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Voxel Conversion
The volume fraction calculation results can directed to a voxel conversion algorithm that uses the material fractions to define mixed materials. Two user parameters affect the mixing. Materials with fractions less than the ignore threshold are not included in the mixture. A new material is not defined if the current mixture is the same as an existing mixture; they are the same if each constituent is within the sameness threshold. The mixing uses materials defined in MCNP format. The converted geometry can be written in MCNP format, either as a lattice or as cells. The voxel conversion is a new feature (as of Moritz version 0.94); it has not been fully tested and is subject to revision. Development plans include formatting the results for use in deterministic transport codes, such as PARTISN, that run on a rectangular or cylindrical grid.
Material Mixing. New materials are defined by mixing for mesh cells that contain more than one material. The isotopes of the new material are added with the isotope fraction multiplied by the fraction of the material in the grid cell. The density of the new material is the fraction weighted sum of the constituent material densities.
Before mixing, the composition fractions and density of all materials are converted, if necessary, to weight fractions and to a mass density. The fractions are normalized so that their sum is unity.
Two thresholds can be used to limit the number of mixtures. Materials with fractions below an ignore threshold are ignored and the remaining isotope fractions are renormalized so that their sum is 1. The algorithm searches for another grid cell with the same material. Materials are considered to be the same if the difference between each isotope fraction in the grid cells being compared is less than the same-as threshold. If the same material is not found in another grid cell, a new material is created.
MCNP Geometry. The MCNP geometry may be written as individual cells or as a lattice. The lattice uses a smaller number of cells but adds somewhat to the execution time. When lattice geometry is written, a single spherical cell and universe is created for each material. These cells are centered on the unit lattice cell and are larger than the extent of the unit cell. The lattice elements are filled with one of these single cell universes according to the material at the lattice element position.
This page shows 2D and 3D views of a voxelized sphere. The colors are mixed between the blue of the sphere and the gold of the surrounding material using the same weighting fractions as used for mixing the materials. Voxels containing pure gold material are not shown in 3D.
This page shows a duct model constructed from portions of 3 tori through a concrete box (green). The blue material inside the duct and at the sides of the box is air. Pure concrete voxels are not shown in the 3D view. The bottom Figures show the model converted to voxels.
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