Measurements of Thermal Disadvantage Factors in Light-Water Moderated PuO2-UO2 and UO2 Lattices

The disadvantage factor for thermal neutrons in light-water moderated PuO2-UO2 and UO2 square lattices were obtained from measurements of thermal neutron density distributions in a unit lattice cell, measured with Dy-Al wire detectors. The lattices consisted of 3.4w/o PuO2-UO2 and 2.6w/o UO2 fuel rods, and the water-to-fuel volume ratio within the unit cell was parametrically changed. The PuO2-UO2 and UO2 fuel rods were designed to realize equal fissile atomic number density. The disadvantage factors thus measured were 1.36±0.07, 1. 37±0.08, 1.40±0.06 and 1.38±0.06 in the PuO2-UO2 fuel lattices, and 1.30±0.06, 1.31±0.08, 1.30±0.08 and 1.33±0.06 in the UO2, for water-to-fuel volume ratios, of 1.76, 2.00, 2.38 and 2.95, respectively. This difference in disadvantage factor between PuO2-UO2 and UO2 fuel lattices corresponds to about 8%. Calculated results obtained by multigroup transport code LASER agreed well with the measured ones.