Development of CAD-MCNP interface program GEOMIT and its applicability for ITER neutronics design calculations

Nasif, H. R.*; Masuda, Fukuzo*; Morota, Hidetsugu*; Iida, Hiromasa*; Sato, Satoshi; Konno, Chikara  

GEOMIT is the CAD/MCNP conversion interface code. It is developed to automatically generate Monte Carlo geometrical data from CAD data due to the difference in the representation scheme. GEOMIT is capable of importing different CAD format as well as exporting different CAD format. GEOMIT has a capability to produce solid cells as well as void cells without using complement operator. While loading the CAD shapes (Solids), each shape is assigning material number and density according to its color on the original CAD data. Shape fixing process is been applied to cure the errors in the CAD data. Vertices location correctness is evaluated first, then a removal of free edges and removal of small faces processes. Binary Space Portioning (BSP) tree technique is used to automatically split complicated solids into simpler cells to avoid excessive complicated cells for MCNP to run faster. MCNP surfaces are subjected to an automatic reduction before creating the model. CAD data of International Thermonuclear Experimental Reactor (ITER) benchmark model has 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 the ITER, include Blanket first wall heat loading calculations, surface fluxes and volume fluxes at different divertor regions as well as TF coils heating.