Nuclear alchemy

Physicists are developing a way to transform highly radioactive nuclear waste that would dramatically reduce the time it stays hazardous. Rob Coppinger reports.

Science fiction has an uncanny knack of forecasting the future. In the early 1990s the US government employed sci-fi authors to dream up a ‘forbidden zone’. The writers, space science professors Carl Sagan and Gregory Benford, and an artist were asked to envisage a monument that could portray visually to anyone approaching it in 10,000 years’ time that this zone should not be dug up, built on or tampered with in any way as it contained stores of decaying nuclear matter.

Ten years later we have still not found a satisfactory way of dealing with nuclear waste by-products besides burying them. Though the world’s first nuclear reactor was built in 1942 in Chicago, high-level radioactive waste will, it seems, remain a serious threat to the environment for more than 10,000 years, twice as long as human civilisation has existed.

This problem undermines the nuclear industry’s efforts to portray itself as a greener way of generating electricity because it produces no carbon dioxide, and has been a constant source of friction between opponents and supporters of nuclear power.

The UK and the US have, between them, spent £2.5bn trying to find a site for storing waste safely for longer than the pyramids have stood in Egypt. Yet the US is still 10 years away from having its selected site operational, even if it started building it today, and the UK has abandoned its work completely.

But now there could be a technical solution to the problem. According to US and European scientists there is a way of reducing the risk of nuclear waste so that secure storage would be needed only for a few centuries or even decades. The process, called nuclear waste transmutation, involves bombarding the waste (which consists of irradiated fuel from nuclear reactor cores) with neutrons in a special reactor, causing the radioactive elements in it to decay into other, less radioactive substances.

Kevin Hesketh, technology leader for British Nuclear Fuels’ Research and Technology’s Advanced Reactor Team, is confident that it can work. As the UK’s representative on a European transmutation project, he says: ‘You will no longer get any accumulation of high-level waste using transmutation.’

The promise of being able to halt the growth in high-level waste output, which represents 90 per cent of any waste stock’s radioactivity, makes transmutation an attractive technology. The US government has been enthusiastic enough to pour money into it year on year. Its Department of Energy recommended in April that around £4.5bn should be spent on developing the technology over the next 20 years.

Dr Denis Beller, Advance Accelerator Intercollegiate Programs Co-ordinator at the University of Nevada, Las Vegas, has been working on the programme since 1998 and has seen a dramatic rise in funding over the past four years. He says that since 1999 the US government has spent $45m (£29m), and in this financial year more than all that combined: $52m (£33m).

Transmutation would involve a series of processes. First, the highly radioactive elements would have to be separated from the reusable uranium which makes up 95 per cent of any fuel rod. Just one per cent of the rod produces 90 per cent of the radioactivity generated, and includes nasty isotopes of plutonium, neptunium and americium, and really long-lived elements such as iodine-131 and technetium-241.These elements would be separated from the reusable uranium with a well-established chemical process used in nuclear fuel reprocessing. They would then be placed in the form of pellets into a reactor chamber where the neutron bombardment would take place. Both the US and European teams are using fast reactors, originally developed until the early 1990s for possible commercial use (see sidebar).

In typical decay an atom of the high-level isotope technetium-99 absorbs a neutron to become the highly unstable technetium-100, which has a half-life of just 16 seconds and decays into the lower level and stable waste material ruthenium-100.

Although a great deal of theoretical research for transmutation was carried out 60 years ago it has only just been made public. In the early days scientists realised they would not have the technology to make it work for decades. Only in the past few years have engineers become aware of what would be needed for a transmutation reactor system. Meanwhile, the nuclear industry embarked on a well-publicised, expensive and fiercely opposed search for sites for a deep repository for the waste (see sidebar).

The view that transmutation could not be achieved changed in the 1980s when Italian physicist and Nobel prize winner Prof Carlo Rubbia championed it. This new attitude gained more weight when the US agreed with Russia, in the early 1990s, to decommission large numbers of nuclear weapons, meaning there were large amounts of plutonium to be dealt with. Hesketh says: ‘Transmutation could potentially change toxic-enriched plutonium into something safer.’

While the US moved forward to turn theory into practice, France initiated an informal European network with no central budget or research institute. Nirex, the UK waste disposal agency, has had a policy of only keeping a watching brief on the technology. It reviews the subject regularly and in a March 2001 report recommended: ‘More research should be carried out, particularly into transmutation rather than simply burying or dumping the waste.’

Beller confidently predicts that once high-level waste has been turned into intermediate there is no reason why the process could not continue to the point where it all became less radioactive than low-level waste. But Hesketh is cautious about this and prefers to predict half-lives of millennia being reduced to centuries.

Even so, the US’s goal is no more ambitious than to have an operational prototype transmutation plant in 20 years. A commercial system is therefore almost as distant as fusion power.

Europe could yet beat the US to creating a prototype plant. The Belgian government’s nuclear research centre SCK-CEN is working on an accelerator-driven system reactor. Named Myrrha, its design and construction could cost up to E450m (£280m). Research on the reactor is funded partly by the EU’s fifth Framework Research Programme, and a detailed design will be completed by 2006. The reactor, which could produce 30-40MW, would be operational by 2012. The US system would be a 100MW system but Myrrha project leader Dr. Hamid Ait Abderrahim is confident the European reactor is can prove the viability of transmutation: ‘Although our system is smaller, we think we will have the information needed to demonstrate that it works, to go ahead with an industrial-scale version.’

Despite this potential competition between the US and EU the two countries may combine their work. It is early days but there are discussions between the US Department of Energy and SCK-CEN on how to take transmutation research forward.The UK is not involved in Myrrha at this stage but researchers at BNFL have contributed to the informal European network. Hesketh says: ‘The UK is helping the French with studies on fuel performance, for example should the waste be in a pellet shape or not?’ The Institute of Transuranic Studies in Germany has already produced such pellets, but only with grams of waste. On an industrial scale a transmutation reactor would need tonnes of pellets, an engineering problem that has yet to be overcome.

‘Studies so far have been measuring the properties of these waste materials and what irradiating does to them. This has led to small quantities, grams of material, being irradiated to see if the outcome matched the theory.’ Other work has focused on the effect the reactor has on the stainless steel fuel assembly, which is the apparatus that holds the fuel waste in position. The research also has to model the entire reactor environment.

The jury is still out on whether transmutation is the Holy Grail governments are seeking to solve the nuclear disposal issue, but the likes of Hesketh are confident the technology could be made to work. Not only will transmutation mean governments are no longer having to look 10,000 years ahead using science fiction, it will reduce the potential risks of burial. As Hesketh says: ‘A whole repository could be percolated by water eventually, over a 10,000-year-plus timespan. Those long-lived elements could contaminate water supplies that could rise to the surface. As a scientist I can’t guarantee what will happen in 10,000 years’ time but you can reasonably guarantee something for 1,000.’

Sidebar: Not everyone is convinced

Not everyone is convinced that transmutation is a viable solution. The fission process that underlies both nuclear power generation and waste transmutation depends on probabilities of atomic collisions and so is not very predictable. Although refined uranium is the starting point the composition of the five per cent of used fuel rod that becomes waste is harder to predict.

While plutonium often represents more than half of that five per cent, the remaining waste, much of it highly radioactive and long lived, can be made of a wide range of radioactive isotopes or nuclides. Americium is just one, with a half-life of 500 years. An isotope of americium found in waste is Am-124. Transmutation could turn this into iodine-131 and technetium-107, which have half-lives of eight days and 21 seconds respectively. Hesketh and others recognise that for many nuclides it will not produce such dramatic reductions in half-life.

Nuclear consultant John Large is not convinced that transmutation will be possible at all. He thinks the process of separating the different waste materials from the used fuel rod will be extremely problematic: ‘I don’t think it is possible with current waste stock. In 10 to 20 years it might be a solution for future reactor systems that can generate waste amenable to transmutation, but today’s reactors’ output is just not suitable.’

He believes the randomness of the waste by-products of the original power-generating fission reaction will complicate the chemical separation and pelletisation processes, and make them expensive and prone to error. Even if pellets could be produced Large is convinced that transmutation will simply create another bewildering array of isotopes in place of the commonly found americium, neptunium, curium and californium. And what will the scientists do then?

Meanwhile, environmentalists are equally sceptical. Greenpeace’s nuclear energy spokesman Peter Roach says: ‘I’ve come across a lot of scepticism about it in the UK. What you’ll end up with is still nuclear waste and it will cost a fortune. It is just more work for nuclear scientists.’

Sidebar: the burial problem

The scale of the problem of the nuclear waste generated during the 20th century is enormous. The UK produces about 70 tonnes of high-level waste, 1,300 tonnes of intermediate-level waste and 5,600 tonnes of low-level waste every year. The UK stockpile of stored waste is around 800 tonnes of high-level, 31,000 tonnes of intermediate and 3,500 tonnes of low-level.

The US chose not to reprocess its fuel rods in the 1970s and now has a 70,000- tonne mountain with 700 tonnes of high-level waste. For the past 24 years the US government has been studying Yucca Mountain, about 100 miles north west of Las Vegas in Nevada, as a possible repository for it all. Last July the Senate finally approved its use. It will cost £37bn ($58bn) to build once it gets the go-ahead from the Nuclear Regulatory Commission.

Although this could be built by 2010 it will still not be a final resolution to the waste issue. Even if Yucca Mountain opened this year US nuclear plant waste output would fill it by 2015 and a third mountain will be needed by 2045.

In the UK Nirex has spent £330m since 1986 on researching the deep geological storage of waste. It abandoned proposals at four UK sites because of intense local opposition before turning its attention to Sellafield in 1989. An application for an underground laboratory to study this option further was rejected in 1997 on the grounds, among others, that the site was unsuitable.

But the waste agency has continued to examine phased deep geological disposal over the past five years. That research has taken a very long-term view of storage survivability and considered issues such as possible ice sheet growth in future ice ages. The last ice age was 10,000 years ago and the next is expected in 23,000 years’ time.

Any final decision on a UK repository will not be made for many years, even if the research is near completion. The Department for the Environment and Rural Affairs initiated a consultation process, of which phase one was completed in March. Phase two begins in January but the process is not expected to finish until 2007.

This lengthy consultation timetable has been criticised in the House of Lords following the September 11 terrorist attack, because of worries about the vulnerability of nuclear waste stored on the surface. But no moves have been made to speed up the process.

Sidebar:the right reactor

Both the US and the European teams are using fast reactors for transmutation. The US researchers are developing a subcritical fast reactor while the Europeans are working on a ‘subcritical combined thermal-fast’ reactor.

A thermal reactor is the type used today for commercial power. It has a moderator, usually graphite, to control the flow of neutrons from the uranium fuel which underlie the fission process.

Fast reactors were developed up until the early 1990s for possible commercial use and have no moderator, to maximise the flow of neutrons and improve fission efficiency.

The European design will attempt to combine the best features of both types.But for transmutation both reactors will be considered ‘subcritical’ because they will use quantities of waste that will not start the self-sustaining chain reaction seen in normal nuclear power stations, in which closely packed fuel rods naturally begin the fission process.

Instead, the US and Europeans will use a particle accelerator to ‘inject’ neutrons into the process to begin fission. The accelerator is used to bombard a tungsten or liquid mercury target with protons to produce two per cent of the neutrons required for transmutation. Beller says this is an in-built safeguard: ‘That two per cent makes the difference between the process working and it not working’.