The IGR reactor has been in operation for more than 50 years [1]. It has been used to study various prototypes in engineering and technology. The procedure for working on the reactor is also improving. The basis for improvement is an understanding of the processes occurring in the reactor, the development of efficient computer codes for performing neutronphysical and thermophysical calculations, and analysis of the particulars of working with different irradiated objects.Knowledge of the changes in the power ratio of a fuel assembly placed in the central experimental channel to the reactor power and stability in recording the power are of great importance in operating the reactor. The questions raised cannot be solved on the basis of a one-point model of the reactor. In [2][3][4][5], it is shown that the computer codes of [6] for calculating the neutron-physical properties of IGR are highly efficient. The need to determine the temperature fields of the masonry by a computational method is substantiated in [5]. A method for calculating the dynamics of temperature fields and neutrons fluxes during IGR startup has been developed on the basis of the computer code of [6] and a code for performing thermophysical calculations [7].In this method, the startup time and the core masonry are divided into a definite number of parts with conventionally equal temperature at each time step, and the initial temperature field in the elements of the masonry (first time step) and the position of the control rods at each time step are given. At the present time the problem of reactor kinetics is not included in the method, so that the data on the actual power are used at each time step.Neutron-physical and thermophysical calculations are performed alternately for each time step. The neutron-physical calculations based on the temperature distribution over the masonry volume and the positions of the control rods are used to find the neutron flux distribution. To take account of the changes in the cross section in graphite [8], the temperature dependence of the graphite characteristics was calculated with step 50 K. In the thermophysical calculations, the increment to the temperature in the elements of the core masonry was determined on the basis of the energy release. The short startup time of the reactor makes it possible to assume that there are no heat overflows between them. Thus, it is shown in [5] that the smoothing time of the temperature field for IGR is several hours.As a rule, at the start of reactor operation the temperature of all masonry elements was set at the same value 300 K. The core masonry was divided into 124 elements along the horizontal and vertical sections with an empty central channel (Fig. 1). A small change of the temperature of the reflector masonry and other external elements, among which the water tank containing the safety-and-control system sensors is important, does not affect the change of the neutron field and is neglected in the thermal calculations.Reactor startup in the "pulsed" mode was conducted ...
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