Four power generating units with i~GP-6 reactors are operating at the Bilibino nuclear power plant. The units were put into operation starting in January 1974 with an interval of one year.The lining of the reactors at the Bilibin nuclear power plant is assembled from graphite blocks with a 198.6 x 198.6 ram cross section, arranged in a square lattice with a spacing of 200 x 200 ram. Each contains a cylindrical opening with a diameter of 88.6 +~ ram, whose axis is displaced relative to the vertical axis by I0 ram in the direction of one of the faces. The height of the graphite blocks can be 150, 300, or 450 nun. However, the height of the graphite block located-in the reactor core is 450 mm (the core is 3 m high), neglecting the ring-shaped end protuberance. The ring-shaped end protuberances at the top of each block, which insert into corresponding recesses in the bottom of a block, prevent transverse displacements of blocks in one column of the lining relative to another. A schematic sketch of a block is displayed in Fig. 1.Either fuel assemblies or the channels of the safety and control rods are placed in the opening of each column of the lining of the reactor core. In the core the fuel assemblies and channels of the safety and control rods consist of cylindrical graphite bushings with diameter of 88 -~ mm and 79.9 -~ ram, respectively, into which either six tubular fuel elements and a drop-down central steel tube (for the fuel assembly) or a tube with the cooling water (for the channel of the safety and control rods) are placed [1]. The reactor contains 273 fuel assemblies and 60 channels for safety and control rods.The gap between the graphite bushings and the graphite blocks in the liner was chosen on the basis of two requirements: it must be small enough so as to allow the removal of heat from the graphite l~ing and large enough to prevent the graphite blocks from jamming the fuel assemblies during the settling of the graphite under the action of neutron irradiation.The w/stematie measurements of the geometric dimensions of the openings in the cells of the graphite |inin~ at the nuclear power plant were begun in 1987, when the reactor in the first power generating unit had already operated for 13 years. The measurements were performed with the aid of an apparatus containing two mutually perpendicular differentialtransformer sensors, which converted their linear displacements into an electrical signal which was fed into a two-eharmel automatic plotter.The reactors in the nuclear power plant operate in a regime of partial fuel reloadings, performed with the reactor shut down, and the spent fuel assemblies are replaced with fresh ones. The design does not allow for rearrangement of the fuel assemblies in the reactor core and rearrangements are not performed. The diameter of the openings in the cells of the graphite cladding was measured after the spent fuel assemblies were removed. During each fuel reloading measurements were performed for two or three reactor cells. It seemed natural to choose for the measurem...
