear compares well to coal, in terms of no emissions into the atmosphere, including no carbon dioxide," he said. The environmental credentials of his company are also boosted by the presence of environmental lawyer and former member of the Centre for International Environmental Law, David MacGraw, he added. Grae muses that Thorium Power may be the "only nuclear company in the world with an environmentalist on the board".