Response of Compacted Bentonites to Thermal and Thermo-Hydraulic Loadings at High Temperatures

Snehasis Tripathy,Panagiotis Stratos,Hywel Thomas

doi:10.3390/geosciences7030053

Snehasis Tripathy, Panagiotis Stratos + Show 1 more

Open Access

https://doi.org/10.3390/geosciences7030053

Copy DOI

Journal: Geosciences	Publication Date: Jul 7, 2017
Citations: 11	License type: CC BY 4.0

Affiliation: Cardiff University

Abstract

The final disposal of high-level nuclear waste in many countries is preferred to be in deep geological repositories. Compacted bentonites are proposed for use as the buffer surrounding the waste canisters which may be subjected to both thermal and hydraulic loadings. A significant increase in the temperature is anticipated within the buffer, particularly during the early phase of the repository lifetime. In this study, several non-isothermal and non-isothermal hydraulic tests were carried on compacted MX80 bentonite. Compacted bentonite specimens (water content = 15.2%, dry density = 1.65 Mg/m3) were subjected to a temperature of either 85 or 150 °C at one end, whereas the temperature at the opposite end was maintained at 25 °C. During the non-isothermal hydraulic tests, water was supplied from the opposite end of the heat source. The temperature and relative humidity were monitored along predetermined depths of the specimens. The profiles of water content, dry density, and degree of saturation were established after termination of the tests. The test results showed that thermal gradients caused redistribution of the water content, whereas thermo-hydraulic gradients caused both redistribution and an increase in the water content within compacted bentonites, both leading to development of axial stress of various magnitudes. The applied water injection pressures (5 and 600 kPa) and temperature gradients appeared to have very minimal impact on the magnitude of axial stress developed. The thickness of thermal insulation layer surrounding the testing devices was found to influence the temperature and relative humidity profiles thereby impacting the redistribution of water content within compacted bentonites. Under the influence of both the applied thermal and thermo-hydraulic gradients, the dry density of the bentonite specimens increased near the heat source, whereas it decreased at the opposite end. The test results emphasized the influence of elevated temperatures (up to 150 °C) on the thermo-hydro-mechanical response of compacted bentonites in the nuclear waste repository settings.

Highlights

Compacted bentonites are proposed to be used as the barrier and backfilling materials in underground disposal of high-level nuclear waste in many countries
Bentonites exhibit swelling during the hydration process, whereas the drying process is accompanied by a decrease in the volume
The results show that heat flow within the bentonite columns under non-isothermal loading condition was not one-dimensional

Summary

Introduction

Compacted bentonites are proposed to be used as the barrier and backfilling materials in underground disposal of high-level nuclear waste in many countries. Due to the high swelling and low permeability characteristics, the bentonite buffer surrounding the waste canisters is expected to retard the flow of radionuclide to the natural barrier systems. The in situ boundary conditions dictate exposure of compacted unsaturated bentonites to elevated temperature from the waste canisters and hydration upon fluid uptake from the surrounding host rock. Bentonites exhibit swelling during the hydration process, whereas the drying process is accompanied by a decrease in the volume. Under confined condition and upon exposure to a hydration source, the development of swelling pressure is expected to provide stability to the waste disposal repositories [1,2,3,4].

Objectives

Methods

Discussion

Conclusion