Accuracy of the Modal-Local Method for Reactivity Determination

Control rod worth measurements through the inverse kinetics equation depend on accurate determination of the amplitude function from detector signals. The modal-local method introduced in a previous study estimates space-time changes in the flux or shape function so that the amplitude function can be determined accurately and efficiently. A simple thermal-hydraulic feedback model is included in the modal-local method for at-power reactivity analysis. The method is tested with two simulated rod worth measurements: a zero-power rod drop experiment and a differential rod worth measurement in a power reactor. The modal-local method reproduces the reactivity obtained with the FX2-TH time-dependent diffusion theory code with an overall accuracy of 1 to 2%, except for simulated detectors located in the immediate vicinity of the rod motion.