Engineered clay buffers

Abstract This paper examines the hydraulic interaction between emplaced engineered clay buffers and host rock formations. This has been achieved via the analysis of the re-saturation behaviour of a large scale in-situ experiment. Experimental results from a testing programme performed by Atomic Energy of Canada Limited, (AECL), are examined via both direct interpretation of the experimental results and numerical simulations of the test. Analysis of seepage rates into an open borehole indicate the presence of an EDZ prior to the emplacement and some healing of the EDZ after buffer placement. It is also found that correct representation of moisture infiltration into the buffer is essential to capture the variations in pore water pressure in the host rock. 

This paper has examined the hydraulic interaction between emplaced engineered clay buffers and host rock formations. This has been... 

Gens, A., Guitnaraes L. do N., Garci a-Molina, A., Alonso, E.E. 2002. Factors controlling rock-clay buffer interaction in a radioactive waste repository. In Engineering Geology. 64 pp. 297-308. 

The engineered barriers of a repository (waste form, container, clay buffer and backfill material and repository stmcmre) which will degrade with time due to various physico-chemical processes, strongly affect the geochemistry of fluids and rocks of the near field environment (redox properties, sorption properties, solubility limits, equilibrium chemistry, etc.) and determine the speciation and the subsequent migration behaviour of the radionuclides into the far field. Extensive... 

Figure 13.33 Schematic diagram of the engineered barrier system (BBS) showing the high-level nuclear waste in its metal container, surrounded by a buffer or backfill (usually of compacted bentonite clay), in contact with the host rock. The BBS and rock affected thermally by the waste are sometimes termed the near field, with more distant surrounding rock termed the far field. After The status of near field modeling. Proc. Technical Workshop, copyright 1995 by OECD. Used by permission.

Engineered barriers generally include a waste form (spent nuclear fuel or glass containing radioactive wastes) encapsulated within a metallic container (e.g., steel and copper), which may be surrounded by a low-permeability, clay-rich buffer and backfiU. Natural barriers include the repository host rock and a volume of rock between the repository and biosphere. The engineered barriers and immediately adjacent host rock are referred to as the near field [1]. 

The primary engineered barrier is the spent fuel itself. It is a sparingly soluble oxide (U02(s)) which dissolves extremely slowly under anoxic conditions, but can corrode to more soluble forms in the presence of dissolved oxygen (O2) (summarized in Miller et al. 1994, Section 4.2). This is also the case for the copper canister, and the steel lining of the canister, should it be breached. The copper canister can also undergo sulphidic corrosion (described below) under reducing conditions. The buffer material is intended to be compacted bentonite clay, with a mixture of bentonite and sand to be used as fill material for the tunnel and shafts. The compacted bentonite buffer is designed to maintain low hydraulic conductivity in order... 

In principle the material properties identified in Table 1 allow an attenuation liner to be engineered for any given landfill. The ideal liner recipe which maximizes the attenuation of the key contaminants in leachate is deduced to be high clay and organic carbon content plus adequate alkaline pH buffering capacity. 

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