Waste disposal

Disposal is emplacement of waste in an appropriate facility without the intention to retrieve, although some countries use the term disposal to include discharges of effluents to the environment [39].

Exempt waste is the waste that meets the criteria for clearance, exemp­tion or exclusion from regulatory control for radiation protection purposes as described in Ref. [40]. Exempt waste is acceptable for disposal in landfill sites used for domestic and industrial waste.

Simple trenches have been used for many decades for the disposal of short-lived low and intermediate level waste. They are generally considered appropriate only for those wastes including disused sources that will decay sufficiently within an anticipated period of institutional control (generally between 100 and 300 years) to represent no risk to the public, as determined by safety assessments. The design and function of such repositories are described in Refs [41-44] . Large-scale (typically thousands of cubic metre capacity) near-surface engineered vault repositories have similar contain­ment objectives and are used for similar types of RAW as simple trenches. Their engineering is intended to allow ease of waste emplacement and increased efficiency in the management and closure of the repository. The issue of post-institutional control intrusion can still be a dominant factor in waste acceptability [7, 11]. For the near-surface disposal option, a perform­ance assessment is also required to determine either that the activity of the RAW can be contained until it has decayed or, if some migration is antici­pated, that consequent doses are acceptable.

Near-surface shafts and/or boreholes can be considered as alternative or complementary to near-surface vaults. These disposal options have the advantages of being economical and also minimizing the probability of human intrusion. If necessary an engineered barrier system (EBS) can be added to the design and construction of these facilities to provide additional protection against radionuclide migration and human intrusion. More heavily engineered near-surface facilities have been designed with the specific intention of reducing the likelihood of intrusion by emplacement of a massive concrete plug or cover over a large shallow shaft or borehole. For example, a reinforced concrete slab at least 1 m thick is considered to be a deterrent to inadvertent intrusion. These intrusion-resistant designs [45] will be helpful if institutional controls break down before the typically envisaged 300-year period. However, they do not offer a sufficient guaran­tee against intrusion to be considered for disposal of higher activity or longer lived waste.

Radioactive wastes that are not acceptable for disposal in near-surface disposal facilities, because they will not decay sufficiently within the period of institutional control, may be suitable for disposal at greater depth in disposal units characterized by one of several configurations [7]. At present, with the exception of deep tunnels and mines, it is uncommon to find con­struction work (e. g., deep foundation engineering) carried out at depths greater than about 30 m [44] , so disposals at depths greater than this are only vulnerable to intrusion by deep drilling for water or mineral explora­tion — a much lower probability. As a result, the intrusion exposure risks posed by higher activity waste disposed of at intermediate depths are small. Shafts or boreholes to depths of several tens of metres or more are rela­tively simple to construct and can offer an attractive disposal option for small volumes of waste such as radioactive sources [44]. Evaluation of such options needs to consider the stability of the hydrogeological system over the time period of concern for containment, which may be several hundreds or thousands of years depending on the types of radioactive sources to be disposed of.

Very low permeability host rocks, with little or no advection of ground­water, can also provide adequate containment without the need for addi­tional EBSs. Some clay and claystone formations at intermediate depths can provide such an environment, and evidence of lack of flow can be obtained from pore water environmental isotope analyses and evaluation of any fracturing that may be present in the rock. The isolation capability of this option depends on the ability to provide good shaft or borehole backfilling and sealing. The use of natural materials that reconstitute the original properties of the penetrated rock formations is recommended for all or some part of the sealing system. This may involve removal of some lining or casing to allow sealing against the host formations. If the disposal borehole/shaft is subject to significant water inflow or the geotechnical characteristics of the geological materials do not allow the excavation to be sufficiently stable, EBSs need to be emplaced to provide a level of contain­ment commensurate with the hazardous life of the waste.

There are some disposal facilities for RAW in large rock cavities at depths of several tens of metres, generally in hard crystalline rocks such as granite (e. g., in Sweden and Finland). They are designed to contain short-lived low

and intermediate level waste. The containment provided by such repositor­ies often comprises massive concrete vaults or silos, with additional EBSs such as clay backfills and buffers. This type of containment should be ade­quate for the disposal of many, if not all, types of RAW. For emplacement of high activity waste in a mined, intermediate depth repository, it is necessary to consider packaging and activity concentrations that suit the thermal char­acteristics of the host rock and EBSs of the repository. In addition, disused mines and/or caverns can be considered for intermediate depth disposal. Such facilities have not been widely used for the disposal of RAW. The objective of using deeper boreholes, at depths typical of geological reposi­tories, would be to achieve greater isolation for limited volumes of RAW, including disused radioactive sources, in an environment that is character­ized by lower flow, more stable chemistry and longer potential return paths to the biosphere, compared with the other options. In a very low permeabil­ity environment (e. g., some clay and claystone formations), there may be no effective water movement at depths of a few hundreds of metres. In such conditions, provided an adequate borehole seal can be constructed, contain­ment of radionuclides is provided by the geological barrier and there is no requirement for supplementary EBSs beyond those needed to emplace the radioactive sources into the borehole and to maintain borehole stability during emplacement operations (casing and cementing). The option is par­ticularly suited to the highest activity and long half-life radioactive sources, for which long containment periods are required (e. g., -10-20 half-lives or more). For example, strong 226Ra sources could require isolation for -20,000­30,000 years. The depth and design of disposal also significantly reduces the likelihood of inadvertent intrusion, resulting in exposures to high concentra­tions of radionuclides before sources have decayed.

Mined repositories, comprising caverns or tunnels with varying types of EBSs, are being developed in many countries that have nuclear power industry wastes to manage [11, 46, 47]. They are designed to contain long-lived low and intermediate level waste, HLW and SFW. The contain­ment provided by all such repositories is expected to be adequate for the disposal of all types of RAW provided that legal and regulatory require­ments on repository inventory permit (some countries have strict con­straints on the types of waste that can be placed in specific repositories which are purely legal and unconnected with safety and performance). In addition, disused deep mines and/or caverns could be considered for geo­logical disposal [46, 47] .

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