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.

The results of investigations of the nuclear safety of the masonry in the AI uranium-graphite (Industrial Association Mayak) reactor are presented. It is concluded on the basis of these results that the masonry is nuclear safe and a radiation certificate is composed. The radiation examination made it possible to determine the level, composition, and distribution of the radioactive contamination of the masonry as well as the level and distribution of the neutron and γ radiation, and to construct a forecast of the change in the activity of radionuclides in graphite as a function of the holding time. These data are necessary for safety analysis and for making decisions about the subsequent stages of decommissioning of the reactor.The radiation examination of graphite masonries of stopped commercial uranium-graphite reactors is conducted in accordance with the safety rules for decommissioning. The results of an examination of the masonries of the AV-1, -2, and -3 reactors are presented in Ref. 1. The AI reactor differs from the AV reactors by the construction [2] and operation. Of the structural differences, the smaller dimensions of the masonry and the larger diameter of the access hole in the graphite blocks must be singled out. According to the operating history, we note that the masonry has been partially disassembled and the graphite columns were replaced in 1956, a measure that had to be taken to liquidate the consequences of an accident with fuel meltdown, and that after 1967 the reactor operated in a plug-free regime. In the period after the masonry was disassembled, one accident during which part of the fuel entered the masonry occurred in 1974. The AI reactor was shut down in 1987.In 2003, core samples were extracted from four cells, one of which is a neighbor of the damaged cell, to investigate the composition, level, and distribution of the radioactive contamination of the graphite. Experimental data on the specific activity were obtained for γ-emitters (Co 60 , 137 Cs, 134 Cs, 155 Eu, 154 Eu, 241 Am), β-emitters ( 90 Sr + 90 Y, 14 C), and α-emitters ( 238 Pu + 241 Am, 239 Pu + 240 Pu, 244 Cm).The radioactive contaminations of the graphite contain products of activation and fission as well as transuranium radionuclides. The latter are located primarily in a region close to the damaged cell. The products of activation of graphite and the impurities contained in it are distributed over the entire volume of the masonry.

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