A comparison between time domain and frequency domain calculations of stationary neutron fluctuations

Unexplained neutron flux fluctuation patterns observed in some reactors were recently investigated by various European institutions. The time-domain code DYN3D is one of the tools used for simulating these fluctuations. Though, the applicability of time-domain codes for modelling small stationary fluctuations remains a discussed question. Aiming at a confirmation that these codes may be applied for neutron noise calculations, two special cases of neutron flux oscillations have been simulated with DYN3D and with CORE SIM, the latter one being validated for the context here. The comparison between the results of these two codes is the subject of this paper. This study demonstrates that time- and frequency-dependent calculations can give qualitatively equivalent results but substantial quantitative deviations may occur. Nevertheless, DYN3D may be considered as qualified for neutron-noise calculations as the deviations are smaller than 20 %. The optimization of the DYN3D setup is a matter of future research.