Study of key nuclei for present and innovatice fuel cycles
In the frame of studies dedicated to molten salt reactors, elastic scattering cross section measurement experiments on 12C, 19F and 7Li nuclei took place at the PEREN platform. These measurements required the making of a slowing-down time spectrometer in graphite, where were inserted targets (~20 kg) containing 19F or pure 7Li. In 2009 7Li targets were processed at LPSC in the glove boxes under controled atmosphere of the PEREN-Molten salts facility, and pure 7LiF salt ingots were obtained. This program brought semi-integral measurements of elastic scattering cross sections of natural Carbon and Fluor in a slowing-down neutron spectrum. A relative measurement of elastic scattering cross sections of 7Li et du 19F allowed the comparison with existing evaluations for these nuclei.
Neutron physics in the slowing-down time spectrometer
Neutron transport in matter can be modelled using the Boltzmann equation, an equation which is notoriously difficult to solve, including for the simplest geometries. To circumvent this difficulty, we have studied the statistical properties of the collision chains performed by neutrons. This approach has lead to two noticeable results: (i) the formulation of the neutron slowing-down times and thermalisation times as a function of their energy. Knowledge of these times is required to quantify the sensitivities of the scattering cross-sections, measured using slowing-down time spectrometers, to neutron energy groups; (ii) the formulation of the number of elastic scatterings required to slow-down a neutron. This solution ends a classical problem in neutron physics and validates the up-to-now accepted approximation.