New Ways of Waylaying Radioactive Waste

The China Syndrome is a sort of urban myth in nuclear circles, that a nuclear reactor core without cooling could become so hot it would melt throught the bottom of the reactor and begin burrowing its way through the Earth to China. Couple of problems: IF it happened, the burrowing blob would only go as deep as the local water table, which (so the myth goes) would cool it and blast it out to contaminate the countryside in a massive steam explosion. On a more practical level, such a blob could never get all the way to China; it would stop and collect at the Earth’s core because of the zero gravity zone there.

Now comes yet another proposal for nuclear material to boldly go where no nuclear material has gone before. As Tom Clarke reports in Nature, University of Sheffield researcher Fergus Gibb is suggesting that high-level waste packed into boreholes drilled 5 km deep in solid granite would generate so much heat that it would melt the surrounding rock, which would then engulf it. As the deadly waste cools, the rock would eventually re-solidify and encasing it. “Granite is the ideal material to form a coffin for nuclear waste – a coffin that will survive for millions of years,” says Gibb. Granite is one of the most erosion-resistant rocks on Earth; some are half-a-billion years old. Gibbs presented research on this subject in his article “Granite recrystallisation: The key to the nuclear waste problem?” published recently in Geology. Of particular interest is his lab work showing that molten granite would recrystalize in a matter of weeks instead of hundreds of thousands of years as previously thought. Despite this, er, hot breakthrough it is doubtful granite storage of radioactive waste will become a reality. “It would be virtually impossible to deliver for political reasons,” notes Charles Curtis of the
UK Radioactive Waste Management Advisory Committee.

Digging holes to stuff radioactive trash in is so gauche. A far more preferable option, of course, would be to merely zap it with a laser (preferably solar powered, of course) that would rid the offending waste of its radioactivity entirely. This idea at first sounds so unlikely that of course it was reported as being accomplished for the first time last week – although the laser wasn’t solar powered, yet. As Peter Rogers of Physics Web writes, physicists in the UK and Germany have now demonstrated a new laser-driven approach to “transmutation” by converting iodine-129, which has a half-life of 15.7 million years, into iodine-128. The half-life of this lighter isotope is just 25 minutes.

In this work, scientists illuminated a small gold target with a 360 Joule laser pulse from the VULCAN glass laser at Rutherford Appleton Laboratory. The laser pulse had a duration of 0.7 picosecond and was focussed to give an intensity of 5×1020 Watts per square centimetre. The laser ionized the gold to form a plasma and then accelerated the electrons in the plasma to relativistic energies. When the electrons struck the solid gold of the target they emitted gamma-rays as bremsstrahlung radiation. A sample of nuclear waste containing radioactive iodine was placed behind the gold target. Transmutation occured when a gamma-ray ejects a neutron from a iodine-129 nucleus to leave behind short-lived iodine-128. Each laser shot produced about 3 million iodine-128 nuclei.

“We have shown for the first time that we can transmute isotopes with lasers,” said researcher Ken Ledingham. “Now we need to scale up our methods so that we can deal with the sort of volumes likely to be produced by the nuclear industry in the future. Using lasers is a relatively cheap and very efficient way of disposing of nuclear waste.” And, it would appear, of turning….gold into iodine.