Power distribution control of reactor core in pressurized water reactors by optimizing xenon oscillation and axial offset of power

The power distribution control in nuclear reactors is one of the important control subjects. The objective of power distribution control is to assure a low peaking factor during reactor operation while maintaining flexibility of plant operation (load-following). The Russian WWER-1000 reactors have been designed for operation in base load and their control strategy is based on the control of peaking factor with the requirement to keep them within prescribed limits using in core instrumentation. In this project the possibility of core control improvement has been investigated based on Constant Axial Offset Control strategy. A simple reactor core control model which contains the main parameters of reactor core physics, has been developed for the control of Xenon oscillation in load following operation. The model is based on one group diffusion equation with moderator temperature and Xenon feedbacks. Two group of control rods including black and gray rods have been defined in the model. The mathematical equations of model with their solution at steady state condition have been obtained. In this model the calculus of variation have been applied for optimization method and the problem of two-point boundary value has been solved by the technique of initial value method. The numerical studies has been conducted with open loop control and optimization of Xenon oscillation and axial offset. The numerical results show that for daily load following operation between 100% and 80% of rated power, optimal solution closely follow the desired load demand and maintains the desires power distribution with a small control effort. Since, we have simplified the model by assumption, the accurate calculation by using proper computer code is necessary. The DYNCO computer code was considered for accurate calculations. This code is used for fast transient calculation in WWER-1000 reactor core. After preparing the code (addition of several subroutines for axial offset calculation and simulation of axial power distribution control methods in load following operation), the axial offset and the important parameters affecting the axial power distribution were calculated by using the code. After simulation and comparison of different methods of power maneuvering in DYNCO code, the following results has been obtained: In WWER-1000 reactors, the axial power shape is changed during the power maneuvering. As a results the value of axial offset exceeds from reference value. Therefor, the automatic control of axial offset is recommended. The best method for power maneuvering is using of Gray rods (Mode G) with 40% overlaps between control groups. This method by using the proposed algorithm, conduct the standard power maneuvering with axial offset control simultaneously, in different burnup stages