Bentonite homogenisation during the closure of void spaces

J.F Harrington,K.A Daniels,A.C Wiseall,P Sellin

doi:10.1016/j.ijrmms.2020.104535

Abstract

In a geological repository, the disposal of radioactive waste will result in the creation of engineering voids. Bentonite is commonly proposed as a sealing material as a result of its high swelling capacity. As the bentonite expands, the non-uniform development of porewater pressure and its coupling to total stress within the bentonite, may impair homogenisation. In this study we present results from five laboratory tests performed on sodium- and calcium-based bentonites to examine their swelling potential and capacity to homogenise over extreme bentonite-to-void ratios. Results demonstrate that even under these extreme ratios, the bentonite is able to swell and ultimately fill each void, creating a small swelling pressure. The swelling pressure development is spatially complex and time-consuming, and does not appear to be influenced by friction. Instead, it is characterised by plastic yielding of the clay with 70%–80% of the volume change associated with clay expansion adjacent to the void. This leads to heterogeneity illustrated by the presence of persistent differential stresses and the non-uniform distribution of moisture contents. Increases in the moisture content were measured but did not always correlate with the development of swelling pressure. This disequilibrium of the system is likely a reflection of the test durations and the slow evolution in the rates of change in swelling and porewater pressure beyond 130 days. Given the length of the experimental tests presented here, the time required to achieve full homogenisation of the clay is likely to be many years, if it occurs at all. Gravity segregation was also present in horizontal tests, further impairing clay homogenisation. However, as presented in this paper, it is possible to define functional relationships describing the bentonite swelling potential across engineering voids of differing size. This information will assist in establishing a safety case for bentonite usage in geological radioactive waste disposal.

Highlights

Geological disposal of radioactive waste is the favoured option for many nations for the removal and isolation of hazardous material from the biosphere.[1,2,3] Disposal designs are dependent on the geology available, with nations usually proposing to use a clay backfill material surrounding the waste in either a clay-based[4,5,6] or crystalline[1,7] hostrock
Following insertion of each sample into the apparatus, the pore pressure was carefully increased in a series of steps to the target value of 4500 kPa; this value was set with the radioactive waste management company Svensk Karnbranslehantering (SKB) as a suitable reference value comparable to the current Swedish repository concept
Given that the rate of pressure change observed during the latter stages of testing is very small, extrapolation of the results suggests that homogenisation of the bentonite would not occur for many years

Summary

Introduction

Geological disposal of radioactive waste is the favoured option for many nations for the removal and isolation of hazardous material from the biosphere.[1,2,3] Disposal designs are dependent on the geology available, with nations usually proposing to use a clay backfill material surrounding the waste in either a clay-based[4,5,6] or crystalline[1,7] hostrock. Bentonite is commonly chosen to be the clay backfill material[13,14,15] because of its favourable physical and chemical properties including low permeability and high swelling capacity.[16,17,18,19] Both sodium (Na) and calcium (Ca) bentonites have been considered for use within Europe,[20,21,22,23,24] a greater emphasis has been placed on Na-bentonites such as MX8025 and FEBEX,[26,27] primarily because of its lower cost.[28] Comparing the two, Marcial and co-workers[29] studied the compression characteristics, observing that the Ca-bentonite still had a larger void ratio and suggesting that the reduced compression was related to the larger diameter of the divalent Ca2+ cation. Ben Rhaïem and co-authors[30] found that Na-bentonite contained an increased number of smaller pores than the Ca-bentonite, suggesting that the Na-bentonite had a higher hydration and swelling capacity

Results

Discussion

Conclusion