The McCad Code for the Automatic Generation of MCNP 3-D Models: Applications in Fusion Neutronics

The Monte Carlo (MC) code MCNP is the reference tool in fusion neutronics, allowing the description and analysis of full and detailed 3-D geometry of a tokamak machine. The geometrical models of the components used are typically available through computer aided design (CAD) files: the main benefits of this system are related to its portability and compatibility with several tools commonly used in engineering analyses. However, at the present stage, the information contained in CAD files cannot be directly provided to MC as inputs, because of the different representation scheme used. This issue leads to the necessity to develop interfaces that can translate them into the correct MC geometrical description. McCad is a software developed by the Karlsruhe Institute of Technology, dedicated to the fully automated generation of the MCNP geometrical models from CAD files (step, iges, and brep formats): it is provided with a graphical user interface allowing the visualization of the geometries and tools for data exchange and modeling. This paper summarizes the results of some benchmark tests performed on JET components and a DEMO reactor aimed at the assessment of the suitability of McCad for fusion neutronic applications. The reliability of the conversion algorithm has been evaluated comparing the results of stochastic MCNP volume calculations carried out using the generated models, and the corresponding volumes provided by the CAD kernel of the interface program. Moreover, the consistency of a converted DEMO MCNP model has been verified through particle transport calculations for the estimation of the neutron wall loading poloidal distribution. Several aspects related to the use of the code have been evaluated such as its portability, performances, and impact of the geometric approximation introduced on the neutronic analyses. Furthermore, a useful feedback for the optimization and enhancement of the McCad interface has been provided.