Chapter 47: Radioactive Waste | A Book: Nuclear Energy

This chapter was published on “Inuitech – Intuitech Technologies for Sustainability” on March 23, 2013.

Radioactivity refers to the spontaneous disintegration of an unstable atomic nucleus, usually accompanied by the emission of ionizing radiation. Radioactive wastes represent the scraps from the use of nuclear materials for the production of electricity, diagnosis and treatment of disease, and other purposes. It is reported that radiation exposure immediately around a properly functioning nuclear power plant (NPP) is about 1 percent greater than the normal environmental background exposure.

The generation of electricity from a typical 1000 MW (e) nuclear power station, which would supply the needs of a city the size of Amsterdam, produces approximately 300 m3 of low and intermediate level waste per year and some 30 tonnes of high level solid packed waste per year.By way of comparison a 1000 MW (e) coal plant produces some 300,000 tonnes of ash alone per year, containing among other things radioactive material and heavy metals which end up in landfill sites and in the atmosphere. Nuclear power generation facilities produce about 200,000 m3 of Low and Intermediate Level Waste (LILW) and 10,000 m3 of High Level Waste (HLW), including spent fuel designated as waste, each year worldwide.

Radioactive wastes come in many different forms including the following:

Protective clothing of people in contact with radioactive materials;
The remains of lab animals used in experiments with radionuclides;
Cooling water, used fuel rods, and old tools and parts from nuclear power plants;
Mill tailings from uranium-enrichment factories;
Old medical radiation equipment from hospitals and clinics; and
Used smoke detectors which contain radioactive americium-241 sensors.

Here is how radioactive waste gets around: The planet’s water cycle is the main way radiation gets spread about the environment. When radioactive waste mixes with water, it is ferried through this water cycle. Radionuclides in water are absorbed by surrounding vegetation and ingested by local marine and animal life. Radiation can also be in the air and can get deposited on people, plants, animals, and soil. People can inhale or ingest radionuclides in air, drinking water, or food. Depending on the half-life of the radiation, it could stay in a person for much longer than a lifetime. The half-life is the amount of time it takes for a radioactive material to decay to one half of its original amount. Some materials have half-lives of more than 1,000 years.

According to the International Atomic Energy Agency (IAEA), radioactive waste is any material that contains a concentration of radionuclides greater than those deemed safe by national authorities, and for which no use is foreseen. Because of the wide variety of nuclear applications, the amounts, types and even physical forms of radioactive wastes vary considerably: Some wastes can remain radioactive for hundreds or thousands of years, while others may require storage for only a short decay period prior to conventional disposal.

To facilitate communication and information exchange among its Member States, the IAEA instituted a revised waste classification system in 1994 that takes into account both qualitative and quantitative criteria, including activity levels and heat content.Radioactive waste is generated in a number of different kinds of facilities and it may arise in a wide range of concentrations of radionuclides and in a variety of physical and chemical forms. These differences result in an equally wide variety of options for the management of the waste. There is a variety of alternatives for processing waste and for short term or long term storage prior to disposal. Likewise, there are various alternatives for the safe disposal of waste, ranging from near surface to geological disposal.

Various schemes have evolved for classifying radioactive waste according to the physical, chemical and radiological properties that are of relevance to particular facilities or circumstances in which radioactive waste is managed. These schemes have led to a variety of terminologies, which may differ from State to State and even between facilities in the same State. In some instances, this has given rise to difficulties in establishing consistent and coherent national waste management policies and implementing strategies, and can lead to less than optimal levels of safety.

In order to address these issues, the classification of radioactive waste has been the subject of international standards on the safety of radioactive waste management. Accordingly, different types of waste may be grouped for operational waste management purposes. For example, waste containing radionuclides with short half-lives may be separated from waste containing radionuclides with longer half-lives, or compressible waste may be separated from non-compressible waste.

1. CLASSES OF RADIOACTIVE WASTE:

Here are six classes of radioactive waste that used as the basis for the classification scheme:

1.1 Exempt Waste (EW):

Waste that meets the criteria for clearance, exemption or exclusion from regulatory control for radiation protection purposes is classified as EW.

Exempt waste 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 landfills or recycled. Such material can be cleared from regulatory control and does not require any further consideration from a regulatory control perspective.Liquid or gaseous effluents discharged to the environment under appropriate regulatory control are somewhat analogous to cleared waste, inasmuch as discharged material requires no further consideration from the perspective of radiation protection and safety. There are, however, some notable differences in the establishment of limitations on effluent quantities suitable for discharge and, in the case of discharge of effluents, confirmatory environmental monitoring is normally carried out.

For radionuclides of natural origin, a different approach was adopted: these values were determined on the basis of consideration of the upper end of the worldwide distribution of activity concentrations in soil.

1.2 Very Short Lived Waste (VSLW):

Radioactive waste that can be stored for decay over a limited period of up to a few years and subsequently cleared from regulatory control according to arrangements approved by the regulatory body, for uncontrolled disposal, use or discharge. This class includes waste containing primarily radionuclides with very short half-lives often used for research and medical purposes.

Very short lived waste contains only radionuclides of very short half-life with activity concentrations above the clearance levels. Such waste can be stored until the activity has fallen beneath the levels for clearance, allowing for the cleared waste to be managed as conventional waste. Examples of very short lived waste are waste from sources using 192Ir and 99mTc and waste containing other radionuclides with short half-lives from industrial and medical applications. Although this Safety Guide focuses on the classification of solid radioactive waste, it should be noted that storage for decay is frequently used in the management of liquid and gaseous waste containing short half-life radionuclides, which is stored until the activity concentration has fallen beneath the applicable levels for discharge to the environment.

The main criteria for the classification of waste as VSLW are the half-lives of the predominant radionuclides and the acceptability of the amounts of longer half-life radionuclides. Since the intent of storage for decay is to eventually clear the material, acceptable levels of concentration of long half-life radionuclides are set by the clearance levels. The boundary for the half-lives of predominant radionuclides cannot be specified generically because it depends on the planned duration of the storage and the initial activity concentration of the waste. However, in general, the management option of storage for decay is applied for waste containing radionuclides with half-lives of the order of 100 days or less.

1.3 Very Low Level Waste (VLLW):

Waste that does not necessarily meet the criteria of EW which also does not necessarily need a high level of containment and isolation is suitable for disposal in near surface landfill type facilities with limited regulatory control. Such landfill type facilities may also contain other hazardous waste. Typical waste in this class includes soil and rubble with low levels of activity concentration. Concentrations of longer lived radionuclides in VLLW are generally very limited.

Substantial amounts of waste arise from the operation and decommissioning of nuclear facilities with levels of activity concentration in the region of or slightly above the levels specified for the clearance of material from regulatory control. Other such waste, containing naturally occurring radionuclides, may originate from the mining or processing of ores and minerals. The management of this waste, in contrast to exempt waste, does require consideration from the perspective of radiation protection and safety, but the extent of the provisions necessary is limited in comparison to the provisions required for waste in the higher classes (LLW, ILW or HLW). Waste with such a limited hazard, which is nevertheless above or close to the levels for exempt waste, is termed very low level waste.

In order to determine whether a particular type of waste can be considered to fall into the class of VLLW, acceptance criteria for engineered surface landfill type facilities have to be derived. This can be carried out either using generic scenarios similar to those applied in the derivation of exemption and clearance levels or by undertaking a safety assessment for a specific facility in a manner approved by the regulatory body. The criteria derived will depend on the actual site conditions and on the design of the engineered structures or, in the case of the use of generic scenarios, on assumptions made to take account of these factors. For this reason, generally valid criteria cannot be defined in this Safety Guide. Nevertheless, it is expected that with a moderate level of engineering and controls, a landfill facility can safely accommodate waste containing artificial radionuclides with levels of activity concentrations one or two orders of magnitude above the levels for exempt waste, for waste containing short lived radionuclides and with limited total activity. This applies as long as expected doses to the public are within criteria established by the regulatory body. In general, for waste containing naturally occurring radionuclides, acceptable levels of activity concentration will be expected to be lower than those for waste containing artificial radionuclides, in view of the long half-lives of naturally occurring radionuclides. Depending on site factors and the design, it may still be possible to demonstrate the safety of waste with higher levels of activity concentration.

1.4 Low Level Waste (LLW):

Radioactive waste which is above clearance levels but with limited amounts of long lived radionuclides is classified as LLW. 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. This class covers a very broad range of waste. LLW may include short lived radionuclides at higher levels of activity concentration, and also long lived radionuclides, but only at relatively low levels of activity concentration.

In previous classification schemes, low level waste was defined to mean radioactive waste that does not require shielding during normal handling and transport. Radioactive waste that requires shielding but needs little or no provision for heat dissipation was classified as intermediate level waste. A contact dose rate of 2 mSv/h was generally used to distinguish between the two classes of waste. Contact radiation dose rate is not used to distinguish waste classes in the present, revised classification scheme, which is based primarily on long term safety. However, it remains an issue that has to be considered in handling and transporting the waste, and for operational radiation protection purposes at waste management and disposal facilities, but is not necessarily a determining factor for the long term safety of a disposal facility.

In the classification scheme set out in this Safety Guide, low level waste is waste that is suitable for near surface disposal. This is a disposal option suitable for waste that contains such an amount of radioactive material that robust containment and isolation for limited periods of time up to a few hundred years are required. This class covers a very wide range of radioactive waste. It ranges from radioactive waste with an activity content level just above that for VLLW, that is, not requiring shielding or particularly robust containment and isolation, to radioactive waste with a level of activity concentration such that shielding and more robust containment and isolation are necessary for periods up to several hundred years.

Low concentrations of long lived radionuclides may be present in LLW. Although the waste may contain high concentrations of short lived radionuclides, significant radioactive decay of these will occur during the period of reliable containment and isolation provided by the site, the engineered barriers and institutional control. Classification of waste as LLW should, therefore, relate to the particular radionuclides in the waste, and account should be taken of the various exposure pathways, such as ingestion (e.g. in the case of long term migration of radionuclides to the accessible biosphere in the post-closure phase of a disposal facility) and inhalation (e.g. in the case of human intrusion into the waste). Thus, radioactive waste suitable for disposal near the surface and at intermediate depths may, in most instances, be differentiated on the basis of the need for controls over time frames for which institutional control can be guaranteed and thus human intrusion into the waste can be prevented.

A precise boundary between LLW and intermediate level waste (ILW) cannot be 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 will be dependent on the actual design of and planning for the facility (e.g. engineered barriers, duration of institutional control, site specific factors). Restrictions on levels of activity concentration for long lived radionuclides in individual waste packages may be complemented by restrictions on average levels of activity concentration or by simple operational techniques such as emplacement of waste packages with higher levels of activity concentration at selected locations within the disposal facility. It may be possible for a regulatory body to provide bounding levels of activity concentration for LLW on the basis of generic site characteristics and generic facility designs, as well as specified institutional control periods and dose limits to individuals.

1.5 Intermediate Level Waste (ILW):

Radioactive waste that is classified because of its 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, waste in this class requires disposal at greater depths, of the order of tens of metres to a few hundred metres.

Intermediate level waste is defined as waste that contains long lived radionuclides in quantities that need a greater degree of containment and isolation from the biosphere than is provided by near surface disposal. Disposal in a facility at a depth of between a few tens and a few hundreds of metres is indicated for ILW. Disposal at such depths has the potential to provide a long period of isolation from the accessible environment if both the natural barriers and the engineered barriers of the disposal system are selected properly. In particular, there is generally no detrimental effect of erosion at such depths in the short to medium term. Another important advantage of disposal at intermediate depths is that, in comparison to near surface disposal facilities suitable for LLW, the likelihood of inadvertent human intrusion is greatly reduced. Consequently, long term safety for disposal facilities at such intermediate depths will not depend on the application of institutional controls.

1.6 High level waste (HLW):

Radioactive waste with levels of activity concentration high enough to generate significant quantities of heat by the radioactive decay process or waste with large amounts of long lived radionuclides that need to be considered in the design of a disposal facility for such waste. Disposal in deep, stable geological formations usually several hundred metres or more below the surface is the generally recognized option for disposal of HLW.

High level waste is defined to be waste that contains such large concentrations of both short and long lived radionuclides that, compared to ILW, a greater degree of containment and isolation from the accessible environment is needed to ensure long term safety. Such containment and isolation is usually provided by the integrity and stability of deep geological disposal, with engineered barriers. HLW generates significant quantities of heat from radioactive decay, and normally continues to generate heat for several centuries. Heat dissipation is an important factor that has to be taken into account in the design of geological disposal facilities.

HLW typically has levels of activity concentration in the range of 104-106 TBq/m3 (e.g. for fresh spent fuel from power reactors, which some States consider radioactive waste). HLW includes conditioned waste arising from the reprocessing of spent fuel together with any other waste requiring a comparable degree of containment and isolation. At the time of disposal, following a few decades of cooling time, waste containing such mixed fission products typically has levels of activity concentration of around 104 TBq/m3. For the purposes of communication pending the establishment of disposal facilities for HLW, national authorities may determine that certain waste constitutes ILW or HLW on the basis of generic safety cases.

2. THE CLASSIFICATION SCHEME:

A conceptual illustration of the waste classification scheme is presented in Figure 01. The vertical axis represents the activity content of the waste and the horizontal axis represents the half-lives of the radionuclides contained in the waste. In some cases, the amount of activity, rather than activity concentration, may be used to determine the class of the waste. For example, waste containing only very small amounts of certain radionuclides (e.g. low energy beta emitters) may be excluded or cleared from regulatory control.Considering Figure 47-04:

Vertically, the level of activity content can range from negligible to very high, that is, very high concentration of radionuclides or very high specific activity. The higher the level of activity content, the greater the need to contain the waste and to isolate it from the biosphere. At the lower range of the vertical axis, below clearance levels, the management of the waste can be carried out without consideration of its radiological properties; and
Horizontally, the half-lives of the radionuclides contained in the waste can range from short (seconds) to very long time spans (millions of years). In terms of radioactive waste safety, a radionuclide with a half-life of less than about 30 years is considered to be short lived. It is beneficial to make such a distinction between waste containing mainly short lived radionuclides and waste containing long lived radionuclides because the radiological hazards associated with short lived radionuclides are significantly reduced over a few hundred years by radioactive decay. A reasonable degree of assurance can be given that institutional control measures to contribute to the safety of near surface disposal facilities for waste containing mainly short lived radionuclides can be kept in place over such time frames. Limitations placed on the activity (total activity, specific activity or activity concentration) of waste that can be disposed of in a given disposal facility will depend on the radiological, chemical, physical and biological properties of the waste and on the particular radionuclides it contains.

3. CONCLUSION:

An important reason for the public’s concern about nuclear power is an unjustifiable fear of the hazards from radioactive waste. Several states have laws prohibiting construction of nuclear power plants until the waste disposal issue is settled. Yet ironically, there is general agreement among the scientists involved with waste management that radioactive waste disposal is a rather trivial technical problem. However, studies show that radioactive waste from nuclear power operations represents less of a health hazard than waste from any other large technological industry.

The whole issue of nuclear waste, as seen by public opinion, is grounded in the fear of detrimental effects on human health, and the environment. Two major questions sum up this uncertainty:

Have all risks been properly identified, including those concerning effects that might arise several thousand years from now?
Do radiation protection regulations, according to current recommendations, really make it possible to vouchsafe real protection over time spans of many millennia?

The answers to these queries are in the affirmative, even though some biological mechanisms do still require to be more precisely detailed.

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