For LLW/ILW, which comprises a wide range of materials, require suitable encapsulant to enclose the waste and act as a matrix for packaging. The choice of encapsulant is not solely dependent on the physical and chemical nature of the waste, there are other factors to be considered.
Firstly, the acceptable criteria for immobilisation of radioactivity; the waste form should be able to physically immobilise the radioactivity before closure of GDF. During early post closure phase, it might be advantageous if the wasteform still contributes to the containment of radionuclides. These include limiting radionuclide release to the groundwater and providing a stable and durable solid product with low leading rates . The second factor to consider relies on the chemical nature of the waste. The encapsulant should be designed to be chemically compatible with the waste and the container. This consideration required knowledge of reactions of the wasteform in a certain timescale, which shall be discussed in later part of the study. Since the radionuclide ionised in random behaviour, materials may breakdown and lead to the release of radioactive substance. These lead to the third factor to be considered, the encapsulant are designed to have a considerable tolerance to radiation doses, which should encountered both the waste being encapsulated and the surrounding waste. Fourthly, the resulting wasteform should display some physical integrity over the changes in the surrounding environment in a certain timescale. Finally, the flow properties of encapsulant are considered. Encapsulant are designed to be able to penetrate through the waste in complex geometries to allow a complete immobilisation of waste.
There are two types of encapsulants commonly used for LLW/ILW, namely cement-based and polymeric wasteform. Their process mechanism and designs are different. Both are suitable to encapsulate ILW/LLW, the matter of choice depends on specific disposal concept.
The technique of cement-based encapsulation is used in many countries . Cement-based encapsulants are especially designed for immobilising waste in the form of sludges, fine particulates and liquid. This is because these materials can be mixed completely with the cement and they do not undergo any adverse chemical reactions with the cement porewater. Ordinary Portland cement (OPC) is the most common type of cement used to immobilise liquid waste. A composite cement system are also commonly employed worldwide, usually containing OPC with additional powders of blast furnace slag (BFS) or pulverised fuel ash (PFA). The mechanism of the forming of cement-based wasteform is due to hydration. Hydration process is complex and highly exothermic, which may cause thermal stresses and lead to cracking. Therefore, the design of OPC as encapsulant always included a small addition of BFS or PFA to reduce the heat output. These substances react with Calcium Hydroxide produced as a...