Standards Development

Scope

This document specifies the test devices, test proceduresdata analysis, test reportquality control of swelling pressure test for buffer material under coupled thermo-hydro-mechanical conditions for the geological disposal of high-level radioactive waste. This is one of three key tests of buffer material under these conditions. Specifically, the test temperature range of those tests is from room temperature to 90 , because the buffer material will experience such temperature conditions during early disposal process.

This document is Part 1 of a three-part series comprising the following:

Part 1 – Swelling pressure test

Part 2 – Hydraulic conductivity test

Part 3 – Triaxial compression strength test

The series is applicable to the whole process of testing of buffer material under coupled thermo-hydromechanical conditions for geological disposal of high-level radioactive waste. This document is applicable to the swelling pressure test specifically. 

Purpose

Deep geological disposal is internationally recognized as a feasible and effective way to dispose of High-level Radioactive Waste (HLW). Repositories are generally designed on the basis of a multiple barrier system concept, which is composed of natural and engineered barriers. As an engineered barrier between the waste container and host rock, the buffer material is an important component in the multiple barrier system to isolate HLW canisters and to limit transport of radionuclides with water flow into the biosphere. The safety functions of the buffer require adequate properties of the buffer material such as

• sufficient density to ensure other required attributes;

• sufficient mechanical stiffness and strength to stabilize HLW canisters within the buffer;

• sufficient swelling capacity to seal engineered voids and gaps remaining in the buffer and to integrate with the surrounding host rock;

• low hydraulic conductivity to limit advective water transport and radionuclide migration;

• adequate thermal conductivity to transfer heat into the far-field and thus to avoid overheating to high temperatures;

• sufficient gas transport capacity allowing gas release without compromising the integrity of the barrier system;

• sufficient sorption capacity to retard release of radionuclides;

• low gas entry/breakthrough pressures to allow gas release without damaging the integrity of the geological and engineered barrier system.

The buffer material is exposed to a coupled thermo-hydro-mechanical-chemical (THMC) process over hundreds of thousands of years. Qualification of a candidate buffer material needs to determine its properties and behaviour under THMC conditions being expected in a repository. During last decades, various kinds of experimental methods and technologies have been developed and applied for characterisation of these different properties of bentonite-based buffer materials. In order to guarantee the quality of the test results, it is necessary to identify and unify suitable test methods, i.e., to establish standards test methods.

With focus on swelling pressure, the justification is as follows:

The swelling pressure is one of the most important properties of buffer materials. The above-mentioned safety functions of the buffer are closely related to the swelling pressure. The buffer materials in the HLW repositories need to provide a suitable swelling pressure for long-term stability and safety of the barrier systems under coupled THM conditions. Therefore, the swelling pressure is a key input for the design and safety assessment of the HLW repositories.

The swelling pressure can be determined by different methods in the laboratory. At present, there are three kinds of methods for testing of the swelling pressure of soils, i.e., the free swell-compression test, the loading swell-compression test and the constant volume test. In the free swell-compression test, water is added, and the soil specimen undergoes free or full swell before it is progressively loaded, as in the consolidation test. In the loading swell-compression test, several identical specimens are used, and these specimens are allowed to swell under different loads, resulting in different volume changes. The constant volume test is conducted by making continuous adjustments after adding water to prevent the volume expansion of the specimen by increasing the vertical stress until no longer vertical deformation is measured. However, due to the difference of stress paths between each method, the testing results of swelling pressure are different.

In addition, some countries such as China and US proposed standards for testing of swelling pressure of soils. However, the existing standards are only suitable for low expansive soil but not for buffer material with strong swelling. This is because for the buffer material with strong swelling, its expansion is very quick after wetting, and hence it is difficult to maintain the constant volume through loading. Under the loading mode, the specimen will undergo complicated expansion-compression processes, which influence the accuracy of swelling pressure data. More importantly, existing standards do not take into account the effect of temperature on the swelling pressure.

Among these methods, the constant volume swell test is the most widely reported and accepted method for swelling pressure measurement of the buffer material in different countries such as China, Finland, France, Germany, India, Japan, Spain, South Korea, Sweden, and UK. Although the same method is adopted by different countries, the specifications (e.g., specimen diameter, specimen thickness, and water circulation system) are different.

It is concluded that the existing test methods for swelling pressure measurements of the buffer material have not been standardized or accepted universally. Moreover, there is no ISO test method at present. Therefore, it is necessary to standardize the swelling pressure test method for buffer material under constant volume conditions. A PPT file with more details is attached for reference. 

n a word, standard test methods are proposed to determine key geotechnical properties and behaviour of bentonite-based buffer materials under THM conditions.

The three-part will provide unified technical specifications for tests of

Part 1 – Swelling pressure (this New Work Item Proposal)

Part 2 – Hydraulic conductivity (future proposal)

Part 3 – Triaxial compression and strength (future proposal)

Thus, for buffer material under coupled thermo-hydro-mechanical conditions, the parameters obtained are comparable with each other. Research institutes and waste management organizations will be able to use this 3-part series to test the buffer materials which would be used in their repositories and then finally evaluate the potential use of the buffer material.