Space-time correction for reactivity determination in source-driven subcritical systems

The presence of a localized spallation source in an accelerator-driven subcritical system leads to significant spatial variations in the power distribution and invalidates the simple point-kinetics approach. To eliminate higher-harmonics contamination in the detector response and to account properly for spatial and spectral effects in reactivity determination, a method directly combining measurements with numerical simulations of the experimental data is developed through a quasi-static formulation. The method provides space-time correction to a variety of traditional point-kinetics techniques and determines the reactivity essentially independent of the detector position, as long as sufficiently accurate information on the reactor configuration is provided. In the current work, the space-time corrections are derived for two well-known point-kinetics methods: area-ratio technique and a-method. Numerical simulations performed with the FX2-TH diffusion theory code along with a space-time analysis of MUSE-4 pulsed source experimental data illustrate the applicability of the proposed methods for the determination of significant subcriticality levels in fast and thermal reactor systems.