Large volume dosimetry for BLOOM in CROCUS
Volume dosimetry is part of the BLOOM experimental program on pile oscillation in CROCUS as an additional prospect. Indeed, in addition to online neutron flux measurements during oscillation, the objective is to measure the activation of a sample after its in-core oscillation, including the reconstruction of its activity profile. As a first step of the present study, scripts for the generation of Monte Carlo models of irradiations have been improved to represent the experiments in a sufficiently detailed manner, as well as the scripts for the analysis of the simulation results. It consisted in improving the modelling and analysing scripts to reduce uncertainties and have a better convergence of the calculations. Simulations are used to estimate the global activity of a sample after its oscillations in the CROCUS reactor, first for radiation protection purposes during the experiments, but also to predict the activity profiles. Axial, radial and azimuthal profiles are thought to be of special interest for validation and assimilation purposes, as they allow feedback not only to cross sections, but possibly to angular distributions as well. In a second step, simulations of spectrometry measurements are carried out to reconstruct the corresponding countings due to the sample profiles, and to design and optimise the experiments. In particular, a collimator has been designed by conducting a parameter study using Monte Carlo simulations. The two selected options of collimators have a thickness of 15 cm for lead, and 9 cm for tungsten, with a 0.2 cm radius hole for both. After test experiments, a pile-oscillation at 50 W during 1800 s has been performed. After a waiting time of 15.76 hours, the measurements performed on two different parts of the sample show a ratio between the lower and the higher activated part (i.e., with respect to the axial profile) of 0.93±0.02 (one sigma). This result is consistent within one sigma with the simulation results of 0.931±0.001. The consistency between experiment and simulations allows to validate the modelling and analysis scripts improved and developed during the internship. According to the simulations, azimuthal discretization is possible, however, the radial one is not. It is due to the small radius of the dosimeter (0.5 cm). Future developments include the use of a better collimator for the activity reconstruction. It should allow a better axial reconstruction and the feasibility of the azimuthal one. The next step of the project consists in the production of a tungsten collimator for the conduction of the spectrometry measurements along the experimental program.
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