Solid waste treatment

The essential purpose of solid low and intermediate level waste treatment is to reduce the volume. Both compaction and supercompaction technolo­gies are described in detail in Ref. [21] . Compaction involves compressing the waste into containers or boxes in order to reduce the volume. Low force compaction is the least expensive and an easier volume reduction process than high force compaction. Compaction units are also amenable to auto­mation, which can improve operational efficiency and radiation protection aspects. High force compactors can give somewhat better reduction factors, whereas supercompactors achieve highest volume reduction which is close to the theoretical density of materials by minimizing the voidance. Both high force and supercompactors typically compress the waste inside of drums. From a technical viewpoint, the same technique may be applied as a treatment or a pre-treatment step, depending on the required sequence in the overall waste management scheme. For instance, shredding could be considered as treatment when applied for volume reduction of waste before packaging, and as pre-treatment when applied before incineration. In another example, low pressure compaction may be applied as a treatment method when used before supercompaction as a pre-compaction step.

Thermal treatment (incineration, pyrolysis, plasma, etc. [11, 21, 22] may provide the best potential for effective volume reduction of generated solid waste. A further advantage of employing thermal treatment is an improve­ment of homogeneity and quality of the waste form obtained after treat­ment and conditioning. Considering the high overall costs of waste disposal and the growing requirements for improved quality of the final waste form, the benefits offered by thermal processing become very significant. Thermal methods may also have disadvantages restricting their applications. The primary consideration is meeting environmental safety requirements, such as gaseous effluent restrictions, which lead to greater complexity and higher costs of these technologies. Higher imple­mentation costs may not justify the application of incineration for relatively small volumes of solid waste. Generally, the permits or licences from regula­tory agencies will stipulate numerical emission limits or known reference standards to be met. Melting waste metal scrap, with resultant homogeniza­tion of the radioactive material and its accumulation in the slag, may be considered as a means of achieving authorized reuse or removal of regula­tory control [22] .

Two emerging technologies, molten salt oxidation and thermochemical treatment, have demonstrated promising performance parameters, although to date they have limited application. Molten salt oxidation is a flameless thermal desorption process [ 22] , The waste is introduced into a bath of molten salts, typically at temperatures between 500 and 950°C. This has the effect of oxidizing the organic constituents of the waste. Carbon dioxide, nitrogen and water are produced. The end product is an organic-free salt residue which captures radionuclides, metals and other inorganics. The pro­duction of acid gas emissions is inhibited by the formation of the stable salts. Thermochemical treatment uses powdered metallic fuel (PMF), such as Al or Mg, which interacts with the waste both chemically and physically through reaction with the water present in the waste. This results in the formation of hydrogen gas and heat; the subsequent combustion is used to destroy the organic material, resulting in solid slag or ash. The hydrogen gas burns because of the presence of enough oxygen, and in co-reaction the waste is combusted leaving a slag-like product. The presence of excess metal powder suppresses the production of corrosive gases. Thermochemical processing technologies are used for treating and conditioning problematic radioactive wastes [23] , Thermochemical processing uses PMFs that are specifically formulated for the waste composition and react chemically with the waste components. The composition of the PMF is designed to minimize the release of hazardous components and radionuclides in the off-gas and to confine the contaminants in the ash residue. Thermochemical procedures allow decomposition of organic matter and simultaneous capture of hazard­ous radionuclides and chemical species [22, 23].

Technical solutions for management of radioactive waste 125

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