The most beautiful machine: A similar situation occurred in Germany when, in 1989, the State government of North-Rhine-Westphalia, after spending 2 billion Euros, stopped the operation of a pebble-bed reactor (Kugelhaufenreaktor), which was then called the most beautiful machine. The high-temperature reactor THTR 300 in Hamm-Uentrop has become a was-a-belle from today to tomorrow. The Chernobyl radioactive cloud that had passed over Europe in 1986 was the main reason for shutting down this is-a-belle machine.
What makes the high-temperature pebble-bed reactor so beautiful? Like any other reactor, the THTR operates on nuclear fission, but the primary fuel is Thorium-232, with 20% Uranium-235 needed to create a critical assembly that sustains the chain reaction. The moderator is graphite instead of light water, and the coolant is helium gas.
Elegantly, the nuclear fuel is mixed with the neutron-moderating graphite (carbon) and formed into spheres the size of a tennis ball. Such an assembly has a negative temperature coefficient: as the temperature increases, the fissionable material-carbon mixture eventually becomes subcritical, giving the THTR an inherent safety feature.
How the THTR operates: When the THTR runs, thermal neutrons breed Uranium-233 from Thorium-232, where the former is fissioned "online." Instead of uranium, the THTR burns thorium - more abundant on earth than uranium - with only a tiny amount of plutonium produced. The theoretical energy efficiency of boiling water reactors (like those at Fukushima) is 47%; that of the helium-cooled pebble bed reactor is 71%, because the beautiful machine not only generates electricity but also high-temperature heat. This heat was intended to hydrogenate the abundant coal on the Ruhr, producing methane and other hydrocarbons.
Fresh carbon-thorium balls are fed into the reactor vessel from above during operation, while burned-out spheres are removed from the bottom and reprocessed. Although the THTR produces a small amount of plutonium, the radioactive waste from burning thorium, due to the shorter half-lives of the fission products, is less of a problem than the waste from light-water reactors.
The Rubbiatron: Carlo Rubbia's energy amplifier may allay concerns about the small amount of plutonium produced. In coupling a thorium assembly with a proton accelerator, nuclear fission is sustained by neutrons from spallation reactions. No Uranium-235 is needed, and no plutonium is produced.
How the THTR operates: When the THTR runs, thermal neutrons breed Uranium-233 from Thorium-232, where the former is fissioned "online." Instead of uranium, the THTR burns thorium - more abundant on earth than uranium - with only a tiny amount of plutonium produced. The theoretical energy efficiency of boiling water reactors (like those at Fukushima) is 47%; that of the helium-cooled pebble bed reactor is 71%, because the beautiful machine not only generates electricity but also high-temperature heat. This heat was intended to hydrogenate the abundant coal on the Ruhr, producing methane and other hydrocarbons.
Fresh carbon-thorium balls are fed into the reactor vessel from above during operation, while burned-out spheres are removed from the bottom and reprocessed. Although the THTR produces a small amount of plutonium, the radioactive waste from burning thorium, due to the shorter half-lives of the fission products, is less of a problem than the waste from light-water reactors.
The Rubbiatron: Carlo Rubbia's energy amplifier may allay concerns about the small amount of plutonium produced. In coupling a thorium assembly with a proton accelerator, nuclear fission is sustained by neutrons from spallation reactions. No Uranium-235 is needed, and no plutonium is produced.
Why is the machine called an energy amplifier? One has to input a certain amount of energy to get the more energy-producing chain reaction going. This also means nuclear fission stops immediately when the accelerator is switched off. Such a proton accelerator could likewise be used to "incinerate" the nuclear waste produced.
When we discussed the Rubbiatron at CERN, one technological issue was insurmountable. As an accelerator operates in a vacuum, there must be a separation (a thin window) between the thorium assembly and the proton machine. Such a metallic membrane will be hit by enormous amounts of radiation and thus be damaged quickly.
Epilogue: With all the present-day nuclear hysteria in Germany, the State government of North Rhine-Westphalia suddenly claimed that 2000 of the 675,000 radioactive thorium-graphite balls used in the THTR had gone astray. Luckily, this turned out to be a hoax!
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When we discussed the Rubbiatron at CERN, one technological issue was insurmountable. As an accelerator operates in a vacuum, there must be a separation (a thin window) between the thorium assembly and the proton machine. Such a metallic membrane will be hit by enormous amounts of radiation and thus be damaged quickly.
Epilogue: With all the present-day nuclear hysteria in Germany, the State government of North Rhine-Westphalia suddenly claimed that 2000 of the 675,000 radioactive thorium-graphite balls used in the THTR had gone astray. Luckily, this turned out to be a hoax!
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| Dr. Printz of the Jülich Research Laboratory proudly presents one of the "missing" graphite-thorium balls (DPA) |
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