Emerging in the early 80’s, decommissioning is more than ever a major issue since hundreds of sites and facilities worldwide will end their operations over the next decades. Decontamination and remediation projects are all the more sensitive since they could last several years and turn out to be highly costly if not well-prepared. The key lies in an adequate contamination knowledge which helps to manage the remediation works and optimize the radiological waste production.
The methodologies commonly applied base their recommended decision-process on more or less complex statistical analyses to validate the remediation target after the waste removal work (guidance for demonstrating compliance with a dose- or risk-based regulation). These techniques ignore the spatial behavior of the contamination and the importance of sample localization upstream. The lack of representation and data processing tools leads to inefficient radiological characterizations, which always maximize the amount of contaminated soils or concrete volumes.
To solve these issues, the French Atomic Energy Authority (CEA) has developed a methodology over the last 10 years with Geovariances’ partnership to fulfill the radwaste categorization. This methodology consists of an ordered sequence of evaluation actions starting with historical and functional analyses, in-situ characterization if relevant using non-intrusive measurement techniques, validation of contamination activity levels and depths with drill-holes and laboratory analyses. The implementation of geostatistics makes possible to give value to the collected data and map the contamination at each step of the sequence to finally get a robust and reliable characterization of contaminated areas. It also provides an efficient way out for sampling network optimization.
The article introduces the geostatistical methodology and illustrates its added value on a real application case dealing with grounds of facilities partially dismantled at the end of the 1950s. 10 years ago, a first exploratory drill-hole confirmed the presence of a deep radiological contamination (more than 4m deep). More recently, 8 additional drill-holes failed to delineate the contaminated volumes according to a preliminary geostatistical analysis. The integration of the former topography and other geological data led to the realization of 7 supplementary drill-holes. This final stage significantly improved the characterization of the radiological contamination, which impacted the remediation project and the initially estimated volumes.