Prompt Alpha and Reactivity Measurements on Fast Metal Assemblies

This paper summarizes and reviews the methods of reactivity determination and measurement of the prompt neutron decay, briefly describes the equipment requirements for such measurements for unmoderated and unreflected metal assemblies and presents experimental results to illustrate the methods. The primary reactivity determination methods used have been (1) stable reactor period measurements, which are usually used near delayed criticality to calibrate the reactivity prior to burst initiation, (2) prompt reactor period measurements which are useful to determine the reactivity early in the excursion, (3) inverse kinetics rod drop measurements which obtain the reactivity as a function of time after a rod or reactor component is removed from the assembly, and (4) prompt neutron decay constant measurements from which the reactivity can be obtained if corrections are made for changes in the neutron lifetime. Inverse kinetics and decay constant measurements are usually used below delayed criticality although decay constant measurements have been performed above delayed critical.