Mitigating the numerical xenon instability in deterministic reactor burnup calculations

Abstract

Depletion methods used in modern core simulators for thermal-spectrum reactor burnup calculations may be predisposed to activating, under the correct conditions, a numerical instability that presents itself in the form of artificial spatial xenon oscillations. In this paper a three-dimensional pressurised water reactor depletion problem is utilised to demonstrate that one of the most widely used predictor-corrected depletion schemes can be afflicted by numerical xenon instabilities in reactor burnup calculations performed with a state-of-the-art nodal diffusion core simulator. A method which results in tightening the algorithmic coupling between the 135Xe transmutation and the neutron flux computation steps is proposed for mitigating the numerical xenon instability. The method is called the Xenon Transmutation Feedback (XeTF) method because it incorporates 135Xe transmutation in the feedback iterations of a neutronics solver. The effectiveness of the method is verified by numerical evaluation of some typical reactor depletion scenarios.