The impact of heavy reflectors on power distribution perturbations in large PWR reactor cores

Highlights: • We study the power sensitivity to a heavy and conventional reflectors of a large PWR core. • At low power, a peripheral perturbation causes a bigger power tilt when a heavy reflector is present. • Difference between heavy and conventional reflectors vanishes increasing core power and BU. • The first two eigenvalues analysis can only explains this different behavior at nearly-zero power. • When the core power increases, this approach fails and utilization of more eigenvalues is necessary. - Abstract: The paper is intended to provide a simple – but general – overview of the question of neutronics coupling in large PWR cores. As an example, we have investigated the sensitivity of the radial power distribution in the core to the nature of the reflector (conventional vs. a "heavy" stainless-steel one). This was done by applying a perturbation to some peripheral assemblies of the UAM GEN-III benchmark with a fresh fuel core. The perturbation was selected to simulate possible assembly bowing effects, causing a variation of the inter-assembly water gap, which can generate an azimuthal asymmetry in the power distribution. This is conventionally referred to as a 'power tilt'. The amplification effect of the reflector on the power tilt for fresh fuel has been addressed in two steps. Firstly, we have compared the power tilt, computed with the deterministic code CRONOS2 (the neutronics diffusion module of the HEMERA chain) to that found with the probabilistic transport code MCNP, so as to validate the CRONOS2 response against a Monte Carlo reference and also obtain a quantification of the amplification effect at zero power. Then the thermal feedback and burn-up effects have been accounted for in the diffusion calculations so as to evaluate the sensitivity of the power tilt to the reactor operating conditions. The difference in the behavior of the cores with a conventional and a heavy reflector has been investigated through the examination of the power distributions and the dominance ratio of the systems, as well as of the assembly-wise infinite multiplication factors. Whilst the results help to explain the higher sensitivity to a peripheral perturbation of a fresh fuel heavy-reflected core compared to a conventionally-reflected one, they do not enable fully comprehensive conclusions to be drawn, especially towards End Of Cycle, due to the model assumptions and the limitation in the eigenvalue expansion.