The Next Generation:
16 March 2009
Nuclear energy produces very little greenhouse gases, but it does have many acknowledge drawbacks. But what if we could build a nuclear reactor that offered no possibility of a meltdown, generated its power inexpensively, created no weapons grade plutonium, and burnt up existing high level waste.
Now a radical new technology based on thorium promises what uranium never delivered, abundant, safe and clean energy. It may sound too good to be true, but such a reactor is indeed possible, and a number of teams around the world are now working to make it a reality. Named after Thor, the Norse god of thunder, thorium could ironically prove a potent instrument of peace as well as a tool to soothe the world's changing climate.
With demand for energy on the increase around the world, and the implications of climate change beginning to strike home, governments are increasingly considering nuclear power as a possible alternative to burning fossil fuels.
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 of the only other naturally occurring actinide, uranium. This means thorium and uranium share several characteristics, 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.
Natural thorium "Th-233" absorbs a neutron and quickly transmutes into unstable "Th-233" and then into protactinium "Pa-233", before quickly decaying into "U-233", 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.
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. and it doesn't produce as many heavy and 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 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.
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. This is especially significant when it comes to plutonium, which has proven very hard to dispose of using conventional means.
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.
So why isn't everyone using thorium reactors. Unlike uranium, which can be left to its own devices to start producing power, thorium needs a bit of coaxing. Thorium 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.
To get around the sub criticality of thorium we need 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.
In light of the potential advantages for reducing the quantity of nuclear waste and preventing the dissemination of bomb making materials, it is not surprising that interest in thorium based fuels has recently undergone something of a renaissance. Although it seems unlikely that economics alone could drive the adoption of thorium fuels, there are no technical show stoppers here. Modifications to the existing commercial infrastructure would clearly be needed, but no fundamentally new technology is required. And the fact that the relevant materials thorium and enriched uranium have a long record of experimental use in reactors lends credibility to the notion that this scheme could one day find widespread application, should policymakers push the nuclear industry in that direction.
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