Abstract :
The paper describes the causes and nature of instabilities in a graphite power reactor. The scientific investigation of these instabilities is of importance in the design of control systems of nuclear reactors. In the present approach a model of the reactor is chosen that is simple enough to be simulated on a laboratory analogue but includes the essential features of the problem. That the results and conclusions from the smaller system are to a large extent valid for practical cases seems to be borne out by earlier investigators. The stability of a cylindrical reactor, divided into two active regions, has been studied, first with one control rod for each region and later with just a single regulating control rod in the reactor. Each region is simulated as a point reactor coupled to the other region by neutron diffusion. The error signal for the control system is obtained from the change in mean fuel temperature. No dead bands or saturation are provided in the control system. For stable operation of a reactor the lower and upper limits for the controller gain and the nature of the oscillations at these boundaries are investigated. With one control rod, spatial oscillations of power are studied. The effects of variation of moderator temperature coefficient, the equilibrium flux level, and the location of the control rod have been examined. With the help of simplifying assumptions, certain of the above cases have been investigated analytically and compared with the computer results.
