An experimental study of the interactions between control rods in a subcritical assembly

Measurements have been made of the reactivity controlled by 1, 2 or 4 control rods, of mild steel, cadmium or boron carbide at various positions in a natural uranium and graphite exponential assembly. Comparison is made with predictions based on: (a) thermal neutron absorption and no fast neutron absorption; (b) thermal neutron absorption together with fast neutron absorption represented by an extrapolation length averaged over a 1/E spectrum; (c) thermal neutron absorption together with fast neutron absorption in which the extrapolation length is averaged by means of age theory. The third method gives good agreement with experiment in media having few voids; theory does not satisfactorily account for the effects of large voids.