No abstract

No abstract

Cores of uranium-graphite reactors use graphite with specified properties produced by high-temperature graphitization of natural materials (coke, pitch, resin, etc.). The graphitization removes impurities because they evaporate faster than the graphite.Nuclear-pure graphite was used in the first industrial reactor (1946), in the Obninsk nuclear power plant (1954), and in many other power units. The total mass of graphite removed from or still in uranium-graphite reactors is approximately 50,000 tons. As it slows down neutrons, graphite has been irradiated, in industrial reactors for example, at an average thermal neutron fhience of up to 3.1022 cm -2.A large number (up to 30) of natural elements exist as impurities in reactor graphite at concentrations of 10"4-10-6%by weight, many of which form long-lived radionuclides as a result of (n, ~,), (n, p), (n, ~), and (n, 2n) reactions. Along with this group of radionuclides there are also actinides and fission products. They are produced for many reasons, but they come mainly from fuel particles that enter the graphite cladding of the fuel elements as a result of accidents and stuck fuel element bundles. The mass and localization of fuel in graphite cladding was determined by nondestructive neutron and x-ray tests [I] after several reactors were shut down and unloaded. Experiments and calculations showed that a total mass of approximately 70 kg of uranium particles with an initial ~ 3% enrichment was in the cladding of the AMB-100 reactor of the Beloyarsk nuclear power plant and 2-10 kg with an initial enrichment of 0.71% in the cladding of industrial reactors. The fuel particles had been irradiated in the open over 20-30 years at a temperature of -550~ and had reacted with purge nitrogen at a rate of -500 m3/hr, as well as with water-steam mixtures when coolant entered the graphite cladding during accidents. As a result, the graphite in several reactors acquired anomalously high radiation properties. However, besides fuel particles from accidents, fission products and actinides that can be explained by natural uranium and thorium impurities are observed in reactor graphite (estimated up to 10-5%), as well as uranium contaminants on the surfaces of the fuel-element cladding. The total of both of these factors gives an upper estimate of about 0.5 kg per 2000 tons of graphite in the cladding of industrial reactors or RBMKs before they are put into operation.Probably radionuclides, 6~ for example, can enter the cladding as a result of corrosion. However, their contribution is negligibly small compared to the background of radionuclides produced from uranium and plutonium fission and from activation of impurities (including 6~ from 59Co at concentrations up to 10-4%). Tables 1-3 show calculated specific activities of long-lived radionuclides in graphite and uranium. For application purposes they are normalized to operational parameters relatively common for all uranium-graphite reactors: 30 years' radiation, 3-1013 sec-t-cm -2 thermal neutron flux, 0.05 spectral hardness...

The results of an examination of the radiation state of the graphite masonry in three commercial uranium-graphite reactors at the Mayak Industrial Association are presented. On the basis of these results, conclusions are drawn about the nuclear safety of the masonry and radiation licenses are composed. The comprehensive radiation examination made it possible to determine the level, composition, and distribution of radioactive contamination of the masonry and the level and distribution of neutron and γ radiation and to predict the variation of the radionuclide activity in the graphite as a function of the holding time. These data are required to make design decisions about further reactor decommissioning stages.A radiation examination of the graphite masonry of commercial uranium-graphite reactors which have been shut down is performed in accordance with the rules for ensuring safety during decommissioning. During the time these reactors were in operation, accidents with melting of the uranium blocks with the natural content of 235 U had occurred. When the consequences of such accidents were liquidated, some of the fuel entered the graphite masonry, forming fragments which were irradiated until operation ceased.An examination of the masonry of AV-type commercial reactors, which were shut down in [1989][1990], has now been completed. The operating organization -the Mayak Industrial Association -as well as its associates the Physics and PowerEngineering Institute, the Russian Science Center Kurchatov Institute, and the Siberian Integrated Chemical Plant (which obtained the graphite samples) participated in this work. The examination included the following steps: selection of radionuclides and determination of their activity in samples of graphite blocks and bushings, calculation of the total radionuclide contents of the masonry, neutron and photon probing of the masonry, determination of the γ-ray exposure dose rate and the fluxes of thermal and fast neutrons in the cells, and determination of the content of transuranium elements and fissile materials in the masonry.

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 excursiontest units at the Beloyarsk nuclear power plant. Their operation was directed towards attaining important economic goals: production of plutonium and radioactive isotopes, developing heat and electric power, and testing materials and equipment. However, 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. It is appropriate to note that government-level standard safety documentation for nuclear power units was only introduced in the 1970s, when several uranium-graphite reactors had already operated for 20-25 years.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 socalled "scale," which was removed 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. Experience in repairing a stack after two fuel-element accidents has been discussed [1]. However, after a large number of accidents, repairs do not completely solve the problems caused by fuel particle dispersion. It was the accumulation of a large amount of fuel fragments and particles that was the main reason for the early shutdown of both reactors in the first series at the Beloyarsk nuclear power plant.If no fuel-element accidents had occurred over the whole reactor operation period, long-lived radionuclides would have formed only from nuclear reactions in carbon and the small amount of atomic impurities that occur in nuclear-grade graphite. Long-term (20-30 year) irradiation of the fuel particles in the graphite contaminates the graphite with fission products and transuranium radion...