Temperature Coefficient of Water Lattices

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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 1×10-4Δk/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× 10-4Δk/k/°C as the water to fuel volume ratio decreases, may be mostly resolved by applying correc-tions 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 0.2×10-4Δk/k/°C over the temperature range of 20°to 70°C.
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