Solid waste

The essential purpose of solid waste treatment is to reduce volume. The main features of solid waste treatment comprise waste pretreatment operations such as segregation according to activity and nature, packaging and size reduction, and final volume reduction. Available solid waste treatment options [20] include such methods as:

· Mechanical treatment (low force and high force compaction).

· Thermal treatment (incineration, thermal destruction, molten glass, molten salt, plasma arc incinerator, slagging kiln, plasma pyrolysis).

· Melting and sintering processes (metal melting, electroslag melting, plastic extrusion, microwave melting, plasma melting, ceramics production).

· Chemical (acid digestion, acid stripping, chemical oxidation, photolysis, electrolysis), biochemical and photo-oxidation decomposition.

The waste concentrates having achieved the highest volume reduction are then routed to the appropriate conditioning step where the final package for interim storage or disposal is obtained.

Dealing with solid waste, the clearance from regulatory control applied to recycling, reuse outside nuclear facilities or disposal as normal waste is an important action to be taken. Such clearance can either be achieved by measurement and sorting, decontamination of contaminated waste materials; or by decay storage of waste contaminated with short-lived radioactive isotopes.

At residual activity levels that would result in an individual dose, regardless of its origin, in order of some tens of microsieverts per year, that are likely to be regarded as trivial, materials can be recycled to unrestricted use or disposed of like conventional waste subject to efficient radiation measurements and monitoring.

TABLE VIII. MAIN FEATURES OF THE SOLID WASTE TREATMENT PROCESSES

Treatment methods Features Limitations

Low force compaction · Relatively low cost.

· Easy to operate. · Low volume reduction factor (3–5).

High force compaction · High volume reduction factor (up to 100).

· Good quality waste form.

· Services may be exported to other Member States.

· High equipment cost.

· Maintenance is costly and frequently required.

· High volume reduction factor.

· Low technology to high technology, matching the need of the user.

· Secondary waste resulting from incineration may require

immobilization in high integrity matrix (e.g. cement).

· Off-gas must be monitored and treated if required.

· Trained workforce is required.

Molten glass/molten salt · High integrity confinement.

· Combines incineration and encapsulation features.

· Complex technology, with demanding workforce requirements.

· Applies to metals when decontamination is not successful.

· Requires large equipment, expensive to maintain.

· Economically may not be justified for small amounts of large items.

Plasma arc incineration · High temperature incineration. · Advanced technology only suitable for advanced nuclear programmes.

Melting and sintering

processes · May facilitate decontamination and waste minimization.

· Suitable for recycling and reuse.

· The volume of waste and technological needs of the country will have to be advanced.

· Trained workforce is required.

· High cost.

Chemical decomposition · Avoids the necessity of high temperature incineration.

· Suitable for biological waste.

· No volume reduction.

FIG. 8. Management flow diagram for solid waste.

Figure 8 is a flow sheet with a basic description of the management of solid radioactive waste. Table VIII addresses the main features and limitations of various forms of solid waste treatment. The figure and table may be used together to make some preliminary decisions about the technical feasibility/desirability of some options for managing low and intermediate level solid waste.

Incineration normally achieves the highest volume reduction and converts the waste to a form which is suitable for subsequent immobilization and disposal. However, incineration of solid waste, as in the case of organic liquid waste, should be considered only after careful evaluation of all features, especially radiological aspects. Incinerators without special air cleaning or ash-handling devices have a relatively low investment cost, but the activity of the waste incinerated needs to be restricted to levels which will not result in exposure of the general population to concentrations which exceed those permitted under national regulations or those approved by the Regulatory Body.

Incineration of combustible waste containing larger quantities of radionuclides requires special off-gas cleaning and maintenance systems involving high investment and operation costs.

Compaction involves compressing the waste into containers or boxes in order to reduce the volume. Compaction is a suitable method for reducing the volume of solid waste. Different types and designs of relatively simple compactors with compressive forces between 10 and 50 tones are available offering varying volume reduction possibilities (typical volume reduction factor 2–5).

Shredding usually performed during pretreatment (including granulation, grinding, and pulping) breaks the waste down into smaller pieces. This process provides disfigurement of the waste and also prepares the waste for other types of treatment. The shredding process serves not only to disfigure the medical waste but to prepare it for distribution through the auger conveyor.

Shredding may provide up to an 80% volume reduction.

Depending on the volume, nature and level of radioactivity, a suitable method could be chosen. For example, if the waste volumes and the activity levels are low, mere compaction and packaging may be sufficient before disposal. On the other hand, if higher volume reduction is desired, the appropriate technology may be supercompaction or incineration, followed by conditioning of the ash in a suitable matrix like cement. Such factors as the availability of disposal space in the repository, the nature of engineered barriers, the regulatory stipulations in respect of waste acceptance criteria for disposal and cost considerations would influence the choice of technology for conditioning.