The peculiarities of development of neutron-physical model of the VVR-M research nuclear reactor in the SCALE calculation code are considered in the article. Models of separate core elements, which influence neutron-physical characteristics of VVR-M, have been developed. Simulation was performed using the CSAS6 control module. Validation of the VVR-M neutron-physical model, built in the SCALE calculation code, has been carried out by comparing the calculated value of the effective neutron multiplication factor with the critical reactor state at the beginning of seven fuel loads with the number of fuel assemblies in the core from 72 to 129. The model is developmed to determine the effective neutron multiplication factor in the reactor, as well as other neutron-physical characteristics, such as neutron spectrum, neutron flux density in various cells of the reactor. Thus, it is possible to conduct numerical experiments to determine the most optimal locations of research channels in the core of the VVR-M, to conduct physical experiments on the irradiation of the research samples, detectors, structural materials, etc. In the article, the simplifications accepted at construction of neutron-physical model of research nuclear reactor VVR-M in SCALE calculation code are presented. The main elements of the model are described: fuel assemblies, beryllium displacer, control rods.

In most algorithms for forming control signals, locks and protection of VVER, the value of the reactor's thermal power (RTP) is used. This article is dedicated to the analysis of the problem of determining the RTP of VVER-1000. The article suggests ways to improve the accuracy of the determination of RTP based on the signals of the neutron flux parameters control system at VVER-1000. The thermal power of the reactor is one of the important safety parameters of VVER-1000, and also this parameter determines the technical and economic parameters of the power unit. The task of increasing the accuracy of RTP determination is especially relevant considering plans to increase RTP of VVER-1000: in the first stage to 101.5 % of the nominal value, and later to 104-107 % of the nominal value, which equals to 3000 MW according to the project. In the article, the main factors influencing the errors of determination of RTP in different ways are considered: according to the thermal parameters of the 1st and 2nd contours and the parameters of the neutron flux in the Neutron Flux Monitoring System (NFMS) and In-core Monitoring System (ICMS). In order to improve the accuracy of determination of RTP in the NFMS, we propose a model that considers the influence on the signal of the ionization chamber of the following parameters: temperature and concentration of boric acid in the coolant, the position of the control rods, burning of fuel, etc. The results of the analysis of the change in RTP during the fuel campaign of VVER-1000 are given, which is determined in different ways.

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The reactor model was used to study the accident that destroyed the RBMK-1000 reactor at Unit 4 of the Chornobyl nuclear power plant on 26 April 1996. The model of reactivity accident of the RBMK-1000 reactor is based on equations of nuclear reactor kinetics, taking into account feedback in reactor reactivity. Reactivity changes as a result of both external influences – the movement of regulatory organs, changes in the reactor inlet coolant temperature, – and as a result of feedback by core parameters – changes in fuel temperature, coolant density, and 135Хе concentration. The model takes into account steam generation in the reactor core, which corresponds to the real physics of processes at the RBMK reactor, and allows obtaining simulation results that best match the recorded data and the consequences of the accident process. The study of reactivity accident on RBMK-1000 reactor is carried out for different combinations of values of control rods efficiency; reactivity coefficients by fuel temperature and coolant density; changes in the reactor inlet coolant temperature; the emergency protection time, as well as the reactor power level before closing the turbine generator stop valve. Different reactivity accident scenarios at RBMK-1000 reactor allow us to determine the most unfavorable combinations of external influences on the course of reactivity accident, namely: start time of main coolant pump rundown, time of activation of emergency protection, power level before the closing of turbine generator stop valves. In most reactivity accident scenarios, first of all, the critical values of fuel enthalpy are reached, at which the process of fuel destruction in the fuel element, destruction of the fuel assembly, and assembly channel start. Important results of studies are 1 – determination of the fact that time of activation of emergency protection after the closing of stop valves of turbine generator significantly affects the value of the maximum neutron power that is achieved during a reactivity accident; 2 – determination of the effect of reactor power before the closing of turbine generator stop valves on the course of the accident; 3 – it is not necessary to achieve supercritical on instantaneous neutrons, supercritical on delayed neutrons is enough to start fuel destruction.

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