Moderating ratio parameter evaluation for different materials by means of Monte Carlo calculations and reactivity direct measurements.

The aim of this work is to determine moderating properties of different materials, in specific the Slowing-down Power (SDP) and the Moderating Ratio (MR). SDP indicates how rapidly a neutron will slow down in the material, but it does not fully explain the effectiveness of the material as a moderator. In fact, a material can slow down neutrons with high efficiency because of its high scattering cross section, but it can be a poor moderator because it also absorbs neutrons with high probability. Thus, the most complete measure of the effectiveness of a moderator is the MR parameter because it takes into account also the absorption effects: the bigger is the moderating ratio values, the more effectively the material performs as a moderator. The first part of the work consisted in the comparison between the SDP and MR parameter evaluated for different materials by means of Monte Carlo simulations and by means of calculations based on their definition formula: these calculations are based on knowledge of material composition and of microscopic cross section (derived from literature). The second part of the work was dedicated to correlate the materials’ MR values with the measured variation of reactivity induced by the insertion of the materials in the core of TRIGA Mark II reactor of the University of Pavia.