Low and high-resolution gamma-ray spectroscopy for the characterization of uranium contamination

Decommissioning is the last step in the life cycle of a nuclear facility. After the evacuation of the facility components, the remaining structures such as concrete walls and floors must be controlled to ensure that no residual contamination remains. As it is a costly and time consuming activity, CEA develops fast measurement methods allowing a complete scanning of very large areas (hundreds of thousands of square meters) in legacy uranium enrichment plants based on gas diffusion, at Pierrelatte nuclear site, France. After fast alpha and beta measurements, HPGe and NaI gamma-spectroscopy detectors are used to characterize more precisely uranium contamination. HPGe has excellent energy resolution but requires very long acquisition times. On the contrary, NaI(Tl) detectors enable rapid measurement, but extracting the contamination signal from the natural background is difficult due to their low resolution. Using an innovative approach based on energy bands of the NaI spectra, contamination activity and enrichment are consistent with that measured with HPGe but in only 15 min vs. 65 h, and with a sufficiently low relative uncertainty of about 15 % and 20 %, respectively, on activity and enrichment. Large area NaI(Tl) detectors will allow cost-effective scanning of the Gaseous Diffusion Plant (UDG) nuclear site (hundreds of thousands of squared meters) within practical times, in complement to alpha and beta contamination detectors.