A General Method of Conserving Mass in Complex Geometry Simulations on Mesh Grids and Its Implementation in BOT3P5.0

The finite difference approach poses a major problem of keeping the exact values of material zone areas and volumes in any geometric simulation for transport calculations. When this requirement is not thoroughly fulfilled, updating density values may be necessary to conserve material zone masses. A method is described that conserves the mass of geometrically complex material zones simulated on both Cartesian and cylindrical mesh grids and its implementation in BOT3P5.0, which is the latest version of the BOT3P code package, publicly and freely available from the Organization for Economic Cooperation and Development/Nuclear Energy Agency Data Bank. BOT3P5.0 lets users optionally require as refined a computation as desired of the possible area and volume error of material zones due to the stair-cased geometry representation and automatically corrects material densities to globally conserve masses. BOT3P5.0 optionally stores on binary outputs the detailed material zone distribution map inside each cell of the mesh grid according to a submesh grid refinement defined in input by the user and the area and volume fraction distribution of the different material zones contained in meshes at zone interfaces. That also allows a local (per-cell) density correction as an alternative to the approach of a uniform density correction on the whole zone domain and makes it possible to perform material zone homogenization locally and transport analyses more accurately.