Fuel temperature coefficients of reactivity for heavy-water-moderated, cluster-type fuel lattices.

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Temperature coefficients of reactivity have been measured up to 600°C on cluster-type UO2 fuel for three kinds of 235U enrichment and on a hollow cluster of sus-cladding tubes by using a hot He gas loop in a heavy-water-moderated, pressure-tube-type critical assembly. A new experimental method has been developed which accurately eliminates the reactivity disturbance caused by heat leakage in the measurement of an extremely small change in reactivity. The fuel (fuel pellet, cladding and pressure-tube) temperature coefficients of reactivity obtained for the temperature range below 300°C are +1.00±0.04, -3.48±0.13 and -6.36±0.25 in the unit of 10-5%Δk/k•°C for 0.2%, 0.7% and 1.5% 235U enrichment, respectively. In the higher temperature region above 300°C, each coefficient shifts to positive side by about 2×10-5%Δk/k•°C. Temperature coefficient of reactivity for the hollow cluster of sus-cladding tubes (cladding and pressure-tube) has a large constant value with positive sign, + (6.42±0.26) × 10-5%Δk/k•°C, all through the temperature range. A calculational model to analyze a hot-loop-type measurement of temperature coefficients with use of WIMS-D code was proposed and could be successfully applied to the present measurement.
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