Transport of 234U in the Opalinus Clay on centimetre to decimetre scales

Abstract

Abstract The Opalinus Clay formation in North Switzerland is a potential host rock for a deep underground radioactive waste repository. The distribution of 238U, 234U and 230Th was studied in rock samples of the Opalinus Clay from an exploratory borehole at Benken (Canton of Zurich) using MC-ICP-MS. The aim was to assess the in situ, long-term migration behaviour of 234U in this rock. Very low hydraulic conductivities of the Opalinus Clay, reducing potential of the pore water and its chemical equilibrium with the host rock are expected to render both 238U and 230Th immobile. If U is heterogeneously distributed in the Opalinus Clay, gradients in the supply of 234U from the rock matrix to the pore water by the decay of 238U will be established. Diffusive redistribution separates 234U from its immobile parent 238U resulting in bulk rock 234U/238U activity disequilibria. These may provide a means of estimating the mobility of 234U in the rock if the diffusion rate of 234U is significant compared to its decay rate. Sampling was carried out on two scales. Drilling of cm-spaced samples from the drill-core was done to study mobility over short distances and elucidate possible small-scale lithological control. Homogenized 25-cm-long portions of a 2-m-long drill-core section were prepared to provide information on transport over a longer distance. Variations in U and/or Th content on the cm-scale between clays and carbonate-sandy layers are revealed by β-scanning, which shows that the (dominant) clay is richer in both elements. Samples were digested using aqua regia followed by total HF dissolution, yielding two fractions. In all studied samples U was found to be concentrated in the HF digestion fraction. It has a high U/Th ratio and a study by SEM-EDS points to sub-μm up to several μm in size zircon grains as the main U-rich phase. This fraction consistently has 234U/238U activity ratios below unity. The minute zircon grains constitute the major reservoir of U in the rock and act as constant rate suppliers of 234U into the rock matrix and the pore water. The aqua regia leach fraction was found to be enriched in Th, and complementary to the HF fraction, having 234U/238U activity ratios above unity. It is believed that these U activity ratios reflect the surplus of 234U delivered from the zircon grains. Some cm-spaced samples show bulk rock 234U/238U activity ratios that are markedly out of equilibrium. In most of them a striking negative correlation between the total U content and the bulk rock 234U/238U activity ratios is observed. This is interpreted to indicate net 234U transfer from regions of higher supply of 234U towards those of lower supply which is, in most cases, equivalent to transfer from clayey towards carbonate/sandy portions of the rock. In contrast, the 25 cm averaged samples all have uniform bulk rock 234U/238U activity ratios in equilibrium, indicating U immobility in the last 1–1.5 Ma on this spatial scale. It is concluded that the small-scale lithological variations which govern U spatial distribution in the Opalinus Clay are the major factor determining 234U in situ supply rates, regulating its diffusive fluxes and controlling the observed bulk rock 234U/238U activity ratios. A simple box-model is presented to simulate the measured bulk rock 234U/238U activity ratios and to give an additional insight into the studied system.