Xenon Oscillation Control in Large PWRs Using a Characteristic Ellipse Trajectory Drawn by Three Axial Offsets

Abstract

We have proposed a very simple xenon oscillation control procedure for axial xenon oscillation control based on a characteristic trajectory. The trajectory is drawn by three offsets of power distributions, namely, AOp, AOi, and AOx, which are defined as the axial offset (AO) of power distribution, AO of power distribution under which the current iodine distribution is obtained as the equilibrium, and that for xenon distribution, respectively. When these offsets are plotted on the X-Y plane for (AOp À AOx, AOi À AOx) the trajectory shows a quite characteristic ellipse (or an elliptic spiral). When the plot is at the origin of the X-Y plane, no xenon oscillation exists. Using the characteristics of the ellipse, the xenon oscillation can be eliminated by guiding the plot to the origin with control rod operation. In this paper, we will present an innovative and very simple procedure to generate the trajectory for the prediction of the oscillation behavior. Using the prediction capability of the xenon oscillation, the optimal control search calculation can be done. Since it is calculated by simple trigonometric functions and an exponential function, the prediction takes less than 1 s with a PC. It is especially effective for radial xenon oscillation control when the control rod motion is not flexible as for axial xenon oscillation control.

Optimal Control Timing Search of Xenon Oscillations in Large PWRs Using a Characteristic Ellipse Trajectory Based on Three Axial Offsets

“…Applicability 13) It has been demonstrated at PWRs in Japan that this procedure can terminate the xenon oscillation completely with one bang-bang control step. However, it is not assured that the initial control rod position can be recovered.…”

“…Although these derivations are presented in Ref. 13), we rewrite them below without neglecting the stability index.…”

“…A search for the optimal timing could be performed by trial and error by reconstructing the trajectory to predict the future behavior and thereby terminate the oscillation as discussed in Ref. 13). The trajectory can be reconstructed as follows.…”

“…In the previous paper, 13) we proposed an innovative and very simple procedure to generate the trajectory for the predicted oscillation behavior, which is expressed by trigonometric functions multiplied by an exponential function. In this paper, we present an optimal control timing search procedure for the bang-bang control step using the prediction capability of the xenon oscillation.…”