A WIMS-Based Calculational Route for Pebble-Bed Fuel

A WIMS-based calculational route for pebble-bed fuel has been established. An outstanding advantage of the WIMS code is its integrated route from basic lattice data to burnup-dependent lattice cross sections. The problem in applying WIMS to pebble-bed fuel is that it lacks spherical geometry. This problem is solved by establishing a number of practical equivalences enabling the replacement of a lattice of spherical fuels by a lattice of cylindrical fuels. A special program was written to convert physical data into WIMS input files, including the Dan-coff factor required for resonance shielding in the multilayer multicell pebble lattice. This capacity provides all that is necessary to generate core-homogenized cross sections directly applicable to core studies. Also generated are zone-homogenized cross sections; in some cases, their use in a transport code results in more accurate core-homogenized cross sections. In terms of the fuel infinite criticality factor, this added accuracy is in the range of 1 to 3 mk for fuel free of absorbers or fuel carrying boron-only absorbers; it is in the range of 3 to 12 mk for fuel carrying hafnium absorbers.