Measurement of the Uranium-238 Doppler Effect in Gas-Cooled Fast Reactor Critical Assemblies

The Doppler effect of a small UO₂ sample was determined for the temperature range from 300 to 1100 K at the core center of the Argonne National Laboratory gas-cooled fast reactor (GCFR) critical assembly. The measurement provided the first data on the important ²³⁸U Doppler effect in GCFR systems. The normalized value of the GCFR ²³⁸U Doppler effect was found to be 54% smaller than the corresponding value in an equivalent liquid-metal fast breeder reactor (LMFBR) assembly. Calculations with ENDF/B-IV data yielded a calculated/experimental (C/E) ratio of 0.83, which is considerably lower than that in LMFBR assemblies. The reason for this misprediction appears to be an underprediction of the low-energy flux, a feature that seems to be general to hard spectrum assemblies. The ²³⁸U Doppler effect was also determined in a “steam-flooded” GCFR assembly. The Doppler effect for the temperature range from 300 to 1100 K increased by 93% relative to the “dry” reference value because of spectral softening. The same calculational methods using ENDF/B-IV data resulted in the much better C/E value of 1.00.