Measurement of the 238U 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