Speaker
Description
Dismantling constitutes the final stage in the life cycle of a nuclear facility, and after the removal of components, the decontamination of civil engineering structures (floors and walls) is an important step in view of its final decommissioning. This decontamination process is costly and time-consuming, therefore employing fast, precise, and reliable measurement methods is crucial to identify and characterize the contaminated areas. Cleaning is then restricted to these zones to limit intervention time and expenses, as well as the volume of produced waste and the associated storage costs. After decontamination, very sensitive measurements are also needed to check that the contamination has fallen below the objective residual activity level. In this context, CEA is developing innovative nuclear measurement methods for detecting uranium contaminations. These methods allow for a rapid scanning of UDG uranium enrichment facilities, in Pierrelatte, France, which include several hundred thousand square meters of civil engineering surfaces. However, natural radiations of uranium and thorium radioactive chains, as well as 40K present in concrete floors and walls, constitute a significant background. This makes it challenging to detect residual contamination when aiming for objectives below 1 Bq/cm². To address these challenges, CEA IRESNE in Cadarache and ISEC in Marcoule, develop combined alpha, beta, and gamma measurements. A 1st-level fast detection of the contamination is carried out with alpha and beta ZnS(Ag) and plastic PVT scintillators, respectively, with a 900 cm2 detection area each and measurement times of less than 3 min. This primary detection is then completed by a 2nd-level low-resolution gamma spectrometry measurement using a cluster of 3×3 NaI(Tl) scintillators, each measuring 4"×4"×2". With a total detection surface similar to alpha and beta scintillators and a longer measurement time, around 15 min, this step confirms the activity level with a smaller uncertainty and provides an estimate of its 235U enrichment. Finally, 3rd-level high-resolution X- and gamma-ray spectrometry with electrically cooled CdTe and HPGe detectors, as well as autoradiography measurements, are also performed on an occasional basis. These measurements require much longer counting times (several hours) and are conducted on smaller detection surfaces. They allow to accurately corroborate the activity of selected contaminated areas estimated by the 1st and 2nd level measurements, and verify the 235U enrichment level estimated with the NaI(Tl) detector. Additionally, they assist in estimating the migration depth of the contamination in concrete and its surface distribution, which is not uniform. All this information helps reduce the uncertainty of the alpha, beta and low-resolution gamma measurements.
