Few group diffusion calculations of a light water moderated lattice

An analysis was made of a light water moderated lattice based on few group diffusion calculations. The lattice under investigation consisted of 468 fuel rods in a square lattice arranged in a cylindrical core of 22.46 cm effective radius and 127.2 cm core height, with a water to fuel volume ratio of 2.918. The fuel was 2.02% enriched UO₂, clad in 0.8 mm thick aluminum tube. The theoretically calculated values for thermal, epithermal and fast neutron flux distributions, as well as the effective multiplication constant λ of the lattice, were compared with experimental data. After detailed analysis of the problems encountered in the course of the study the value of 0.9935 was determined for λ. Uncertainty in the nuclear data for fast neutrons would appear to constitute the greatest factor of error in λ. The discrepancies between the calculated and experimental activation distributions of the thermal, epithermal and fast neutrons amounted for about 20, 10 and 15%, respectively, in the reflector region adjoining the core. The fact that these discrepancies cannot be removed by multigroup P₁ calculations would point toward insufficiency of the diffusion or P₁ calculation this region. It was also concluded that the following items could not be neglected without causing an error in λ of more than 0.3% in the particular lattice under investigation: (1) Heterogeneity in the fast neutron energy region. (2) Intra-cell change of thermal neutron spectrum. (3) Macroscopic spectral change near the core-reflector interface. (author)