Improved algorithms -a system of modern methods for controlling the energy release of the core of a VVÉR-1000 reactor -are described. Experience in adopting the improved algorithms in the No. 2 unit of the Khmel'nitskii nuclear power plant is presented.Improved algorithms (I-algorithms) comprise a system of modern methods for controlling the energy release of a VVÉR-1000 core, including a new arrangement of groups of regulating organs and methods for moving them in the core, an improved procedure for monitoring the energy release in the core, modern methods for providing information to the operator, and the control algorithms themselves (the sequence of actions taken by the operator). The improved algorithms are adopted in the new units of a nuclear power plant in order to optimize reactor control taking account of the prospects for operating on a daily load schedule. The fundamental possibility of operating on a daily schedule was confirmed by tests, performed in 1998 on the No. 5 unit of the Zaporozh'e nuclear power plant [1]. But these same tests showed that reactor control must be improved. Such algorithms were adopted, in part, in the No. 1 unit of the Volgodonsk nuclear power plant in 2001 [2] and completely adopted in the No. 2 unit of the Khmel'nitskii nuclear power plant in 2004. Work is now being performed to adopt the algorithms in the No. 3 unit of the Kalinin nuclear power plant.In the present paper, the special features of the improved algorithms and certain aspects of their adoption into operation are examined for the No. 2 unit of the Khmel'nitskii nuclear power plant.Monitoring of the Energy Release. The three-dimensional energy release field in a core is represented by the matrixwhere j is the fuel assembly number; i is the number of the layer in the vertical direction (from the bottom of the core); w(i, j) is the energy release intensity in the cell (i, j) in the core; and w av is the average power intensity of the cells.

The integral and spatial xenon transient processes in the No. 1 unit of the Tianwan nuclear power plant (China) have been studied experimentally. A measurement method which is unconventional for VVER-1000 was tested in the investigations of the integral processes: the course of the xenon process was recorded according to the variation of the critical concentration of boric acid in the reactor at the same time as the concentration was calculated in real-time. The spatial transient processes were studied for the diametric and axial free xenon oscillations of the energy release in the core. It was confirmed experimentally that axial deformations of the energy release affect the power of the reactor as well as the associated operational particularities of the automatic power regulator.Processes which are associated with a change of the 135 Xe concentration in the core have a large effect on VVER-1000 operation [1]. Xenon transient processes can be conventionally divided into integral -those involving a change of the average concentration of xenon in the core, and spatial -those involving a redistribution of the concentration within the interior volume of the core.Integral xenon processes result from a change of the power of the reactor. Their effect on the operation of the reactor is characterized by a reactivity increase or decrease and, correspondingly, a decrease or an increase of the xenon concentration. The following stages of the integral xenon processes are distinguished according to the character of the dominant effects: poisoning -xenon concentration increasing as a result of 135 I decay; unpoisoning -xenon concentration decreasing as a result of xenon decay; and xenon burnup -xenon concentration decrease as a result of neutron absorption by xenon.Calculations and experiments show that in the absence of control the xenon processes can be accompanied by power oscillations, which can be divergent and can result in reactor shutdown or power increasing above the admissible level [2]. In practice, ordinarily, the reactor power is kept at a prescribed level; the change of the xenon concentration is compensated by changing the 10 B concentration by moving the absorbing control rods and changing the boric acid concentration in the first-loop water.When the reactor shuts down, poisoning and subsequent unpoisoning occur in the core. In the process, correspondingly, the degree of subcriticality increases and then decreases. When the initial boric acid concentration in the first loop is

Currently operated VVI~R-1000 reactors operate in base-load support, and their standard data-acquisition and computing systems are designed in the main to maintain a steady state in the reactors. However, there are often deviations from base load operation, and a reactor may operate in a transient state several times during a run, which requires management in states far from equilibrium. Also, new designs for nuclear stations containing VVER-1000 envisage load following on the daily power graph.The operator should thus provide optimum management over conditions more complicated than base load ones, which requires appropriate data support. Such facilities have been incorporated at foreign nuclear power stations such as the BEACON system produced by Westinghouse [1].The need for similar systems for VV~R stations is reflected in the requirements of the state standard. For example, according to GOST R 50088-92 for VVI~R stations, when the reactor operates in the transient state, there should be provision for calculating the current state and forecasting its behavior, and also support to the operator to choose the optimum management.The "reactor simulator" program has been written for simulating load-following modes of Wt~R-1000 reactors. The first version is available, which does not require direct installation in the nuclear station. Instead, it is used off-line for examining load-following WI~R-1000 states and also for analyzing typical situations and for forecasting planned load following at nuclear stations.The functions of the "reactor simulator" program are briefly as follows. General Information. The program is a universal means of simulating the VVI~R-1000 in nonstationary states and is intended for forecasting in managing stations and also for use in design and research to improve fuel cycles and reactor management algorithms. Fortran-95 is used. The program works efficiently on personal computers of PC-AT/486 class or above.Reactor Model. Tile part dealing with the neutron physics (with a two-group diffusion model) and with the accumulation of wastes and the variations in t35Xe and 149Sm concentrations is the same as that in the BIPR-7 program [2]. The xenon kinetics are simulated together with calculations on the nonstationary 149Sm concentration and the fuel burn-up. In initiation and forecasting modes, one considers the nonequilibrium 135Xe concentration, while one considers the equilibrium one in load burn-up and archive translation modes.A major feature of the program is that the working control rod groups can occupy any positions, not merely ones that are multiples of the height step in the net used in the BIPR-7, and this is attained by use of a special interpolation for the neutron-physics constants.Basic Functions. The program implements the following basic functions: 1) calculations on reactor states and nonequilibrium fuel burn-up and 149Sm and 135Xe concentrations; 2) calculating the linear fuel pin power; 3) adjustment to the current state on the basis of the history and forecasting the subsequ...

The salient features of improved algorithms adopted at the Tianwan nuclear power plant for controlling the energy released in a VVÉR-1000 core are examined. The optimal configuration of the controlling groups is chosen in the first two power-generating units, the reactor power can be varied automatically under the control of an automatic power regulator, the boron regulation system makes it possible to determine the first-loop makeup automatically, and a modern version of the Imitator Reactora program has been installed. The results of testing the algorithms in the No. 1 unit in regimes with single and cyclic (daily) power maneuvers are presented. The tests of single power maneuvers were combined with dynamic tests of equipment. The operation of the power-generating unit in a daily load schedule was tested separately. Five daily load-change cycles were conducted during these tests.Improved algorithms consist of a complex of modern means adopted in new power-generating units at nuclear power plants for controlling the energy release in a VVÉR-1000 reactor. This complex includes a new arrangement of groups of control rods and methods for moving them inside the core, a procedure for monitoring energy release, means for providing information to the operator, and the control algorithms themselves (the sequence of actions taken by the operator). Such algorithms have been partially adopted in the No. 1 unit of the Volgodonsk (2001) nuclear power plant and completely adopted in the Nos. 2 and 3 units of the Khmel'nitskaya (2004) and Kalinin (2005) nuclear power plants, respectively, and the first two units in the Tianwan nuclear power plant (2007).Fundamental Features of the Algorithms. By definition, an increase of the axial offset corresponds to a redistribution of the power release into the top half of the core. The concepts of instantaneous (AO) and equilibrium (AO*) offsets, offset-offset, and offset-power phase diagrams are used [1,2]. The information provided to the operator is based on the use of the Imitator Reaktora computational program [3]. Free controlling groups of regulation rods with numbers 10, 9, and 8 ( Fig. 1a) are used to control the energy released in the core under normal conditions. These rods can be moved in manual or automatic regimes, individually, or with motion transferred from group to group downward or outward by 50 and 100% of the core height, respectively. Group 10 (working group) is always present in the core, and groups 9 and 8 are inserted into the core when the reactor is unloaded and when xenon fluctuations are suppressed. Xenon fluctuations are suppressed by maintaining a constant or equilibrium offset [4] or by spatially localizing the xenon processes [5].

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...