Subcritical Reactivity Calculations

Abstract

Simulations of subcritical CANDU reactor cores show that both the dynamic and fixed-source reactivities are generally more negative than the eigenvalue reactivity, at least when solid rods are used to supply the reactivity suppression.  Full-core reactor physics simulations using the different RFSP calculation methodologies (eigenvalue, dynamic, and fixed-source) are used to study situations in which these reactivity definitions differ significantly.  It is found that the fixed-source and dynamic reactivities always match quite well but the eigenvalue reactivity can be significantly closer to criticality in core configurations with a large flux tilt.  A transient that returns a highly asymmetric core configuration to criticality shows that the apparent additional shutdown depth seen in dynamic and fixed-source reactivity calculations coincides with a larger moderator poison coefficient.  The dynamic, fixed-source, and eigenvalue calculations all predict a return to criticality, to within a small fraction of the delayed neutron fraction, at the same core configuration.