Small-Sample Doppler Effect Measurements and their Interpretation in Fast Reactor Spectra

The pile oscillator technique was used to measure the Doppler effect with heated small samples in neutron spectra typical of steam cooled fast reactors. Samples of UO₂ in various enrichments, especially depleted UO₂, were measured up to 1000°K, and also samples of PuO₂ diluted with Al₂O₃. The results were analyzed by a method which properly accounts for the resonance interaction between the hot sample and the cold environment. The results with the UO₂ samples where ²³⁸U gives the major contribution to the Doppler effect were in agreement with calculations within about 10%, though changes of the effect between different assemblies, in some cases, were not well reproduced. The experiments with PuO₂ samples were designed to give integral information on alpha of ²³⁹Pu in the range 0.1 to 5 keV. It is shown that experiment and calculation agree within about 25% if high alpha values similar to those of Gwin are used.