Among the elements of nuclear reactors is graphite masonry, which is used as a neutron moderator. The operation of these reactors leads to the formation of irradiated graphite, for which there is no optimal decontamination technology yet. The biggest hazard in irradiated graphite comes from carbon-14, a substance with a half-life of 5,700 years, which is a product of neutron irradiation of nitrogen-14 from a nitrogen-helium mixture. Graphite-14 is deposited on the surface of graphite blocks, from which it is then extracted for safe disposal.
The experts from InnoPlasmaTech, which specializes in the decontamination of nuclear power plants, have proposed their own method for extracting graphite-14. The method envisages an irradiated graphite block being placed in a chamber filled with argon plasma, after which carbon-14 gets removed from the graphite surface by spraying argon ions. At the same time, the graphite block is heated by plasma to a temperature of 1,700 degrees Celsius, as a result of which other radionuclides move from the graphite to the surface, from which they are also removed by spraying.
The new ion-plasma technology makes it possible to avoid the formation of secondary nuclear waste without damaging the integrity of the graphite blocks. Its principles are also applicable to the reprocessing of nuclear fuel. For that purpose, spent nuclear fuel pellets are placed in a separation pipe equipped with cylindrical liners. Argon, an inert gas, is passed through the pipe in order to perform plasma separation of the spent fuel pellet. The surface temperature of the pipe can vary from 2,600 degrees Celsius where the pellet is located to room temperature at the other end.
Due to the temperature gradient and saturated vapor pressure differences, various chemical elements are deposited on different parts of the separation pipe (removable liners), which is why they can be separated with an accuracy of at least 99%. The removal of liners makes it possible to obtain newly-separated components of nuclear fuel. These include uranium and plutonium, which can be reused, and strontium for long-lasting betavoltaic batteries.
“The proposed approach makes it possible to significantly reduce the costs of decommissioning uranium-graphite reactors and not only make the reprocessing of spent nuclear fuel tens of times cheaper, but also make it more eco-friendly. The next stage in the development of the ion-plasma technology will involve tackling a number of scientific and design objectives: developing and building a prototype device and conducting operations under conditions close to reality at nuclear power facilities,” Anna Petrovskaya, candidate of physical and mathematical sciences, is quoted as saying by the Russian Science Foundation.
