A WIMS-based calculational route for pebble-bed fuel

This paper reports that 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 Dancoff 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