Xenon-induced axial power oscillations in the 400 MW pebble bed modular reactor

Abstract

This study evaluates the stability of the 400 MW Pebble Bed Modular Reactor (PBMR) core design with respect to axial xenon-induced power oscillations. Since the two-dimensional capabilities of the reactor dynamics code TINTE exclude the possibility of modelling azimuthal xenon transients, the study is limited to the axial xenon stability behaviour of the PBMR equilibrium core, with some minor notes on radial xenon stability. The primary aim of this work is to provide the first quantitative investigation into the degree of inherent axial damping in the PBMR annular core design, as well as the xenon oscillation stability under power load-follow operational conditions. It is shown that the TINTE code in its current form can be used with sufficient accuracy to model the axial variations that occur in the power density, iodine and xenon concentration levels, as well as the time-dependent feedback that exists between these parameters during load-follow transients. The detailed TINTE spatial power density and xenon concentration data were used to quantify the amplitude, period and linear damping properties of xenon-induced axial power oscillations. The representative operational load-follow transients that are investigated show well-damped xenon behaviour, and all local and global power and fuel temperature results are well within the prescribed safety limits. These calculations lead to the conclusion that the current PBMR design is inherently very stable against xenon-induced axial power oscillations, and that the design shows an adequate margin to the instability transition point in this respect. No active xenon oscillation control system is therefore recommended for the PBMR.