Hybrid fusion-fission reactors to run on nuclear 'sludge'
Most nuke waste actually useful fuel, say boffins
Texas-based boffins say they have figured out a cunning new method of dealing with America's nuclear waste, using fusion technology - which at the moment can't produce power - to turn 99 per cent of fission reactors' waste into useful energy.
"Most people cite nuclear waste as the main reason they oppose nuclear fission as a source of power," says Swadesh Mahajan, nuke scientist at Texas Uni. He might be wrong there - the roots of the antinuclear movement very frequently lie in opposition to nuclear weapons - but there's no doubt that waste is a major concern.
In particular, the cost of long-term waste storage accounts for a major proportion of the price of fission-generated electricity.
Power from burning rubbish, fusion style.
But the Texas Uni boffins reckon that high-level nuclear waste is actually a major resource, handled in the right way. Countries such as France and Japan already reprocess and re-use some of their waste, but America doesn't: which is one reason why the US has such a large waste stockpile.
The first step, according to Mahajan and his colleague Mike Kotschenreuther, is to use the waste again in America's 100 light water reactors (LWRs), so generating large amounts more energy. This will reduce the waste volume by three-quarters, leaving a final 25 per cent of transuranic high-level waste which can't be made to fission by the level of neutron radiation found inside a normal reactor* - rather as the heaviest fractions of crude oil are very difficult to burn. The Texas boffins refer to these wastes as "sludge".
But highly radioactive waste can't be used for road tar as heavy hydrocarbon fractions are - it needs to be expensively stored or ideally got rid of, by breaking it down through fission into smaller atoms.
To do this, it needs to be blasted with neutrons, even more vigorously than it already has been in the LWRs.
"To burn this really hard to burn sludge, you really need to hit it with a sledgehammer, and that's what we have invented here," says Kotschenreuther.
That's where the fusion tech comes in. Tokamak fusion reactors are excellent for emitting lots of neutrons - their downside, so far, is that sustaining the ring of neutron-spraying plasma inside the tokamak doughnut requires more energy than the fusion reaction gives out. As a result, such reactors are currently only used for research.
Kotschenreuther and Mahajan, however, propose putting heavy transuranic "sludge" around a specially-pimped tokamak, fitted with a "Super X Divertor" allowing the neutron emissions to be cranked right up without destroying the containing doughnut. The neutron blitz would hit the surrounding heavy wastes, fissioning them and yielding energy, though the tokamak would be using energy up.
The scientists aren't yet going firm on whether the hybrid fusion/fission reactor would be a net user or generator of power, but seem sure that if it does draw juice overall it won't be much compared to that generated at the LWR stage of the process. Also, it would be cheaper than storing the wastes.
At the end, the high-level waste needing to be stored would be one per cent of the original amount.
"We have created a way to use fusion to relatively inexpensively destroy the waste from nuclear fission," says Kotschenreuther.
There have long been plans to deal with nuclear wastes by neutron bombardment, of course, but these have normally centred on the use of normal fission-driven processes to provide the neutrons rather than fusion. Such "fast breeder" reactors have been built by many countries. However, waste processing has been only one of their goals - fast breeders have also been historically seen as a means of artificially creating fissionable fuel and/or weapons material, though this aim has lost relevance as it is expensive and troublesome compared to mining or waste-processing. (There turns out to be more uranium ore in the world than people thought back in World war Two.) Most commercial-scale fast breeder facilities worldwide have shut down in recent decades.
But Kotschenreuther seems to suggest that hybrid fusion/fission processing would be much cheaper, so much so as to be cheaper than simply storing wastes. He and his colleagues believe that one hybrid fusion-fission reactor could process the transuranic "sludge" output of ten to 15 LWRs. The special Super-X-Divertor tokamak cores of these facilities would be only the size of a small room, according to the scientists.
Mahajan, Kotschenreuther and their colleagues obviously look forward to the day when fusion reactors can produce abundant energy on their own, avoiding the need for scarce uranium. On that long-anticipated day, the human race's energy problems will largely be over - fusion, the same process that powers the sun, involves no carbon emissions, no troublesome wastes and no finite resources. The only fuel needed would be isotopes of hydrogen, the most common element in the universe, easily extracted from water.
But the advent of fusion power has been expected for a long time, and the human race needs to start getting off fossil fuels now.
"The hybrid we designed should be viewed as a bridge technology," says Mahajan. "Through the hybrid, we can bring fusion via neutrons to the service of the energy sector today. We can hopefully make a major contribution to the carbon-free mix dictated by the 2050 time scale set by global warming scientists."
The nuclear boffins say that research tokamak facilities in the UK and US are interested in testing out their Super X Divertor tech. Their research is published in the January issue of Fusion Engineering and Design. An abstract and DOI details can be viewed here. ®
*The normal method of getting an atom to "fission", or split into smaller ones, is to bang a flying neutron into it. The neutron comes from another unstable atom which has split on its own. In normal fission reactors, the neutron-split atoms then release more neutrons to split more atoms, so making the process self-sustaining.
In a weapon, by cramming together a lot of highly refined fission-y atoms very quickly using ordinary explosives, the process can be made to go runaway so fast that the lump of material has no time to melt or otherwise separate itself out and so reduce the intensity of the neutron hail as would normally happen. The result is a nuclear explosion.
No doubt most of you remember all this from school - sorry to waste your time.