Behaviour of Cementitious Materials in Long Term Storage and Disposal of Radioactive Waste

The Coordinated Research Project resulted in an active interchange of experiences among leading research groups on the behaviour of cementitious materials in long term storage and disposal of radioactive waste. It has enabled to access valuable information on the underlying science and technology of cementitious materials used radioactive waste management.
Cements are a suitable material for the immobilization of a variety of waste constituents due to their favourable chemical and physical properties. The cement hydration products formed favour sorption and substitution of cationic, anionic and neutral radwaste species into cement solids while the microstructure affords physical immobilization. Cements have been extensively modified by reactive admixtures to enhance physical properties and tailor the immobilization potential for specific radwaste species or groups of species and counter the potentially harmful effect of inactive waste species.
Interactions between a cement system and the waste stream have, however, been shown to be complex, and research programmes in this area are carried out to fully understand and quantify these. Available research has highlighted that, by controlling the internal chemistry, microstructure of the matrix and the hydration products formed, through incorporation of reactive admixtures, choice of curing temperature and water /cement ratio, systems may be developed selectively to enhance immobilization of a specific waste component or group of components.
More research is required on the physical and chemical effects of waste ions on the cement structure during solidification; the formation of exotic hydration products, the speciation of waste ions and the resistance of the constituent solid phases to degradation. These interactions must be understood because apparently slight changes in matrix chemistry could result in significant change in immobilization capacity and, if this capacity is reduced, accelerate release of radioactive material into the biosphere. Waste-cement interactions, and the time dependence of the immobilization potential, are the focus of much current research.
Although numerous data are available on the release of radionuclides from various waste forms, modelling studies of the interactive processes of degradation in cementitious material is a much larger task than could be encompassed in this CRP. The local characteristics of the disposal site impose specific constrains on the evolution of barrier performance with time. Both simple and complex models are being developed: simple models as tools for semi-quantitative assessments and more complex and sophisticated models to couple processes, including reactive transport, with other chemical and physical degradation mechanisms.
To verify the durability of the waste forms, several leaching methods have been defined but none have been subject to sufficient critical scrutiny to be considered as standard. Tests are available for both static and dynamic conditions, but irrespective of the test method, analytical work is required following leach tests. Variables affecting the leaching rate during testing have to be agreed as appropriate. Defining “appropriateness” creates problems where data have to be kept generic because no site has been defined and the expected hydrogeological conditions are not known. Among the main variables to be considered are: the flow rate, the time of leaching, the effects of the temperature and the leachant composition.
Novel materials need a better benchmarking. Existing test methods may not give comparable results with different classes of materials.
Among the data to be measured, the most important is leaching characteristic. Under static leaching conditions, the concentration of dissolved species increases with time, until the solution concentration reaches its solubility limits. But it may not be easy to draw conclusions about the applicability of this type of testing to waste form behaviour in low- flow conditions. Surface alteration is a second effect to be considered: monoliths typically react with their service environment so as to form chemically and physically graded structures: reaction is physically inhomogeneous. The waste forms may also be affected by radiation and the products of radiolysis thus affecting the chemical composition of “water” in the waste.
Testing and the extrapolation of short term test data to centuries (or longer) has proved to be very complex. Reliable data on the kinetics of all but the simplest processes are missing. Quality assurance and criteria for acceptance have proven to be another problematical area. Although encouraging progress is reported on non- destructive testing (NDT), additional NDT test methods are required. Criteria for acceptance for final disposal need to be appropriate to the disposal site and activity of the package.
The capacity to model all the effects involved in the dissolution and alteration of the waste form, in conditions similar to the disposal site, and to predict behaviours of cementitious materials used, is the final goal of the research undertaken by some research groups. As described in this document, modelling is still in its developmental stage. Nevertheless it may be the way forward as compression of the time factor is difficult reliably to achieve by accelerated testing. However, it is suggested that the data base for modelling needs to be upgraded and enhanced, and that modellers link to experimental studies designed to verify the key conclusions of mathematical models.
Encouraging progress is reported in the CRP. While many tasks remain incomplete there is a greater understanding and definition of the remaining problems and of approaches and methodologies needed to solve them.