Radioactive waste (RAW) classification

For practical purposes, radioactive waste is classified into different classes depending on actual management needs. A number of parameters are con­sidered in classification schemes, the most important of which are shown in Table 1.1 .

Radioactive wastes are typically classified accounting for potential clear­ance, decay storage or disposal, e. g. final point of waste disposition (IAEA, 2009). Key parameters in the IAEA (International Atomic Energy Agency) classification scheme are radionuclide half-life and radioactivity content. The radionuclides are divided into long-lived and short-lived, where a radi­onuclide with a half-life longer than that of 137Cs (30.17 years) is considered to be long-lived, whereas those with shorter half-lives are considered short­lived. The activity content is a generic term that covers activity concentra­tion and total activity and is used in classification schemes accounting for the generally heterogeneous nature of radioactive waste (IAEA, 2009). The activity content can range from negligible to very high, e. g. very high con­centration of radionuclides or very high specific activity. The radioactivity contents are always analysed compared to exemption levels (IAEA, 2004), e. g. the higher the activity content above those levels the greater the need to contain the waste and to isolate it from the biosphere.

The IAEA classification is shown schematically in Fig. 1.4 and has as lowest by activity content the exempt waste (EW). Exempt waste (EW) is

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Property

Origin

Radiological

Physical

Chemical

Biological

Parameter

Source,

Criticality.

Physical state (solid,

Potential chemical

Potential hazard.

Manufacturer

Half-life.

Heat generation.

Intensity of radiation. Activity and concentration of radionuclides.

Surface contamination. Dose factors of relevant radionuclides.

liquid, gas).

Size, volume and weight.

Compressibility

Dispersibility.

Volatility.

Solubility. Miscibility.

hazard. Corrosion resistance. Corrosivity. Organic content. Combustibility. Reactivity.

Gas generation. Sorption of radionuclides.

Decomposition rate and products.

 

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1.4 Schematic classification of radioactive wastes aiming for clearance, decay storage or disposal.

that radioactive waste that meets the criteria for clearance, exemption or exclusion from regulatory control for radiation protection purposes which are given in IAEA publications (IAEA, 2003b, 2009). The criteria for exemption were established by the IAEA following the ICRP (Interna­tional Commission on Radiological Protection) recommendations and prin­ciples used to derive exemption levels for radioactive materials. Generically they are based on an expected individual effective dose not higher than 10 pSv/annum and a collective effective dose not higher than 1 person Sv/ annum. Exemption levels were established for both concentration and total amount of radionuclides based on the individual and collective dose. These were determined for each radionuclide taking account of all possible path­ways to humans including assessment of individual and collective doses. Exemption levels are published in the International Basic Safety Standards for Protection against Ionizing Radiation and for the Safety of Radiation Sources (IAEA, 2003b). Sources of radiation are exempt from control if at a distance of 0.1 metres, the dose rate is below 1 pSv/h. Clearance levels are defined by the national regulatory authorities; however, since these take into account internationally approved recommendations, quantified clear­ance levels (with some exceptions) are similar in all countries. EW contains such small concentrations of radionuclides that it does not require provisions for radiation protection, irrespective of whether the waste is disposed of in conventional landfill sites or recycled, so EW is, in practice, considered as a non-radioactive material.

The IAEA classification scheme defines five classes of radioactive waste: very short-lived waste (VSLW), very low level waste (VLLW), low level waste (LLW), intermediate level waste (ILW) and high level waste (HLW).

VSLW is that radioactive waste which can be stored for decay over a limited period of no longer than a few years with subsequent clearance from regulatory control. Clearance is carried out according to existing national arrangements, after which VSLW can be disposed of, discharged or used. VSLW includes waste containing primarily radionuclides with very short half-lives which are most often used for research and medicine.

VLLW is that radioactive waste which does not necessarily meet the criteria of EW, but that does not need a high level of containment and isola­tion and because of that is suitable for disposal in near surface landfill type facilities with limited regulatory control. Typical VLLW includes soil and rubble with low levels of activity concentration.

LLW has higher activity contents compared to VLLW, but with limited amounts of long-lived radionuclides in it. Such waste requires robust isola­tion and containment for periods of up to a few hundred years and is suit­able for disposal in engineered near surface facilities. LLW covers a very broad range of waste with long-lived radionuclides only at relatively low levels of activity concentration.

ILW is that radioactive waste that, because of its radionuclide content, particularly of long-lived radionuclides, requires a greater degree of con­tainment and isolation than that provided by near surface disposal. However, ILW needs no provision, or only limited provision, for heat dissipation during its storage and disposal. ILW may contain long-lived radionuclides, in particular, alpha emitting radionuclides that will not decay to a level of activity concentration acceptable for near surface disposal during the time for which institutional controls can be relied upon. Therefore ILW requires disposal at greater depths, of the order of tens of metres to a few hundred metres. A precise boundary between LLW and ILW cannot be universally provided, as limits on the acceptable level of activity concentration will differ between individual radionuclides or groups of radionuclides. Waste acceptance criteria for a particular near surface disposal facility depend on its actual design and operation (e. g., engineered barriers, duration of insti­tutional control, site-specific factors). A limit of 400 Bq/g on average and up to 4,000 Bq/g for individual packages for long-lived alpha emitting radio­nuclides has been adopted in many countries. For long-lived beta and/or gamma emitting radionuclides, such as 14C, 36Cl, 63Ni, 93Zr, 94Nb, 99Tc and 129I, the allowable average activity concentrations may be considerably higher (up to tens of kBq/g), although they are specific to the site and disposal facility (IAEA, 2009).

HLW is the radioactive waste with levels of activity concentration high enough to require shielding in handling operations and generate significant quantities of heat by the radioactive decay process typically above several W/m3 . HLW can also be the waste with large amounts of long-lived radio­nuclides that need to be considered in the design of a disposal facility. Disposal in deep, stable geological formations usually several hundred metres or more below the surface is the generally recognised HLW disposal option.

As can be seen, the IAEA classification scheme is rather generic and has no exact limits in defining radioactive waste classes. Existing national regu­lations give more exact figures (Ojovan and Lee, 2005). In the UK, radioac­tive wastes are classified as VLLW, LLW, ILW and HLW (Table 1.2).

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