Sunday, April 10, 2011

Was-a-Belle

When I visited Brookhaven National Laboratory (BNL) on Long Island, NY late in 1983 the US government had just stopped the construction of a 200+200 GeV superconducting proton collider called ISABELLE (the Intersecting Storage Accelerator + "belle") after having already spent 200 Mio U$ on infrastructure and superconducting magnets. Frustrated my American colleagues had renamed the killed project Was-a-belle.

The most beautiful machine: A similar situation happed in Germany when in 1989 the State government of North-Rhein-Westfalia after having spent 2 Billion Euros stopped the operation of a pebble bed reactor (Kugelhaufenreaktor) what was then called the most beautiful machine. From today to tomorrow the high temperature reactor THTR 300 in Hamm-Uentrop had become a was-a-belle. The main reason for shutting down the is-a-belle machine had been the Chernobyl radioactive cloud that had passed over Europe in 1986.

What is it that makes the high temperature pebble bed reactor so beautiful? As any other reactor the THTR works on nuclear fission but the main material burned is Thorium-232 although 20% of Uranium-235 is needed to create a critical assembly sustaining 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) formed into spheres of the size of a tennis ball. Such an assembly has a negative temperature coefficient i.e. as the temperature increases the fissionable material-carbon mixture eventually becomes uncritical i.e. the THTR had an inherent safety feature.

How does the THTR operate: When the THTR is running thermal neutrons breed Uranium-233 from Thorium-232 where the former is fissioned "on-line". Instead of uranium the THTR burns the on earth more abundant thorium with only a small amount of plutonium produced. The theoretical energy efficiency of boiling water reactors (like those in Fukushima) is 47%, the one for the helium cooled pebble bed reactor is 71%,  for the beautiful machine did not only generate electricity but also high temperature heat. This heat was intended to be used for the hydrogenation of the abundant coal on the Ruhr producing methane and other hydrocarbons.

During operation fresh carbon-thorium balls were fed into the reactor vessel from above while burned out spheres were taken out from the bottom and reprocessed. Although the THTR produces a small amount of plutonium the radioactive waste from the burning of 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: For those who are still worried about the small amount of plutonium produced Carlo Rubbia's energy amplifier may alley their concerns. In coupling a thorium assembly with a proton accelerator the nuclear fission is sustained by neutrons produced from spallation reactions. No Uranium-235 is needed and no plutonium is produced. Why is the machine called energy amplifier? One has to input a certain amount of energy to get the more energy producing chain reaction going. This also means: when the accelerator is switched off nuclear fission stops immediately. Such a proton accelerator could likewise be used to "incinerate" the nuclear waste produced.

When we discussed the Rubbiatron at CERN one technological issue seemed to be insurmountable. As an accelerators 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 in a short time.

Epilogue: With all the present day nuclear hysteria in Germany the State government of North-Rhein-Westfalia suddenly claimed that 2000 of the 675 000 radioactive thorium-graphite balls used in the THTR had gone astray. However, this turned out to be a hoax!

Dr. Printz of the J├╝lich Research Laboratory proudly presents
one of the"missing" graphite-thorium balls (DPA)

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