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An axial polynomial expansion and acceleration of the characteristics method for the solution of the Neutron Transport Equation

Abstract : The purpose of this PhD is the implementation of an axial polynomial approximation in a three-dimensional Method Of Characteristics (MOC) based solver. The context of the work is the solution of the steady state Neutron Transport Equation for critical systems, and the practical implementation has been realized in the Two/three Dimensional Transport (TDT) solver, as a part of the APOLLO3® project. A three-dimensional MOC solver for 3D extruded geometries has been implemented in this code during a previous PhD project, relying on a piecewise constant approximation for the neutrons fluxes and sources. The developments presented in the following represent the natural continuation of this work. Three-dimensional neutron transport MOC solvers are able to produce accurate results for complex geometries. However accurate, the computational cost associated to this kind of solvers is very important. An axial polynomial representation of the neutron angular fluxes has been used to lighten this computational burden.The work realized during this PhD can be considered divided in three major parts: transport, acceleration and others. The first part is constituted by the implementation of the chosen polynomial approximation in the transmission and balance equations typical of the Method Of Characteristics. This part was also characterized by the computation of a set of numerical coefficients which revealed to be necessary in order to obtain a stable algorithm. During the second part, we modified and implemented the solution of the equations of the DPN synthetic acceleration. This method was already used for the acceleration of both inners and outers iteration in TDT for the two and three dimensional solvers at the beginning of this work. The introduction of a polynomial approximation required several equations manipulations and associated numerical developments. In the last part of this work we have looked for the solutions of a mixture of different issues associated to the first two parts. Firstly, we had to deal with some numerical instabilities associated to a poor numerical spatial or angular discretization, both for the transport and for the acceleration methods. Secondly, we tried different methods to reduce the memory footprint of the acceleration coefficients. The approach that we have eventually chosen relies on a non-linear least square fitting of the cross sections dependence of such coefficients. The default approach consists in storing one set of coefficients per each energy group. The fit method allows replacing this information with a set of coefficients computed during the regression procedure that are used to re-construct the acceleration matrices on-the-fly. This procedure of course adds some computational cost to the method, but we believe that the reduction in terms of memory makes it worth it.In conclusion, the work realized has focused on applying a simple polynomial approximation in order to reduce the computational cost and memory footprint associated to a Method Of Characteristics solver used to compute the neutron fluxes in three dimensional extruded geometries. Even if this does not a constitute a radical improvement, the high order approximation that we have introduced allows a reduction in terms of memory and computational times of a factor between 2 and 5, depending on the case. We think that these results will constitute a fertile base for further improvements.
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Submitted on : Thursday, November 15, 2018 - 10:53:05 AM
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Laurent Graziano. An axial polynomial expansion and acceleration of the characteristics method for the solution of the Neutron Transport Equation. Discrete Mathematics [cs.DM]. Université Paris-Saclay, 2018. English. ⟨NNT : 2018SACLS389⟩. ⟨tel-01923317⟩

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