Application of the “Reactivity Constraint Approach” to Automatic Reactor Control

The “reactivity constraint approach” is described and demonstrated to be an effective and reliable means for the automatic control of power in nuclear reactors. This approach functions by restricting the effect of the delayed neutron populations to that which can be balanced by an induced change in the prompt population. This is done by limiting the net reactivity to the amount that can be offset by reversing the direction of motion of the automated control mechanism. The necessary reactivity constraints are obtained from the dynamic period equation, which gives the instantaneous reactor period as a function of the reactivity and the rate of change of reactivity. The derivation of this equation is described with emphasis on the recently obtained “alternate” formulation. Following a discussion of the behavior of each term of this alternate equation as a function of reactivity, its use in the design and operation of a nonlinear, closed-loop, digital controller for reactor power is described. Details of the initial experimental trials of the resulting controller are given.