A simple theoretical investigation of the effect of a moderator present in the central region of a hollow fuel element

In order to predict the thermal neutron flux increment in the fuel region of the cell, due to the transition from the situation without, to the one, with a central moderator positioned in the central region of a hollow fuel element, a simple theoretical model is used in combination with several experimentally obtained magnitudes, taken in positions of the cell where access is possible. Though the theoretical model is based on the one group diffusion theory and on a number of simplifications, the proposed method gives results which are in fair agreement with the experimentally observed ones.