Development of a modal reduced-order model of xenon oscillations in PWRs

Journal article, 2027

This work presents a modal reduced-order modelling approach to predict the stability of xenon oscillations in large Pressurised Water Reactors (PWRs). The model, labelled 2G-HET, is developed and compared to a simplified model (2G-HOM). The 2G-HET model takes the two-energy group and three-dimensional macroscopic cross sections, homogenised at the assembly or sub-assembly level, as well as the reactor geometry, as input. It calculates spatial eigenmodes and their coupled time-dependent amplitudes for neutron flux, iodine, and xenon concentrations. In contrast, the 2G-HOM model uses cross sections homogenised over the entire core-reflector region and analytically calculated eigenmodes.

The 2G-HOM and 2G-HET are compared against each other based on simulated cross-section data from the Ringhals-4 PWR in Sweden. The analysis shows the significant influence of mode coupling on power and axial offset deviations, with 2G-HET capturing detailed spatial dynamics absent in 2G-HOM. Specifically, the heterogeneous treatment reveals couplings between even and odd modes, not present in the homogeneous model. In a case study, the 2G-HET model showed stable decaying oscillations with modal periods in the interval [31.5;39.3] h. Comparisons with 2G-HOM over a range of power feedback coefficient strengths (FB-scaling) show similar asymptotic behaviour between the two models. However, 2G-HOM have an onset of stable oscillations at 25% lower FB-scaling value compared to 2G-HET. It also consistently displays faster decaying oscillations compared to 2G-HET.

The results highlight that high spatial resolution plays an important role when simulating xenon oscillations following reactor perturbations using three dimensional modal expansion models. The findings show that 2G-HET captures more features of the system, which substantially alters the stability prediction and mode-coupling behaviour. Capturing these features has potential applications in ensuring safe and flexible reactor operation by capturing more complex dynamics of xenon oscillations which cannot be reproduced in simpler models.