The Study of Radionuclide Retardation in Fractured Rock by Means of In Situ Resin Impregnation

Abstract

Traditional in situ tracer tests estimate contaminant retardation by analysis of the degree and form of tracer breakthrough after transport through the rock. Unfortunately, this approach does not allow direct examination of in situ retardation mechanisms and, in the case of strongly retarded radionuclides, is highly impractical as tracer breakthrough may take months to decades. An alternative method to study retardation is therefore required in such a case and Nagra and PNC have recently employed one such variant to study radionuclide transport in fractured crystalline rock. Here, direct, detailed, examination of in situ radionuclide retardation following tracer injection is carried out by immobilising and recovering the intact fracture and associated rock matrix [1,2].The material can then be studied in the laboratory by standard surface analytical and radiochemical methods and the degree and form of radionuclide uptake can be readily assessed.

As part of this work, Nagra and PNC have invested significant effort over the last four years in developing appropriate means of immobilising water-conducting fractures and undisturbed low porosity crystalline rock matrix in a manner which minimises physico-chemical disturbance[3]. After examining a range of options, it was decided to employ in situ resin impregnation as the immobilisation medium as this produced the best results with respect to minimising physico-chemical disturbance of the system while at the same time ensuring impregnation of very fine water saturated pore space. In addition, the polymerised resins improve the rigidity and strength of the rock such that the water saturated structures (pores, fractures or fault gouges) survive the subsequent overcoring and sub-sampling.

Two experiments will be discussed: the first has been recently completed in Nagra's underground laboratory in the central Swiss Alps (the Grimsel Test Site, or GTS) and the second is currently ongoing at PNC's Kamaishi In Situ Test Site (KTS) in north-east Japan.

In the GTS, retardation of radionuclides is being studied in the Radionuclide Retardation Project (RRP) and two resins have been formulated for different aspects of the study. An epoxy resin has been injected into a complex water-conducting shear zone in a granodiorite following the injection of a cocktail of strongly retarding radionuclides (including ⁶⁰Co, ²³⁷Np, ^{234. 235}U, ⁹⁹Tc, ¹⁵²Eu, ¹¹³Sn and ⁷⁵Se [1,2]). To negate the hydrophobic nature of the epoxy resin, a trick has been imported from soil science where isopropanol is first injected to replace the water and only then is the epoxy resin injected. Laboratory tests showed that neither the isopropanol nor the resin should disturb the in situ radionuclide distribution, a result which has since been verified in the field. In parallel with this work, the low porosity (<1%) granodiorite rock matrix behind the shear zone is being examined by means of an in situ injection of an acrylic resin. The very low viscosity of the specially developed acrylic resin allows impregnation (and subsequent visualisation) of the connected microporosity of the matrix, so allowing detailed in situ examination of the depth of available matrix behind the shear zone.

These methods have been further refined in the KTS and are currently being applied to several different types of water conducting features. The form and type of connected porosity in the associated granodioritic rock matrix is also being examined in detail [4]. As with the GTS work, the results of the in situ experiments will be compared with laboratory data on retardation and matrix diffusion to assess the transferability of the large volume of laboratory data to the field.

The development of the various resins will be discussed along with the applicability of these specially developed resins to other rock types. Finally, the results of the recently concluded GTS tests and the ongoing KTS tests will be presented.