Evaluation of core characteristics with two-dimensional lattice calculations

Sato, Osamu*; Maeda, Akio*

Two-dimensional cell calculation is necessary to exactly treat the major heterogeneous structure of fuel pins and a wrapper tube in fast reactor fuel assemblies. JNC has been developing the new SLAROM, which includes a two-dimensional cell calculation routine. In this report, validity and effect of two-dimensional calculation were investigated. The validity was checked by comparing neutron flux in fuel assemblies and assembly averaged cross sections with those calculated by Monte Carlo code GMVP. The comparisons were made for the calculation model called "concentric layers of hexagon with equi-distance pin-rod array"(IGT=13) with a fuel assembly of JOYO and were also made for the model of a two-dimensional slab called "Quadrant of an octant symmetric assembly" (IGT=8) with SCF and DCF fuel cells in ZPPR=9. The calculated results with the new SLAROM and GMVP were agreed within statistical errors for the JOYO fuel assembly and the adequacy of the collision probability calculations in the new SLAROM was proved for a hexagonal fuel assembly. For a two-dimensional slab geometry, the assembly averaged absorption cross sections and total cross sections were 20% higher than those with Monte Carlo calculations below 24keV at which the large resonance of sodium exist. Further investigations are required to explain the differences. The results of core calculations with CITATION were performed and compared with the assembly-averaged cross sections using one-dimensional cell model and with the cross sections using two-dimentional cell model. THe differences caused by the cell calculation model were larger for radial blanket regions than for fuel regions. It revealed that the fewer numbers of fuel pins in radial blanket bring out the larger error of one-dimensional cell model.