Imitator reaktora") is a computational program intended for modelling the operation of the VVER-1000 reactor [1]. Its prototype is the well known program BIPR-7 [2], which is used mainly for planning fuel loading of the core. Unlike the earlier program, RS is oriented toward providing real-time information support to the operator of a nuclear power plant.At first, the functions of RS supplementary to BIPR-7 were developed with the aim of providing means of visualization and interactive control of the program simulating the control of the reactor by the operator of a nuclear power plant. At the same time, the program was checked using operational data from reactors at nuclear power plants, so that the following changes were made in the neutron physics subprogram:• the ability to calculate the state of the reactor with arbitrary penetration of the control rods into the core (BIPR-7 only permitted discrete positions coinciding with the boundaries of the computational layers); this is achieved by interpolating the properties of the fuel for a computational cell with absorbing control rods partially inserted into it. This enhanced the accuracy of the calculation of the height distribution for the energy release without having to increase the number of computational layers along the height of the core; • the program can be adjusted to real, generally nonstationary distributions of the concentration of 135 Xe in the core [3].By moving the controls or changing the boundary conditions at the end reflectors of the core (two alternative modes of running the program) the calculated and measured (i.e., specified by data from the internal control system of the reactor, SVRK) axial offsets of the distribution of the energy release can be brought into agreement. Hence, after simulating the reactor operation for a certain time the calculated and real distributions of the xenon concentration in the core converge, as is necessary if the accuracy of the calculation of the current state of the reactor is to be increased and its operation is to be predicted reliably; • a functionality has been developed for evaluating the microscopic distribution of the energy release [4] by superimposing the generally nonstationary distribution of the local power obtained with RS on a data base of values of the microdistribution obtained from the PERMAK program [5] for a set of stationary states of the reactor. This functionality is currently being modified for on-line operation. The RS program has been certified twice by the Russian Federation GAN, in 1998and 2002 (Certification No. 138 of February 21, 2002.Since 2001, RS has been developed as a means of information support for the operators of a nuclear power plant. In this capacity it is an essential part of the improved algorithms for controlling the energy release of the reactor core used in most new VVER-1000 power production units [6,7]. At present, RS is in use as part of a modernized internal reactor con-

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