Development of a transport solver for DYN3D on the basis of CCCP with orthonormal flux expansion

DYN3D is a well-known and widely used computer code for reactor physics simulation of nuclear power plants, in particular for reactors with hexagonal fuel assembly structures. It has been developed in Helmholtz-Zentrum Dresden-Rossendorf, Germany. The standard version of the DYN3D code can be used for investigations of transients in light water reactors cores with hexagonal or quadratic fuel assemblies. In order to determine the pin with the maximum power in selected assemblies, a two-dimensional pin power reconstruction can be performed based on the node homogenized neutron flux. A superposition of global diffusion solution of the full core calculation with the assembly pin powers obtained in the cell calculations is used therefore. This method is implemented in DYN3D for reconstruction of power inside selected assemblies. An improved onset would be a hybrid solution, the coupling of the full core diffusion solver with an advanced transport solver on fuel assembly base. This method can be used to directly determine the power distribution for each rod inside fuel assemblies by applying a transport solver using unstructured mesh and boundary conditions extracted from the full core diffusion solution. Nowadays, this mentioned methodology is under development. In the present work an advanced multigroup transport method of current coupling collision probability (CCCP) with orthonormal flux expansion inside the calculation regions is being developed and tested for cylindrical, hexagonal geometries and for assemblies of hexagonal cells. The results of test calculations demonstrate very good agreement with the results obtained from Monte Carlo calculations. Multigroup calculations for hexagonal assemblies with cross-sections prepared using the HELIOS code show good agreement with HELIOS reference solution, too. These convincing results encourage the implementation of this advanced pin power calculation method into DYN3D as future pin-power determination method using currents from nodal solution as boundary conditions.