Measurement of Anisotropic Neutron Diffusion Coefficients in Square Lattices of Aluminum in Light Water by the Pulsed Neutron Method

Neutron diffusion coefficients were measured in square lattices of aluminum channels in light water in both the axial and the radial directions by the pulsed neutron technique. The diameter of the channels was 15 mm and the pitch of the lattice was 19 or 24 mm. Good agreement was observed between the experimental values of the axial diffusion coefficient, D_a, and those calculated by the two-dimensional discrete S_n method. In this calculation, the value of the diffusion coefficient was interpreted as the slope of the decay constant as a function of the geometrical buckling in the axial direction of the channels. Also, the measured values of the radial diffusion coefficients agreed well with those calculated by the well-known Benoist practical formulas. The relation between the extrapolation distance and the effective transport length in the axial direction, _ℓa and 3D_a/V was numerically investigated. The ratio of the former to the latter is found to be considerably higher than the value of 0.71 used hitherto.