Design and Use of the Reactivity Computer

The basic, underlying variable of reactor control is the reactivity. We may regard the reactivity as a measure of the deviation from equilibrium of the neutron balance in a reactor. Or more precisely, we may define the reactivity as the average excess number of neutrons produced per fission that cause further fission, "excess" being the number over and above the one-neutron-per-fission that is required to maintain a self-sustaining fission reaction. Reactor control may be described as the process of changing the neutron environment within a reactor so as to produce momentary excursions in the reactivity from its equilibrium value of zero. The measurement of these excursions in reactivity is the subject of this paper. Two electrical analog computer designs are described which compute the reactivity from electrical signals which are provided by conventional reactor instrumentation systems. One computer utilizes a neutron flux input signal; the other employs a log-rate (of neutron flux) input signal. Applications of these computers in control system analysis and reactor instrumentation are discussed.