Nouvelles méthodes numériques pour le traitement des sections efficaces nucléaires.

Abstract : Nuclear data allow to describe how a particle interacts with matter. These data are therefore at the basis of neutron transport and reactor physics calculations. Once measured and evaluated, they are given in libraries as a list of parameters. Before they can be used in neutron transport calculations, processing is required which includes taking into account several physical phenomena. This can be done by several softwares, such as NJOY, which all have the drawback to use old numerical methods derived from the same algorithms. For nuclear safety applications, it is important to rely on independent methods, to have a comparison point and to isolate the effects of the treatment on the final results. Moreover, it is important to properly master processing accuracy during its different steps. The objective of this PhD is then to develop independent numerical methods that can guarantee nuclear data processing within a given precision and to implement them practically, with the creation of the GAIA software. Our first step was the reconstruction of cross sections from the parameters given in libraries, with different approximations of the R-matrix theory. Reconstruction using the general formalism, without any approximation, has also been implemented, which has required the development of a new method to calculate the R-matrix. Tests have been performed on all existing formalisms, including the newest one. They have shown a good agreement between GAIA and NJOY. Reconstruction of angular differential cross sections directly from R-matrix parameters, using the Blatt-Biedenharn formula, has also been implemented and tested. The cross sections we have obtained at this point correspond to a target nucleus at absolute zero temperature. Because of thermal agitation, these cross sections are subject to a Doppler effect that is taken into account by integrating them with Solbrig's kernel. Our second step was then to calculate this integral. First, we have elaborated and validated a reference method that is precise but slow. Then, we have developed a new method based on Fast Fourier Transform algorithm. Comparisons with the reference method suggest that the precision of our method is better than the one achieved with NJOY, with comparable computation times. Besides, we have adapted this method to the case where target nuclei are in a condensed state (solid or liquid). For this latter case, an alternative implementation was done to obtain cross sections by integrating the S(a,b) law that characterize the chemical binding effect on collisions between neutrons and matter. Finally, a method was developed to generate an energy grid fine enough to allow a linear interpolation of cross sections between its points. At this point, we have at our disposal the minimum amount of information required to produce input files for the Monte-Carlo transport code MCNP. Such data have been translated into the correct format thanks to a module of NJOY. Calculations have been performed using our input files on several configurations, to demonstrate that our methods can actually be used to process modern evaluated files. In parallel, as part of a collaboration with Institut Laue-Langevin, we have participated in the treatment of experimental measurements of the S(a,b) law for light and heavy water. With GAIA, we have combined experimental values with values from a molecular dynamics simulation, with the objective to avoid using a molecular model in the domain where experimental values are available. This has only been a first step, but the values obtained improves the predictions of the model of ILL reactor. As a conclusion, during this PhD, new numerical methods were developed and we have shown that they can be used in practical cases.
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Submitted on : Monday, October 27, 2014 - 3:36:17 PM
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Ghislain Ferran. Nouvelles méthodes numériques pour le traitement des sections efficaces nucléaires.. Physique Nucléaire Théorique [nucl-th]. École Polytechnique, 2014. Français. ⟨tel-01077764⟩

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