Characterization Methodology After a Remediation Operation - 23352

In France, the assessment of the radiological remediation of soil or building surface is a process involving different actors, namely the nuclear facility's operators and the French authorities. As a technical expert, the Radioprotection and Nuclear Safety Institute (Institut de Radioprotection et de Surete Nucleaire, IRSN) is often mandated by the French authorities to perform this assessment after the operators' characterization step. The aim of an assessment of radiological remediation is to release the soil or the building for restricted use or for unrestricted public use, release based on remediation criterions. The IRSN's characterization methodology is based on different steps: historical study of the site, strategy definitions for direct in-situ measurements and for sampling, data assessment, etc. The IRSN feedback shows that direct on-field measurements need to be performed even for an alpha/beta contaminated site, due to some lack of knowledge on the nuclear facility life. Hence, gamma flux maps are systematically conducted with portable radiation survey equipment (scintillator, dose rate meter...) and for environmental characterization, with a hand-held portable gamma spectrometer (NaI detector) allowing access to gamma spectrum. Concerning building, measurements with portable contamination monitors are also performed to measure alpha and beta-gamma radiation. One of the difficulties in the assessment of the radiological remediation of soil or building, is the determination of the number of direct measurements and samples to collect. For that, we use different tools based on statistics or geostatistics. In the case of the assessment for a building structure used in the uranium cycle, the predictive number of on-field contamination measurements was determined by using the standard ISO 2859-1. The application of this standard was done considering the surface of the wall and/or the floor, the sensitive area of the portable contamination monitor and an accepted risk to non-detect an abnormal point. The localization of the measurements was chosen on one hand randomly and on the other hand on point knowing to accumulate contamination. The floor was subdivided in smaller areas according to the history and operator's measurement results. The advantage of the ISO 2859-1 standard is the choice of the accepted risk, which could be different according to the user experience. For one of the areas, a contaminated zone has been detected thank to IRSN methodology. In the case of the assessment of the radiological remediation of soil contaminated by Cs-137, the determination of the number of direct in-situ measurements and soil sampling in surface and in depth, was done by using the MARSSIM method and geostatistical tools. The MARSSIM method was used for the determination of the number of the direct in-situ measurements with a 2-L NaI detector and for surface sampling. Thus, for a 790 m² area, 64 in-situ measurements and 29 soil sampling were performed. The measurement results allowed to generate a probability 2D map of non-exceeding a Cs-137 activity threshold. For this map, a co-kriging method was used, integrating information from sampling and in-situ measurements. Then, from on-field and in-laboratory measurements, the localization and the number of depth sampling were determined using geostatistical tools. The theoretical number of boreholes was established at 13 by considering a mainly vertical theoretical migration of Cs-137. The possible location of the potential contaminated zones was assessed from the probability map of exceeding the threshold, and finally adjusted from an on-field geo-radar analysis indicating in-field structures which impact diffusion process of contamination (fractures, fault...). The association of the co-kriging exploitation by geo-statistical tools and geo-radar measurements has really been useful to optimize the sampling strategy. (authors)