A new characteristics algorithm for 3D transport calculations

In this paper, we present recent developments based on a characteristics method applied for solving general 3D geometries in the case of isotropic boundary conditions. Assuming isotropic sources and scattering, this characteristics solver involves the calculation of the region-to-region angular flux by scanning the tracking file containing the integration lines. The scalar flux is computed by collecting all mean angular fluxes in terms of the entering angular flux and the source of the region. At the boundary, the entering angular fluxes are linked to the emerging angular fluxes by isotropic albedos. The transport solution is similar to the one obtained by the standard collision probability method. The main advantage of this treatment is to get rid of collision probability matrices which have a dimensionality of the size of the square of the number of regions. For multigroup calculations, the rebalancing scheme was enhanced to take into account the external currents for any value of the albedos. Numerical comparisons are also presented in order to show the accuracy of the characteristics method as compared to the standard collision probability treatment for 3D supercells in the lattice code DRAGON. Several calculations for the incremental cross sections of adjusters and liquid zone controllers show that the characteristics results are accurate for the usual supercell calculations in a CANDU reactor.