NEUTRON FLUXES IN A SLIGHTLY SUBCRITICAL REACTOR IN THE PRESENCE OF EXTERNAL SOURCES

Safety of nuclear plants, either when shut down or during the subcritical approach to criticality, depends on both the neutron flux and the reactivity as well as on the functional relationship between these two parameters. In a subcritical state, the fluxes strongly depend on the sources which maintain them, but the reactivity is insensitive to these sources. Inhomogeneous calculations with sources must thus be performed to evaluate the functional relationship existing between these parameters. This paper sketches a computational method based on a perturbation approach for calculating the fluxes from data belonging to the critical state only. The principle of the measurement of the reactivity of a slightly subcritical system is also presented as an application of the technique described here.