SCIENCE AND TECHNOLOGY
Breaking Hanford down
January 22, 2000
USING nuclear reactions to generate electricity is a messy business. Reactors that exploit nuclear fission (in which energy is generated by splitting uranium atoms) have produced thousands of tonnes of spent fuel and other radioactive by-products. Meanwhile, research into fusion power (in which energy is generated by fusing hydrogen atoms together at very high temperatures) has left behind a different kind of debris: a trail of experimental reactors, none of which has yet reached the break-even point where the amount of energy that comes out exceeds the amount put in. Bernard Eastlund, a physicist and veteran of the American nuclear industry, has proposed an ingenious way to deal with these nuclear leftovers. The experimental fusion reactors, he suggests, could be used to clean up the waste generated by the fission reactors.
Dr Eastlund first came up with the idea for what he calls a "fusion torch" in 1968, in collaboration with an electrical engineer called William Gough. At the time, it looked as though practical fusion reactors were just around the corner, and the two physicists suggested that their surplus plasma-gas heated to around 10m0C, so that individual atoms have their electrons stripped off-might be used for recycling household and industrial waste.
The idea works as follows. First, start with the plasma, confined in a doughnut-shaped magnetic "bottle" called a tokamak. Next, throw in some unwanted material. This will be reduced almost instantly to a soup of electrons and nuclei. It will also cause the magnetic bottle to overflow and emit a stream of plasma through a special outlet. The plasma stream is then passed over a series of metal plates, held at particular temperatures, and arranged in descending order of temperature.
Atoms of elements with boiling points below the temperature of a particular plate pass over it, just as water runs over a warm river-bed. But eventually, each atom encounters a plate at a temperature lower than its boiling point, at which point it sticks to the plate, just as water freezes when poured on to dry-ice. In effect, the various plates act as an atomic distillation system which sorts the plasma into its constituent elements. Household waste, for example, would be reduced to pure carbon, oxygen and hydrogen, along with a handful of heavier elements (tungsten, iron, aluminium and so on) which could then be re-used.
The idea of the fusion torch attracted a lot of attention, and won its inventors an award from America's Atomic Energy Commission. But nothing came of it, for two reasons. First, fusion research failed to progress as quickly as expected, so there were no power stations producing streams of surplus plasma to fuel fusion-torch recyclers. And second, researchers in the field disliked the idea of lobbing tennis shoes and old tyres into their pristine reactors. So the idea lay dormant for many years.
The growing mountain of nuclear waste from fission reactors, however, has recently rekindled interest in the scheme. Last year, following a request from the American Department of Energy, Dr Eastlund delivered a detailed proposal explaining how the fusion torch could help to dispose of this waste.
At present, the plan is for it to be reprocessed using solvents and chemical processes to separate it into high-level and low-level components. The high-level waste would then be enclosed in glass canisters and buried deep underground while the low-level waste was stored above ground in special secure facilities. The cost of treating the waste from the decommissioned plutonium-manufacturing plant in Hanford, Washington, alone is estimated at over $40 billion. Nationally, the energy department expects the bill to be around $400 billion.
According to Dr Eastlund, breaking nuclear waste up into its constituent elements using a fusion torch would have a number of advantages over the chemical approach. For a start, it would produce much less waste. Chemical reprocessing at Hanford would generate 22,000 tonnes of high-level waste and 500,000 tonnes of low-level waste. For a fusion torch the figures are around 5,000 tonnes and 1,000 tonnes respectively. This is because the waste consists of barrels of sludge containing compounds such as nitrides, oxides and water that are not, themselves, radioactive. Another advantage is cost. The fusion torch could, he reckons, handle the waste at the Hanford plant for around $10 billion.
A number of things have changed since the fusion torch was originally proposed. Experimental fusion reactors reached temperatures of 10m0C in the mid-1980s, making the idea feasible. Many of those reactors, however, have now been mothballed, so using them as fusion torches would probably encounter little opposition from their owners. Dr Eastlund has also been able to draw on subsequent research on turning solids into plasmas. (This method is, in fact, used to apply coatings to the insides of tokamaks.) And the advent of supercomputers able to simulate tokamaks should also speed up the process of building a working prototype.
Dr Eastlund's proposal, which is now being circulated more widely within the department, lays out a $70m research programme that would involve recommissioning a tokamak at the University of Texas to test the concept, and then proceeding to a prototype system (capable of handling a barrel or two of waste per day) within five years. Waste-handling expertise would be provided by Mississippi State University. If successful, the system could then be scaled up by building several such machines.
But the wheels of government turn slowly, though Dr Eastlund says interest at the energy department increases whenever there is a nuclear incident elsewhere in the world. Backing the fusion-torch would be politically difficult, because it would be an admission that chemical reprocessing, the currently favoured approach, has its drawbacks. There would also be legal hurdles to overcome as reprocessing nuclear waste is one of the most regulated industries in America. Even so, Dr Eastlund hopes that he will soon be able to hand on the torch to someone who can make real use of it.
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