The theoretical minimal time modal control strategy to suppress xenon oscillations in nuclear reactors was found to be of the Bang-Bang type. Such control policy implies instantaneous variation of the control poison between two extreme values. The switching action depends on exact knowledge of the location of the reactor state in the phase plane. The state is related to the measured axial offset, and the concept of axial offset phase plane is introduced. The main features of this phase plane can be constructed using a semi-operational method. Using the Carnegie-Mellon University xenon spatial control simulator, optimal and off-optimal control policies were tested and the capability to suppress the oscillation was demonstrated. Some of the attractive features of this suggested method are the simplicity of control policies, use of reactor output data, and the ability to initiate the control action once the oscillation is detected and to predict beforehand the outcome of the control decision, thus increasing the operator capacity to modify his decision.
