Novel fine flux integration methods for the DIABOLO simplified transport solver in COCAGNE
As part of its activity, EDF R and D is developing a new nuclear core simulation code named COCAGNE. This code relies on DIABOLO, a Simplified P{sub N} (SP{sub N}) method to compute the neutron flux inside the core for k{sub eff} eigenvalue problems. In order to complete complex simulations involving a large number of successive eigenvalue calculations within acceptable CPU times, the DIABOLO solver uses computational meshes that contain a restricted number of cells (e.g. 4x4 cells per assembly). Solutions obtained on these relatively coarse computational meshes are then spatially interpolated to obtain physical quantities such as neutron production rates on finer meshes (e.g. pin-by-pin power production). This paper describes two novel methods, named Straw-hat and Poisson, that allow the neutron flux to be integrated over fine mesh cells. Unlike the original method used in COCAGNE, these new methods compute accurate fine flux integrals trough the interpolation of the current DOFs of DIABOLO'S mixed dual RTk finite elements. Assessed on a set of 3-D PWR realistic core configurations, these methods are shown to bring about significant improvements over the original integration scheme: for smooth enough flux distribution, the Poisson method improves the pin-by-pin production rates accuracy by as much as one order of magnitude. (authors)