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...
The operation of the system monitoring the vacuum seal of the external claddings of the fuel elements in fuel assemblies of the I~GP-6 reactors at the Bilibino nuclear power plant (nAlnei" system) is described in detail in [1]. The principle of operation consists of sampling the reactor gas (nitrogen), which flows over the outer claddings of the fuel elements (just as in the reactor in the Obninsk nuclear power plant, the fuel elements are ring-shaped), transporting it to the electroprecipitation chambers, where the decay products of the fission products present in nitrogen (cesium and rubidium radionuclides) settle on rotating disks which deliver the settled sample to a/3-particle detector. Fuel elements of each of the 273 fuel assemblies in the reactor are monitored in this manner.In [1] it was noted that in some fuel assemblies the activity of the settled radionuclides is two to three times higher than the usually recorded level. It was suggested that the claddings of the fuel elements are not vacuum sealed.A careful analysis showed that the high activity of the radionuclides deposited from the reactor gas is observed only in the five central cells of each reactor, and it is 2-5 times higher than in the other cells.After the scheduled unloading of any fuel assembly and its replacement with a fresh assembly, the activity of the radionuclides deposited during the sampling of the gas from the corresponding cell remained essentially unchanged and high. Therefore, this is not an indication of the appearance of microcracks in the claddings of the fuel elements. The search for the reasons for this phenomenon led to the following.During assembly, in accordance with the design, to improve the monitoring of the reactors as they reached the critical state, beryllium oxide tablets were loaded into the graphite masonry. They were placed in the nine central ceils at the vertical center of the core. Each reactor contained about 70 kg of beryllium oxide. Fuel assemblies were placed in five cells (cells 12-I1, 11-10, 11-11, 11-12, and 10-11 in accordance with the numbering in the cartograms [2]), and cooling channels for the rods in the safety and control system were placed in the four other cells (cells 12-10, 10-12, 12-12, and 10-10).Since no reactions could occur on the beryllium or oxygen nuclei, which would give volatile radioactive products that would precipitate in the sensors of the nAinei M system, it could only be assumed that uranium or thorium impurities were present in the tablets. Estimates made assuming that the resonance absorption of neutrons by the impurity nuclei is not blocked showed that in this case, several years after the reactors started operating, the accumulation of 239pu ill the uranium and 233U in the thorium reaches several percent. Therefore it seemed that the thorium or uranium impurity in the beryllium oxide tablets, even in quantities of 10 -a %, could explain the observed features.To confirm or reject this assumption, a neutron-activation analysis of beryllium oxide tablets which were not lo...
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