Whole core transport calculation for the VHTR hexagonal core

This paper is to implement a kernel to deal with the hexagonal geometry to the DeCART code which performs a whole core calculation by coupling the radial MOC transport kernel with a axial nodal kernel. The implementation includes a modular ray tracing module based on a hexagonal assembly and a multi-group CMFD module to perform an efficient transport calculation. Modular rays are generated by simply extending the scheme for the rectangular geometry to a hexagonal geometry. The CMFD module is designed to deal with unstructured cells such as a gap cell. The void problem that appears when the CMFD calculation is performed for the void cell group is resolved by a lumped CMR scheme. The fuel BLOCK and the 2-D core problems are solved by using 190g and 47g cross section libraries, respectively, and the solutions are compared with the MCNP solutions. In the fuel BLOCK problems, DeCART shows less than 200 pcm of an eigenvalue and less than 0.7 % of pin power errors. In the 2-D core problem, DeCART shows about 200-600 pcm of eigenvalue and less than 0.5 % of assembly power errors. DeCART takes less than 5 hours on a PENTIUM-IV 3.0 GHz personal computer for the 2-D core problems. These results indicated that the hexagonal module of the DeCART code worked very well within an affordable computing time.