MODELING OF CANDU REACTIVITY CONTROL DEVICES WITH THE LATTICE CODE DRAGON

The lattice code DRAGON has been used for modelling the normal operati ng conditions in CANDU reactors. The integral transport equation is fi rst solved using the collision probability (CP) formalism for 2-D clus ter geometries representing standard CANDU cells. Whereas the usual tr acking procedure only permits isotropic reflection at the cell boundar y, we investigate the effects on a completely reflected cluster cell o f the cyclic-tracking procedure that can also treat specular reflectio n. For CANDU reactivity devices located perpendicularly to the fuel ch annels, the standard CP formalism is applied to 3-D supercell geometri es containing zones of mixed cylindrical and rectangular geometries. A symmetric two-bundle model allows most of the surfaces to be located in the moderator regions, thus reducing discrepancies introduced by th e assumption of isotropic boundary currents. Using a single basic defi nition for the tracking files, efficient algorithms for computation an d normalization of CP pertinent to these cell and supercell models are also described. Numerical results include reactivity worths for adjus ter rods and zonal control units (ZCU) of a typical CANDU reactor.