|Fig. 1: One-stage waste vitrification technology. |
In the United States, the main sources of nuclear waste include both low-level waste (LLW) generated from the dismantling of nuclear weapons, and high-level waste (HLW) such as spent nuclear fuel from nuclear reactors as well as the liquid waste generated from the reprocessing of spent nuclear fuel.  In order to retain radioactive elements in the event of water intrusion into the repository, solid waste forms such as ceramic, cement and glass have been an important area in nuclear research. Although nuclear waste is often stored in a container isolated from the environment, waste containment material still governs the retention of active species. 
Around the world, borosilicate glasses have been the most widely adopted material for the immobilization of both HLW and LLW. In addition to the chemical durability, mechanical integrity and thermal stability, borosilicate glasses are flexible with waste loadings and possess the capability of incorporating most of the waste elements. The process of incorporating HLW into borosilicate glasses is known as vitrification. Fig 1 illustrates the overall one-stage waste vitrification technology. The first step involves the evaporation of excess water from the liquid waste followed by the vitrification process, generating glass waste blocks and secondary waste. 
An example of the vitrification process is a technique called "In-Can-Melter" developed in the USA as shown in Fig.2. The canister does not only serve as a glass manufacturing vessel, but prevents contact between waste and groundwater after burial. The calciner is where thermal decomposition occurs, converting oxysalts such as nitrites and hydroxides to oxides which are incorporated into the waste form. The resulting calcine-frit mixture is then heated to around 1000 degrees Celsius by the zoned furnace to form a molten glass. When the canister is 90% filled, the canister will be removed and replaced by an empty one for continuous operation. The corrosion-resistant canisters will then be put away for long-term disposal. 
|Fig. 2: In-Can-Melter vitrification process. |
One of the critical parameters that affects radionuclide retention is radiation effects. In general, waste form materials are subjected to alpha (α), beta (β) and gamma (γ) self-radiations. During the first 500 years of storage, β-decay of fission products is the primary source of radiation, generating heat and resulting in elevated temperatures. After 1000 years, α-decay of actinides becomes the prominent radiation source. These β- and α-decay can lead to radiation damages such as atomic displacement, ionization effect and radioactive transmutation in nuclear waste glasses. The structural rearrangement caused by the radiation effects can result in significant changes in physical and chemical properties, potentially reducing the ability of the waste materials in retaining radionuclides. Specifically, the damages are minimal in glass waste forms compared to that of ceramics, experiencing less than ±1.2% volume changes and a factor of 5 increase in leach rates. 
Currently, borosilicate glasses have been developed and produced on a technological scale. In gaining public acceptance of nuclear energy, appropriate treatment and safe storage of nuclear waste is the key factor.
© I-Tso Chen. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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