Radiation characteristics of fuel residues in the graphite stacks of decommissioned uranium-graphite reactors

Power units with channel-type uranium-graphite reactors occupy an important place in nuclear power, both in their number and in length of operation. Among them, the longest operating are the current unit of the Obninsk nuclear power plant, the first in the world, many industrial reactors which have now be decommissioned, and two first-series excursion-test units at the Beloyarsk nuclear power plant. As operational experience has shown, starting in the 1950s, safety problems of units with uranium-graphite reactors were not completely resolved, as evidenced by past accidents and the accident at the Chernobyl nuclear power plant. The operation of reactors always was and is now much more important for the economy than building and decommissioning them. Both the planning and the operation of the units did not consider future decommissioning, which complicated already complex problems of shutdown and the safe preservation and burial of the units in place without completely dismantling them. The last problem (dismantling) evidently will be solved much later. Individual fuel elements failed during the operation of almost all the uranium-graphite reactors, in particular the first series at the Beloyarsk nuclear power plant. The failures with the worst consequences were accidents that formed so-called {open_quotes}scale,{close_quotes} which was removedmore » by grinding that formed fine particles and fuel fragments, which remained in the graphite stack. Study of the archives shows that an average of 0.5 kg fuel was lost in each such accident. The presence of unsealed fuel residues and fragments in the core of a reactor operating at power increases the release of gaseous aerosols and radionuclides, breaks down the graphite cores, which are contaminated with fuel fragments, and diffuses and disperses fuel particles and fission fragments throughout the graphite stack.« less