A transport multicell method for two dimensional lattices of hexagonal cells

The constant increase of stock of Plutonium originated from classical LWR facilities have aroused a renewed interest in undermoderated reactors and, with this, a change on bundle design from the usual light-moderated, X-Y pin-fuel layout to the typical, fast-reactor, hexagonal lattice. To provide a tool for routine, design calculations of pin-fuel assemblies in a ''hexagonal'', beehive-like geometry we have applied a multicell, collision-probability technique. A flat-flux expansion on axially symmetric zones has been used, together with a uniform double P1 expansion for the boundary angular fluxes, to construct an interface-current method for the calculation of two-dimensional hexagonal cells. Collision and escape probabilities are computed in a volume-preserving, cylindrical cell model, while one of three separate PSS models is used for the calculation of the transmission probabilities. When only one zone has been cylinderized then use of the heterogeneous PSS model is equivalent to an exact calculation in the actual two-dimensional cell geometry (without cylinderization). Comparison between the different approximations and a Monte Carlo calculation are presented for a typical undermoderated assembly