It is often argued that, whatever the other merits of nuclear fission as a means of generating electricity, building new nuclear reactors would only exacerbate the existing problem of what to do with radioactive waste. On the other hand, most of those who support nuclear new build tend to argue that the amount of new waste which would be generated is rather small compared to that which has already accumulated, so it is essentially immaterial. In any case, there seems to be a slow move in most countries towards accepting that deep burial in an area with appropriate geology represents the safest and most workable option for disposal.
Unfortunately, what both groups miss is the fact that the high-level waste which seems so much of a problem is in fact a valuable resource. In the current climate, where moves towards a Europe-wide bioeconomy are based on the sensible premise that there really should be no such thing as waste, since uses can always be found for by-products of any process, it seems illogical to exclude the ‘waste’ material from nuclear reactors from this philosophy.
Decay of unstable isotopes of certain elements releases energy; rather large amounts of it from small quantities of material (e=mc2). Naturally occurring uranium – consisting mainly of the 238 isotope – is processed to achieve a high concentration of U-235 which, as well as decaying naturally, is also fissile when it absorbs a neutron. Most current nuclear reactors are constructed using fuel rods which in combination produce a self-sustaining pattern of fission (a controlled chain reaction), which generates a large amount of heat. It is this heat which produces steam and so provides the motive power to generate electricity.
However, the U-235 fuel does not simply burn away, as does gas or coal. The enriched uranium used as fuel still contains a significant amount of the non-fissile isotope U-238, some of which, when bombarded with neutrons from the decay of U-235, is converted into the plutonium isotope Pu-239. This behaves almost identically to the existing U-235 fuel and provides yet more heat; about one-third of the total from the reactor.
Nuclear reactors can be configured in a number of ways, with fission of U-235 simply being the most widely used one. It is also possible to set them up so that the fast neutron flux creates more Pu-239 than it consumes: the so-called fast breeder reactor. The UK had a technically-successful fast breeder programme at Dounreay, in northern Scotland, set up in the early days of the nuclear power industry, when the availability of sufficient quantities of uranium fuel was unclear.
The fact that fast breeder technology has not become mainstream tells us that the present generation of water-cooled, uranium-fuelled reactors is the favoured option because of its relatively simple fuel cycle. But there are two downsides: only about 1% of the theoretical energy yield is obtained from the fission of U-235 alone, and there is a legacy of high level nuclear ‘waste’ to be dealt with. (On a tangential issue, we should note that perhaps 90% of waste is low or intermediate level and offers little real risk; it is the smaller amount of highly radioactive high level waste which is the cause of concern. It requires active cooling and is comprised of radioactive isotopes with very long half lives.)
The UK has the world’s largest stockpile of plutonium extracted from spent fuel (about 120 tonnes in the form of oxide), much of it originally intended to fuel a projected fleet of sodium-cooled, fast neutron reactors. There are also large quantities of spent uranium fuel, stored at Capenhurst (the plutonium is at Sellafield). There are specific concerns about plutonium, the main choice for use in atomic weapons, so the Sellafield stockpile gives an additional worry.
There may be some headaches, but on the other hand there are also some very real opportunities. The value of spent uranium fuel rods and extracted plutonium in the UK alone has been estimated at over £100 billion. Of course, to convert it into usable fuel is not a cheap task, but it is technically quite feasible by remote handling. Since constructing a nuclear waste disposal site would also be a major engineering project and would provide no real benefits, the reprocessing costs are likely to look quite reasonable in comparison. If the full potential of the fuel was used, existing stocks could give us perhaps 500 years of electricity generation, according to DECC chief scientist Professor David Mackay.
There are, of course, alternatives to such an idealised scenario. Construction of a new mixed-oxide nuclear reprocessing plant at Sellafield has recently been mooted. This would combine uranium and plutonium into a form in which they can fuel existing reactors. However, this is not a cycle which could be repeated continuously, so the energy gains are relatively modest and high level waste is not reduced by much.
Another option is to build a GE Hitachi Nuclear Energy PRISM (Power Reactor Small Modular) system to burn the plutonium stockpile, which is actively being considered for Sellafield. This would generate some electricity, but its main purpose would be to destroy the plutonium over a rather short period (five years has been suggested) and so eliminate the possibility of it getting into the wrong bomb-making hands (Are fast-breeder reactors the answer to our nuclear waste nightmare?).
On balance, though, this seems a rather negative approach to deal with the problem. It addresses a particular safety and security issue, but provides no other benefit. It is surely better to regard both spent uranium and plutonium as valuable resources which could provide secure, low-carbon electricity for many years to come, and for the government to encourage at least a full evaluation of the pros and cons of maximising the energy extracted from uranium.
The Scientific Alliance
St John’s Innovation Centre
Cowley Road
Cambridge CB4 0WS
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