Science: A nuclear opportunity goes to waste: Peter Hodgson looks at how the Japanese are taking the lead in a field where British research once held prominence

MOST OF our household waste is disposed of by burying it underground or burning it in municipal incinerators from which the heat energy can, on occasion, be put to good use. But nuclear waste is different, we are often told.

Conventional wisdom has it that the highly radioactive fission products in the spent fuel rods discharged from nuclear reactors are a special hazard.

For 30 years, the rods from Britain's first-generation Magnox reactors have been consigned to the Sellafield reprocessing plant where they are dissolved in nitric acid, and the uranium and plutonium, which can both be re-used, extracted.

Sellafield's controversial new thermal oxide reprocessing plant - Thorp - which is complete but not yet in operation, is designed to treat the spent fuel rods from second-generation advanced gas-cooled reactors and from American-style light-water reactors in a similar way.

However, reprocessing still leaves behind a highly radioactive liquor containing the fission products. British Nuclear Fuels, which operates the Sellafield reprocessing plant, has bought French technology designed to evaporate this liquid and cast the waste into solid glass blocks. Because they contain so much radioactivity, the blocks have to be cooled and government policy is that they should then be stored for at least 50 years behind heavy shielding in a purpose-built air-cooled building at Sellafield.

The best we can then do, according to the nuclear industry, is to bury them carefully in stainless-steel containers deep underground. The half-lives of many of the fission products are tens of thousands of years, and there is nothing we can do to alter that.

However it is possible to 'burn' nuclear wastes, and research is already under way in Japan to find how this can best be done. There are two possibilities, both aimed at destroying the long-lived nuclides by nuclear reactions. These long-lived radioactive elements are mainly the 'actinides' - atoms created inside a reactor by the nuclear transmutation of the elements.

The first method is to use what is called an actinide burner reactor. In this, the actinides are irradiated by an intense beam of neutrons from a fast reactor. The neutrons, which are electrically uncharged, can penetrate the inner nucleus of many atoms. Many reactions can take place, and these transmute the long-lived actinides to stable or short-lived nuclei, which can be easily stored until they decay. Many of the reactions themselves produce neutrons that can transmute or 'burn' nearby actinides. In this way, the burner acts as nuclear reactor, except that it needs an external beam of neutrons.

The other method is to irradiate the unwanted actinides by a beam of high-energy protons. Protons are similar particles to neutrons, except that the proton carries an electric charge. The proton forms the nucleus of hydrogen, the simplest of all atoms. Beams of these protons hit the actinide nuclei and break them up into many pieces, in a reaction called spallation. The products are also stable or short-lived nuclei.

However, before these nuclear incinerators can be built, careful studies of efficiency, cost and safety have to be made. Some of the burning reactions that will occur are well known, but many are not. It is expensive and often impossible to measure the reaction data needed, so research is in progress to see how best they can be calculated in future. The Japanese are among the world leaders in this branch of nuclear physics.

Until recently there was also a strong research programme dealing with these problems in this country, both in government establishments and in universities. However, the recent cuts in the fast-reactor programme have severely curtailed this work, and it is likely to cease within a few years.

Once again we see the closing of an area of research where Britain has long been among the world leaders. Nevertheless, it will continue in Japan, and in a few years we may hope to see an efficient and environmentally friendly solution to the problem of the disposal of nuclear wastes.

Dr Peter Hodgson is a Fellow of Corpus Christi College and lecturer in theoretical nuclear physics at Oxford University.

(Photograph omitted)