Measurements and analyses of the ration of 238U captures to 235U fission in low-enriched UO2 tight lattices

The ratio of 238U captures to 235U fissions (C8/F5) was measured for four kinds of light-water moderated UO2 lattice cores; 1.42S, 1.00S, 0.75S and 0.56S. These cores have a central tight lattice test region surrounded by a normal lattice driver region. In the test region, the volume ratios of water-moderator to fuel are 1.420, 1.000, 0.750 and 0.564, respectively. Measurements were carried out using uranium metal foils set in a fuel rod at the center of the test region. The radial distributions of relative C8 and F5 rates in the fuel rod were also measured for the 1.42S and 0.56S cores. Calculations were performed by using the continuous energy Monte Carlo code MVP with the JENDL-3 library. The calculated C8/F5's were larger than the experimental ones, and the discrepancy between the calculated in-fuel C8 distribution and the experimental one was observed. From the results of the sensitivity analysis, it is found that the decrease of the resonance 238U capture cross section and the increase of the resonance 235U fission cross-section reduced the discrepancy between the calculation and the experiment in the C8/F5 ratios and their in-fuel distributions, respectively. (author)