The Use of Clay as an Engineered Barrier in Radioactive-Waste Management — A Review

Abstract

Geological disposal is the preferred option for the final storage of high-level nuclear waste and spent nuclear fuel in most countries. The selected host rock may be different in individual national programs for radioactive-waste management and the engineered barrier systems that protect and isolate the waste may also differ, but almost all programs are considering an engineered barrier. Clay is used as a buffer that surrounds and protects the individual waste packages and/or as tunnel seal that seals off the disposal galleries from the shafts leading to the surface.Bentonite and bentonite/sand mixtures are selected primarily because of their low hydraulic permeability in a saturated state. This ensures that diffusion will be the dominant transport mechanism in the barrier. Another key advantage is the swelling pressure, which ensures a self-sealing ability and closes gaps in the installed barrier and the excavation-damaged zone around the emplacement tunnels. Bentonite is a natural geological material that has been stable over timescales of millions of years and this is important as the barriers need to retain their properties for up to 106 y.In order to be able to license a final repository for high-level radioactive waste, a solid understanding of how the barriers evolve with time is needed. This understanding is based on scientific knowledge about the processes and boundary conditions acting on the barriers in the repository. These are often divided into thermal, hydraulic, mechanical, and (bio)chemical processes. Examples of areas that need to be evaluated are the evolution of temperature in the repository during the early stage due to the decay heat in the waste, re-saturation of the bentonite blocks installed, build-up of swelling pressure on the containers and the surrounding rock, and degradation of the montmorillonite component in the bentonite. Another important area of development is the engineering aspects: how can the barriers be manufactured, subjected to quality control, and installed?Geological disposal programs for radioactive waste have generated a large body of information on the safety-relevant properties of clays used as engineered barriers. The major relevant findings of the past 35 y are reviewed here.

Gas migration in pre-compacted bentonite under elevated pore-water pressure conditions

“…In such cases, 70 laboratory testing demonstrates that the applied pressure gradient will directly control the 71 resulting gas flow rate (Villar et al, 2012). It has also been suggested that, for sand 72 bentonite mixtures with a suitably high sand content, similar behaviour may also occur in 73 reasonably well-saturated bentonite (Sellin and Leupin, 2013). However, in the case of fully 74 saturated, pure bentonite, water is less mobile and pathway dilatancy is expected to occur 75 instead (Sellin and Leupin, 2013).…”

“…It has also been suggested that, for sand 72 bentonite mixtures with a suitably high sand content, similar behaviour may also occur in 73 reasonably well-saturated bentonite (Sellin and Leupin, 2013). However, in the case of fully 74 saturated, pure bentonite, water is less mobile and pathway dilatancy is expected to occur 75 instead (Sellin and Leupin, 2013). In such cases, gas migration at in situ conditions is not 76 thought to occur unless the applied gas pressure, P g , exceeds the total stress, σ, experienced 77 by the clay (resulting from the sum of the applied water pressure and the swelling pressure, 78 Π) Harrington and Horseman, 2003).…”

“…In some cases, the 32 potential for gas generation rates to exceed the rate of dissipation by diffusion must be 33 considered SKB, 2006). Gas pressures may continue to elevate in 34 such a scenario, until either (i) gas generation reduces sufficiently or (ii) a free gas phase is 35 able to enter the surrounding engineered clay or host formation, in which case advective 36 transport may occur (Sellin and Leupin, 2013). It is important to ensure that gas pressure 37 build-up will not result in undesired effects, such as the transportation of radionuclides or 38 mechanical damage to repository infrastructure (SKB, 2006).…”

A thermal, sorptive and spectral study of HDTMA-bentonite loaded with uranyl phosphate

“…An aluminum phyllosilicate clay called bentonite, which is characterized by a high sorption capacity and an ability to swell under the influence of sorbed water, has been evaluated as a good material for the construction of ecological-engineering barriers for safe storage of nuclear wastes [4]. In our earlier experiments, we assessed the sorption affinities of hexadecyltrimethylammonium-bentonite (known as HDTMA-bentonite) toward U(VI) ions [5,6].…”

Experimental Study of Montmorillonite Structure and Transformation of its Properties under the Treatment of Inorganic Acid Solutions

“…These systems are used for radioactive waste depositories of different activity and intended to provide storage safety for several hundreds or thousands years due to the high adsorption capacity and low water permeability [1][2][3].…”