In a conventional nuclear reactor the input fuel is uranium-235 (U-235) which is part of a much larger mass of uranium – mostly U-238. This U-235 is progressively “burned” over about three years to yield a lot of heat. This fission of U-235 causes some of the U-238 to turn into plutonium-239, which behaves almost identically to U-235. So, some of the U-235 effectively renews itself by producing Pu-239 from the otherwise waste U-238.
In a “breeder reactor”, the reactor is configured to "breed" more Pu-239 than it consumes; so that the system can run indefinitely – in a sense, making the process “renewable”. However, there does need to be steady input of reprocessing activity to separate the fissile plutonium from the uranium and other materials. This is fairly expensive but basically straightforward and well-proven.
As well as uranium and plutonium, thorium can be used as a nuclear fuel. The process is similar to, but more efficient than, a uranium breeder reactor.
The process is normally started with a radioactive material such as U-233, U-235 or PU-239. The thorium absorbs neutrons from the seed material and then decays, “breeding” U-233, which is an excellent reactor fuel.
Thorium has several advantages over uranium as a reactor fuel.
- Unlike a uranium reaction, a thorium fuel reaction does not produce weapons-usable plutonium.
- Thorium is much more abundant than uranium. The amount of thorium in the Earth’s crust is similar to the amount of lead – about four times as much as uranium.
- All of the mined thorium can be used as a nuclear fuel; whereas only about 0.7% of mined uranium (U-235) can be used.
- The waste from a thorium reactor is toxic for about 500 years – compared with tens of thousands of years for uranium reactor waste.
- The volume of waste produced is less than half that produced by a uranium reactor.
- Thorium could be used in new and even existing reactors without having to make major changes to reactor designs.
The disadvantage of using thorium as a reactor fuel is that the thorium reaction needs a constant input of neutrons from another source to keep it going – which adds to its complexity and cost.
One way of providing the necessary protons is from another radioactive material. The material needed to start the thorium reaction can be the waste from uranium reactors – thorium reactors can actually dispose of the waste from conventional reactors. It is estimated that four thorium reactors in each of the United States and Russia could dispose of those countries’ plutonium waste.
The other way of providing the protons to seed a thorium reactor is from a particle accelerator (powered by the thorium reactor). Such a reactor has absolutely no possibility of a Chernobyl-style meltdown because any malfunction would cause the particle beam to switch off and the reactor to stop.
Australia has the world’s largest reserves of both thorium and uranium. Other countries with large thorium reserves include India, the United States, Turkey and Norway.