Post-depositional reactivity of the plutonium in different sediment facies from the English Channel - an experimental approach

¹ Laboratoire de Radioécologie de Cherbourg-Octeville, IRSN, 50130 Cherbourg-Octeville, France
² UMR CNRS 6143 Morphodynamique Continentale et Côtière, Université de Caen, 14000 Caen, France

Abstract

The post-depositional reactivity of the plutonium was investigated in bottom sediments collected in the central English Channel using an experimental approach. Medium to coarse grained surficial sediments and fine-grained undisturbed sediment cores were spiked with ²³⁸Pu. After a one-month incubation, pore waters were extracted under strictly anoxic conditions and dissolved plutonium was analysed by liquid scintillation techniques. Kd values ranged from 10² to 10⁵ in surficial sediments undergoing oxic diagenesis, and were shown to be grain-size dependant. In fine-grained sediments undergoing anoxic diagenesis, Kd values ranged between 10³ and 10⁵. Kd profile suggests an active recycling of the plutonium in the topmost sediment layers, and subsequent efficient uptake process at depth, tentatively acido-volatile sulphide (AVS) precipitation. This hypothesis was further confirmed by a correlation between the AVS content and the loosely bound plutonium extracted using an oxidising/complexing solution from sediment subsamples. In surficial sediments, by contrast, loosely bound plutonium was likely to be associated with the carbonate phase as surface complexes.