All BBER-1000 reactors are currently operating in their base regime, which permits only single load changes which are isolated in time. However, experience in performing such individual power maneuvers as well as computational investigations show that reactors of this type can operate in a maneuvering regime, specifically, with power changes occurring daily [ 1 ]. In the period from March 29 to April 6, 1998 power maneuvers were performed on the fifth power generating unit of the Zaporozh'e nuclear power plant. In the course of these tests a typical daily-weekly cycle was conducted in which the grid load was followed by night-time load reduction to 80% nominal power (Nnom) on work days and shutdown on holidays. The purpose of the tests was to finish the control algorithms experimentally, to check the working capacity of the technological systems and equipment, and to assess the accuracy of the computational simulation of reactor operation.Control Algorithm. In contrast to the power manuever in the base regime, which consisted of switching the reactor from one stationary state into another, for daily power maneuvers the reactor is always in a nonstationary state on account of transient xenon processes. This requires continuous control of the reactivity and energy-release distribution in the core. This feature of the maneuver regime can lead to extreme accumulation of liquid radioactive wastes accompanying water exchange in the boron regulation system, In this connection a control algorithm with minimization of water exchange was optimized in the present tests.A power maneuver was performed using the following control actions: two groups of control rods -the working group and a control group with vertical distribution of energy release (controlling group) were moved; their position in the core is shown in Fig. 1; temperature regulation -variation of the water temperature at the reactor entrance (Tin) within the range of the corresponding variation of steam pressure in the second loop (P2) 6-6.2 MPa; introduction of distillate into the first loop in the quantities required to compensate burnup during the preceding days, together with spontaneous "reactivity overshoot" as a result of the burnup of xenon at the power increase stage.The control of the energy-release distribution reduced to preventing the development of xenon fluctuations of the axial offset. Since in the present tests power is lowered mainly as a result of the motion of groups, and under existing regulations this does not permit maintaining a constant offset, a different principle of offset stabilization, based on spatial localization of xenon processes, was used. According to this principle, to decrease the reactor power the vertical distribution of energy release is formed using groups in a manner so that the power of the bottom half of the core does not change and the entire change occurs in the top half-in this case the xenon transient processes occur in a small volume and cannot give rise to strong spatial xenon fluctuations. In addition, the requ...

The safe and efficient operation of a nuclear power plant is directly related to the introduction of fundamentally new methods of monitoring and diagnostics in technological processes. This concerns monitoring and diagnostics of nuclear fuel, specifically, fuel assemblies. One such new method is monitoring nuclear fuel during reloading to determine the degree of burnup. In [1,2], burnup is estimated from measurements of the characteristic γ radiation from fuel assemblies. Using a different method -passive reconstructive tomography -information can be obtained not only about burnup of fuel assemblies but also about the distribution of fission products in each fuel element. Tomography was proposed in [3,4] for PWR fuel assemblies in [5] for VVÉR fuel assemblies. The goal of tomography is to monitor the integrity of fuel assemblies during storage and accounting for and monitoring nuclear materials. In the present paper, tomography is considered as a means for solving practical problems of diagnostics [6]: it makes it possible to calculate the burnup of fuel inside an assembly, monitor the seal of fuel-element claddings, and estimate the initial enrichment of fresh fuel. Tomographic examination of fuel assemblies is conveniently combined with reloading of fuel, when tomography can be performed in real-time without changing the reloading schedule. The basic principle of tomographic analysis of fuel during reloading consists of measuring the characteristics of the γ radiation fields from fuel assemblies followed by reconstruction of the distribution of fission products over the volume of a fuel assembly, using methods of passive reconstructive tomography.Computer tomography requires a detector with high resolution (the best such detector is a CdZnTe detector) or a collection of spatially separated detectors, a digital γ-spectrometric channel, and a computer of average capacity for processing and interpreting the results. There are several ways to form spatial projections of the characteristic radiation field of a fuel assembly: discrete angular displacement of the fuel assembly being monitored around its axis, arranging a large number of detectors around the fuel assembly being monitored, and arranging γ-ray detectors at several angular positions around a fuel assembly. Irrespective of the method of computer tomography, the radial displacement of a detector or the fuel assembly being monitored is difficult from the structural standpoint. Consequently, in what follows, computer tomography is studied only for angular projections of the characteristic radiation from a fuel assembly.When a detector is placed at the nth observation point at a distance R from the axis of a fuel assembly, the measured intensity of γ radiation from the ith isotope with energy E γ at the position of the detector is (1) where A mi is the activity of the ith isotope for the mth fuel element, taking account of its actual state; k iγ is the yield of the γ line with number γ for the ith isotope; w mn is the coefficient of the contribution of the mth...

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