On the Control of Spatial Xenon Oscillations

Abstract

The control of spatial xenon-induced oscillations in large power reactors is considered from the point of view of practical operator manual control. A control strategy is developed based on control theory concepts and considerations of the physics of the problem. It is shown that oscillations can be eliminated by a simple control action consisting of positioning a control rod in a specified location for a specified length of time; upon retrieval of the control rod to its equilibrium position, the flux, xenon and iodine distributions will have returned to equilibrium conditions. A control equation is derived from which the control rod insertion time and the duration of control can be calculated. For large pressurized water reactors of current and anticipated designs, control rod insertion times are in the range from one to four hours before the peak in the oscillation, while control times vary from one-half to two hours. Digital diffusion theory simulations are described which tend to verify the control concepts developed in the paper. Constraints in local power peaking are introduced by considering control at off-optimal times. The study provides guidelines for operator control which is near optimal in the sense that control actions are minimized in number and are most effective in terms of eliminating the oscillation.