Development of CAD-MCNP Interface Program GEOMIT and Its Applicability for ITER Neutronics Design Calculations

GEOMIT is a computer-aided design (CAD)/MCNP conversion interface code. It was developed to automatically generate Monte Carlo geometrical data from CAD data due to the difference in the representation scheme. GEOMIT is capable of importing as well as exporting different CAD formats. GEOMIT has the capability to produce solid cells as well as void cells without using the complement operator. While loading the CAD shapes (solids), each shape is assigned a material number and density according to its color on the original CAD data. A shape fixing process has been applied to cure the errors in the CAD data. Vertex location correctness is evaluated first, and then a removal of free edges and removal of small faces processes. A binary space portioning tree technique is used to automatically split complicated solids into simpler cells to avoid excessively complicated cells to allow MCNP to run faster. MCNP surfaces are subjected to an automatic reduction before creating the model. CAD data of the ITER benchmark model have been converted successfully to MCNP geometrical input. MCNP input model validations have been carried out by checking lost particles and comparing volumes calculated by MCNP to those of the original CAD data. Different test cases have been evaluated for ITER, including blanket first wall heat loading calculations, surface fluxes, and volume fluxes at different divertor regions as well as toroidal field coil heating.