A.Even if the last six of the former 17 nuclear power plants in this country go offline at the end of next year, the radioactive relics of a once flourishing nuclear energy industry will be preserved for a long time to come. Because to the question “What to do with the radioactive waste?” There are still no really satisfactory answers. Worldwide, the mountain of spent fuel rods is growing by around 260,000 tons per year. In Germany alone, 17,000 tons are in cooling basins or vitrified in castor containers in interim storage facilities and await an uncertain future. Because still no suitable repository has been found in which the longest-lasting waste products from nuclear fission can be safely stowed deep in the earth – and that for at least a million years, as required by the German Site Selection Act. After that, the radionuclides have reached the activity of natural uranium and are considered harmless. Only in Finland will there be a repository for radioactive waste in three years. In the Onkalo facility on the island of Olkiluoto, at least 6500 tons of long-lived radionuclides should find space at a depth of 500 meters.
The problem is exacerbated by the fact that 50 new nuclear power plants are currently under construction worldwide. The volume of explosive waste will continue to grow accordingly. An idea that looks like modern alchemy sounds tempting: the transmutation of long-lived, highly radioactive waste into substances with significantly shorter half-lives by irradiating them with fast neutrons from an accelerator. Instead of hundreds of thousands of years, the idea is that the radioactive waste would then only radiate for a thousand years. The final storage time was thus shortened to a comparatively manageable period. This could also significantly reduce the amount of waste to be stored.
What has previously only worked in the laboratory – in the 1990s, the Nobel laureate in physics, Carlo Rubbia, succeeded in converting radioactive plutonium using neutrons produced with the aid of an accelerator – is now to be tested for the first time in Belgium for its large-scale feasibility. The European transmutation facility “Myrrha” is currently being built near the city of Mol on the site of the SCK-CEN nuclear research facility. If it is completed – as planned – in around ten years, it will essentially consist of an approximately 400-meter-long linear accelerator for protons including a neutron source and a subcritical nuclear reactor.
The principle sounds simple (see graphic): Protons, which are brought to high energies in the particle accelerator, knock fast neutrons out of a special material (spallation). These neutral core components hit the fuel elements with the radioactive waste in the reactor and induce nuclear fission. This results in radionuclides whose half-lives are significantly shorter than those of the long-lived radionuclides in the fuel elements.
The great hurdles of Myrrha
Plutonium and the minor actinides neptunium, americium and curium – elements that are produced in large quantities as undesirable by-products in a conventional nuclear reactor – are used as fuel. The isotopes are extremely long-lived and, as alpha emitters, are radiotoxic. They cause severe radiation damage if they are released and enter the body via the air we breathe or via the food chain.
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