A Pulsed Activation Measurement of the Doppler Effect of Uranium-238 over the Temperature Range 300 to 3115 K

The Doppler effect for ²³⁵U-enriched UO₂ fuel pellets has been measured by the Pulsed Activation Doppler (PAD) technique in a TRIGA reactor. A combination of static electrical preheating and pulsed fission heating during irradiation was used to perform the measurements at temperatures extending from 300 K to the melting point of UO₂ (3115 K). The ²³⁵U enrichment in the experimental samples investigated ranged from 0.22 to 12% by weight. Measurements were made at the highest temperatures ever achieved in Doppler experiments and represent the first activation Doppler measurements under partially molten conditions of UO₂. Two sizes of pellets were used in the work, with nominal surface-to-mass ratio values of 0.63 and 1.08 cm²/g, respectively. The experimentally determined values of the Doppler ratio were in good agreement with resonance integral ratios determined from GAROL calculations and extrapolations of the low-temperature Hellstrand correlation. Because the technique involved transient heating, an equivalent static temperature (θ_eff) was defined for each experimental pellet. Explicit correlations for the Doppler ratio as a function of the parameter were determined from the present data. It is shown that a linear function in the parameter adequately describes the behavior of the data.