radioactive waste management

radioactive waste management Radioactive waste is a hazardous waste substance, the toxicity of which is derived from the radioactive decay of some of its components at a rate exceeding established thresholds. The presence of significant radioactive decay is the only feature distinguishing radioactive wastes from other wastes. They can be in liquid, solid, or gaseous form; flammable or inflammable. They can also have an additional significant chemical toxicity. In this case they are referred to as mixed wastes.

Radioactive wastes are generated at all facilities where radioactive substances (radio-isotopes) are produced or, otherwise, handled directly. These include facilities supporting the industrial, agricultural, and medical application of radio-isotopes, research establishments (including most large universities), and all facilities involving the nuclear fuel cycle. The latter include the mining, milling, and enrichment of uranium, the fabrication of nuclear fuels, their ‘burning’ in nuclear reactors, and the reprocessing of spent fuel. Radioactive wastes also arise from the decommissioning of nuclear facilities and from the environmental remediation of abandoned sites. Phosphate ore extraction for fertilizer production, as well as drilling for oil and gas, can also result in the abnormal concentration of natural radio-isotopes in waste streams. The latter, however, are not normally classified as radioactive waste, although their radioactivity levels may sometimes reach those of the waste categories mentioned below.

Radiological characteristics

From the point of view of radioactive waste management, three main forms of radioactive decay need to be considered:(1) alpha (α) decay, during which a heavy, positively charged particle is emitted;(2) beta (β) decay, during which a light, negatively charged particle is emitted; and(3) gamma (γ) decay, during which a massless, charge-free particle (photon) is emitted.These particles have different penetrating powers: the heavier the particle, the less the distance it is able to travel through materials and thus the thinner the shield that is needed to protect oneself from it. Thus anyone could handle without risk a thin-walled metal container of α-emitting waste, but would be in serious danger in front of the same container filled with γ-emitting waste. By the same token, while γ-emitting substances are dangerous upon both internal and external exposure, α- and β-emitting substances tend to be harmful only upon ingestion or inhalation and accumulation in critical organs.

The β- and γ-emitters of interest in radioactive waste management decay typically with a half-life of roughly 30 years or less, while the α-emitters have a much longer half-life, extending to hundreds of thousands of years. Half-life is defined as the time needed to reduce the initial amount of the radioactive substance by half. Thus, the presence in the waste of significant concentrations of α-emitters is a relevant criterion for considering their long-term isolation from the biosphere. It must also be noted that, associated with the emission and the slowing down of α-, β-, or γ-particles, there is an energy transfer which may entail a significant temperature increase in the waste and surrounding medium. Heat generation rate is therefore another important criterion in the management of radioactive waste.

Classification

A certain variability exists in the way national organizations take into consideration the radiological characteristics of waste for classification purposes: this can cause confusion when examining information about waste management programmes and practices or data published in the technical literature. In the context of a packaged radioactive waste ready for shipment to a storage or disposal facility, a traditional but generic and qualitative classification scheme is as follows:(1) Low-level waste (LLW): radioactive waste that does not require shielding during handling and transportation.(2) Intermediate- (or medium-) level waste (ILW): radioactive waste that requires shielding but needs little or no provision for heat dissipation during handling and transportation.(3) High-level waste (HLW): radioactive waste that requires shielding and provision for heat dissipation during handling and transportation. This category includes spent fuel in countries where the latter is not reprocessed.(4) Alpha waste: low- or immediate-level waste with significant contamination from long-lived α-emitting substances. In North America this category of waste is also referred to as TRU (TransUranic) waste.A special category of radioactive wastes, which are not packaged in individual containers for disposal and therefore escape the above classification scheme, are the so-called uranium (and/or thorium) mill tailings. These contain low but long-lived levels of radioactivity and some of the chemicals used in the uranium/thorium separation processes. They are managed by stabilization in situ on account of their relatively large volumes.

Finally, some countries have an additional category of exempt or very low-level waste, that is, wastes with measurable levels of radioactivity, albeit below regulatory concern, which can be disposed of without radiological restriction in approved discharge facilities.

Waste generation and composition

Radioactive wastes arise, for the most part, from the day-to-day operation of facilities where radio-isotopes are handled; that is, operations where parts have to be replaced, samples have to be taken and cleaning has to be performed. The volume, amount, and type of waste vary with each waste-producing facility. However, uranium/thorium mill tailings are generated only during the extraction and refining of the original ore, spent fuel is produced only in nuclear reactors, and HLW only following the reprocessing of spent nuclear fuel. Low-level waste typically consists of paper towels, laboratory glassware, used syringes, rubber gloves, cloth overalls, air cleaning filters. Intermediate-level waste is of a more industrial nature, having an important component of scrap metal, evaporator sludge, spent resins used for the clean-up of contaminated waters, and parts of used radio-isotope sources in medical equipment. High-level waste consists of glass blocks into which the high-level liquid streams from the reprocessing plant are stabilized. Ninety-nine per cent of the total radioactivity produced in the nuclear fuel cycle is concentrated in the spent fuel or, where spent fuel is reprocessed, in the HLW; the remaining 1 per cent is dispersed within the alpha and the low- and medium-level wastes.

In nuclear countries, the facilities of the nuclear fuel cycle produce, collectively, the greatest fraction of the radioactive waste. This general situation is portrayed in qualitative terms in Fig. 1. More quantitatively, and taking the European Community (EC) as an example, for 1991–95 the annual rate of production of radioactive waste (excluding mill tailings) was estimated to be about 81 430 m³/year—75 000 m³ of which (91.5 per cent by volume) was low- and medium-level waste, 6500 m³ (or 8 per cent) alpha waste, and 430 m³ (or 0.5 per cent) high-level waste. Waste generation is expected to increase by twofold after the year 2000 owing to the dismantling of obsolete nuclear facilities, including nuclear power plants. In any event, the volumes to be managed will still be limited with respect to those of other wastes, namely, 1000million m³/year of general industrial waste in the EC, of which 10 million m³ are classified as being toxic.

Waste treatment

Industrially produced radioactive waste needs to be treated and stabilized for interim storage and final disposal. In particular, liquids are not allowed in disposal containers.

For low- and medium-level waste, as well as for alpha waste, various industrial methods are applied, as needed, to achieve:(1) volume reduction through incineration, evaporation, centrifugation, chemical separation, and compaction;(2) waste stabilization through the addition of sorbing additives and filler materials or, for some waste streams, cementation or bitumenization; and(3) waste packaging in special containers, depending on the waste type.In the case of high-level waste, the nuclear industry has pioneered the stabilization of the liquid streams through vitrification into borosilicate glass blocks. This technique is now being applied to other hazardous wastes of non-nuclear origin. Alternatively, spent fuel removed from the reactor is already in solid form and needs no additional treatment before disposal, although a packing material is sometimes added in the containers to fill the void space.

Waste storage and disposal

The relatively low volumes of radioactive waste make their management and disposal viable both from economic and environmental points of view. As an example, in France the single 12-hectare disposal facility at La Manche was able to meet the disposal needs of that country's entire production of low- and medium-level waste up to the year 1992. Other useful characteristics of radioactive waste which alleviate the management task are that:(1) there exists only a limited number of critical radio- isotopes; and(2) all radio-isotopes decay with time into stable (non-radioactive) substances.An interim decay period is often useful to lower the γ- and β-activity of most wastes significantly, making them easier to process and to handle. Thus all waste stabilization and disposal activities are preceded by some period of interim storage. Some low-activity wastes are allowed very simply to decay before they are discharged as non-radioactive waste. When it is not feasible to wait for the waste to decay significantly, isolation from the biosphere is practised. This is accomplished by placing the waste within a shallow or deep geological facility, sited and built following an appropriate site characterization programme and environmental impact studies.

Disposal of low- and intermediate-level waste

Until the late 1970s some countries practised both shallow-land and sea disposal of low- and intermediate-level waste, while most other countries have kept the accumulating waste in interim storage. Since the early 1980s sea disposal has no longer been considered an acceptable option, and shallow-land disposal practices have improved with the implementation of engineered barriers. For instance, at the Centre de l'Aube in France, pre-approved, standard waste drums are stacked on each other in concrete cells which are built on a concrete pad a few metres below the ground. The cells are sealed by pouring additional concrete and covering them with impermeable clays to which vegetation-bearing soils have been added. Drainage channels are built within and around the concrete pad, and sampling wells are used in order to monitor potential leaks of radioactive elements. The site will be monitored for roughly 300 years after closure; that is, until the waste decays sufficiently to allow unrestricted access. During that time the information about the source and composition of each waste drum is preserved. Similar disposal concepts have been implemented in other countries, for instance at Rokkasho-Mura, Japan, and El Cabril, Spain.

Deeper geological repositories of low- and intermediate-level waste have also been built and are in operation. They will minimize further the risk of human intrusion, should the continuity of present institutional control be interrupted. Two typical such repositories, at a depth of about 80 m, are the Olkiluoto repository in Finland and the Forsmark repository in Sweden. The repository at Konrad, in Germany, is located in a pre-existing iron mine at the unusual depth of 1000 m.

Disposal of high-level waste, spent fuel, and alpha waste

Geological disposal at depths between 200 and 1000 m is the option normally reserved for waste producing higher levels of heat (HLW and spent fuel) and for long-lived waste (alpha waste). The deeper the disposal, the smaller is the danger of inadvertent human intrusion, the longer is the timescale for geological change, and the less the need for active institutional controls. For maximum passive safety of these deposits, a series of nested barriers is used to prevent the migration of radio-isotopes. These barriers consist of the waste form, the waste containers, earthen materials emplaced around the waste containers, materials sealing the disposal vault and shafts, and the host geological formation itself. Some of the waste containers developed so far have a projected life of several hundreds of thousands of years. Among the identified earthen barriers are bentonite clays, which would swell upon contact with groundwater, ensuring a high resistance to the migration of the waste substances as well as very limited groundwater flow in the proximity of the waste. The host formation would ensure the longest migration (and, therefore, decay) times and the largest volumetric dilution of the waste substances.

No final deep geological repository of HLW, spent fuel, and alpha waste is operating as yet. The most advanced national programmes are at the siting stages of these repositories and only the so-called WIPP facility for the disposal of alpha waste from the defence programmes of the United States is nearing its licensing phase. Following the completion of the conceptual design phase, attention is now being given in many countries to the construction and operation of underground research and development laboratories. These are meant to provide hands-on experience of how a deep repository can be excavated and managed confirmation, in situ of the performance of the safety barriers, and demonstration to interested parties of what a repository might look like. The view is taken that the implementation of deep geological disposal is necessarily an incremental process and that the timing of the implementation will depend not only on technical factors but also on public acceptability. The exact timing required to reach consensus will vary with each country's specific culture, and this will possibly take decades. One factor in favour of long implementation timescales is that, especially for HLW and spent fuel, there is no special urgency for the disposal of the accumulated waste, because the waste volumes are very limited and proven interim storage technologies are available. On the other hand, the disposal task cannot be delayed indefinitely for it behoves the generation that produces the waste to put a safe containment in place, or at least to provide future generations with the means and the technology for its disposal.

C. Pescatore