Millimetric fuel rod displacements: an experimental study of the impact on local thermal neutron flux in the CROCUS reactor

The impact on the thermal neutron flux due to a few mm displacement of a single fuel rod in the periphery lattice of the CROCUS zero-power reactor was measured. These high-resolution experiments were carried out using a miniature neutron detector positioned in the variable inter-pin space. The experimental data obtained show that a variation of [-2.3, +2.2] ± 0.2 mm at the detector level with respect to the nominal lattice position results in a variation of the thermal neutron count rate of [+3.62, -5.47] ± 0.07%, for a total relative variation of 9.09 ± 0.07 % over 4.5 ± 0.3 mm. Serpent 2 Monte Carlo simulations were performed for comparison purposes: the deviations between relative simulations results and measurement data are in the range of ±2.1%, and within 1 to 2 standard deviations.