Enriched UO2 Research Articles

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 det alled 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.

Study of a Slightly Enriched UO2 Lattice with H:U=0.42 — Measurement and Analysis

Parameter measurements in a 1.3% enriched UO2 lattice with H:U = 0.42 have been performed. These measurements are an extension of an experimental program in the TRX critical facility of the Bettis Atomic Power Laboratory. Earlier measurements were made for a wide range of water-to-uranium (H2O:U) volume ratios (1:1 to 8:1) using 4-ft (1.2-m)-high slightly enriched, 0.387-in. (0.98-cm)-diam uranium metal or oxide fuel rods clad with aluminum. The new data have been compared with current analytic techniques, using both P-1 and P-3 multigroup analysis in the epithermal neutron energy range and Monte Carlo multigroup methods for thermal neutrons. This extremely undermoderated lattice provides a very stringent test for both the computational methods and the neutron cross sections used. The quantities measured were: the ratio of epithermal-to-thermal radiative captures in U238 (ρ28); the ratio of captures in U238 to fissions in U235 (the modified conversion ratio, CR*); the ratio of U238 fisions to U235 fissions (δ28); and the ratio of epithermal-to-thermal U235 fissions (δ25). In addition, activations were obtained with thermal-neutron detectors of widely different spectral response.The results indicate that the calculational methods predict the parameters very well, except for δ28. The discrepancy in δ28 may be due to inadequate U238 inelastic scattering cross sections, but this conclusion requires additional study. Monte Carlo calculations of thermal-neutron detector activations show that use of either the Nelkin or Koppel kernel gives results that agree with the data.

Temperature Coefficient of Water Lattices

Temperature coefficients of slightly enriched UO2-H2O lattices were measured in the Ozenji Critical Facility and analyzed in terms of critical parameters. The lattices studied were mainly those of 2.5% enriched UO2, 10 mm in diameter and clad in 0.8 mm thick Al tube, and with water to fuel volume ratios of 2.5 and 1.5. Analyses were made with Deutsch's four factor critical equation and also with three group one dimensional diffusion calculations. The result shows the importance of the influence of the reflector, which is the largest positive contribution to the temperature coefficient, being about 1×10-4 Δk/k/°C. This was experimentally demonstrated by measuring the coefficient for a core with an inner reflector water gap. Analyses also show that the discrepancy between experiment and calculation, which increases from 0.2 to 0.7×10-4 Δk/k/°C as the water to fuel volume ratio decreases, may be mostly resolved by applying corrections for the space-dependent spectrum within the cell using the THERMOS code and for the effect of Dancoff factor with the TUZ-ZUT code. Calculation with these corrections applied reproduces experiment within an accuracy of about 0.2×10-4 Δk/k/°C over the temperature range of 20° to 70°C.

Temperature Coefficient of Water Lattices

Temperature coefficients of slightly enriched UO2-H2O lattices were measured in the Ozenji Critical Facility and analyzed in terms of critical parameters. The lattices studied were mainly those of 2.5% enriched UO2, 10 mm in diameter and clad in 0.8 mm thick Al tube, and with water to fuel volume ratios of 2.5 and 1.5. Analyses were made with Deutsch's four factor critical equation and also with three group one dimensional diffusion calculations. The result shows the importance of the influence of the reflector, which is the largest positive contribution to the temperature coefficient, being about 1×10-4 Δk/k/°C. This was experimentally demonstrated by measuring the coefficient for a core with an inner reflector water gap. Analyses also show that the discrepancy between experiment and calculation, which increases from 0.2 to 0.7×10-4 Δk/k/°C as the water to fuel volume ratio decreases, may be mostly resolved by applying corrections for the space-dependent spectrum within the cell using the THERMOS code and for the effect of Dancoff factor with the TUZ-ZUT code. Calculation with these corrections applied reproduces experiment within an accuracy of about 0.2×10-4 Δk/k/°C over the temperature range of 20° to 70°C.

Spatial distribution of the repeatedly scattered γ-rays from collimated sources of 198Au, 60Vo and 24Na

The variational nodal method is generalized by dividing each spatial node into a number oftriangular finite elements designated as subelements. The finite subelement trial functions allow for explicit geometry representations within each node, thus eliminating the need for nodal homogenization. The method is implemented within the Argonne National Laboratory code VARIANT and applied to two-dimensional multigroup problems. Eigenvalue and pin-power results are presented for a four-assembly DECD/NEA benchmark problem containing enriched UO2 and MOX fuel pins. Our seven-group model combines spherical or simplified spherical harmonic approximations in angle with isoparametric linear or quadratic subelement basis functions, thus eliminating the need for fuel-coolant homogenization. Comparisons with reference seven-group Monte Carlo solutions indicate that in the absence of pin-cell homogenization, high-order angular approximations are required to obtain accurate eigenvalues, while the results are substantially less sensitive to the refinement of the finite subelement grids.

Two Region Studies in Slightly Enriched Water-Moderated Uranium and Uranium Dioxide Lattices

An experimental and analytical study of the flux distribution of two-region core configurations has been made for the TRX facility. The purpose of this study was to obtain an estimate of the sizes of critical configurations that would yield the same values of the basic reactor parameters in the inner region as a critical core consisting entirely of the inner region material and geometry. Several two-region cores have been constructed and experimental measurements of thermal utilization, resonance escape probability, and fast fission effects have been performed. Slow and fast neutron activation distributions have also been obtained. Two inner regions were constructed utilizing 1.3 w/o enriched UO2 fuel 0.384 in. in diameter and with a density of 10.53 gm/cm3. A third inner region utilized 1.3 w/o enriched uranium metal fuel with a diameter of 0.387 in. Light water served as the moderator and reflector in all cases. The experimental and theoretical results indicate that by utilizing two-region cores, measurements of microscopic parameters can be made for a wide variety of fuel sizes, fuel enrichments, and water-to-uranium volume ratios without the construction of full critical cores for each combination.