Natural analogue study of radionuclides in Paleogene sediments in the Netherlands

In the Netherlands, Paleogene clay layers are eligible for the final disposal of radioactive waste. Geoscientific information from these deposits is essential to assess their suitability. An important aspect is the prediction of the geochemical behavior of radionuclides in these clay layers. Such behavior can be studied by laboratory experiments. However, these experiments are relatively short-term compared to the expected migration period of radionuclides if released from the engineered containment radioactive waste and it is very challenging to imitate disposal representative conditions in an above ground laboratory. An additional, alternative approach to predict radionuclide behavior is by studying their natural analogues that are present in the sediment of the host formation.

In this study, Paleogene deposits of two drillings were investigated. Se, U, Th, Cs, and Eu, being natural analogues for radionuclides in radioactive waste, are addressed in more detail. A wide range of analyses, such as XRD, XRF, TGA, SEM, sequential extractions and LA-ICP-MS, were performed to assess the mineralogy and associated trace elements.

The Eocene-Oligocene interval in the first core, located in the province of Zeeland, was glauconite-rich and showed an alternation of clayey and sandy layers. The top 10 m contained carbonates which were absent in the lower part of this interval. Th, Cs, Eu, Se and U correlated with clay minerals. Se and U also correlated with pyrite. The second core (Miocene-Eocene interval), located in South-Holland, also had alternating clay, silt and sand layers. In this core, Th, Cs and Eu correlated with clay minerals while Se and U only correlated to S and P.