The following article will guide you about how to dispose radioactive wastes.
Disposal Practices for Gases:
Radioactive gases arise mainly in reactors, spent fuel processing, isotope production, and research and development facilities. The general principles are the same for all procedures that depend upon dispersion into the atmosphere.
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The maximum permissible emission rates—or in some cases the MPC at the stack mouth—is given in the regulations governing the plant or laboratory. It is then the responsibility of the operator to ensure that emissions are kept as far below the permissible level as may be practicable. Numerous methods are available, other than variation of stack height, for achieving this end (Fig. 7.3).
1. Filtration:
It is advisable to filter contaminated air near to the source of the activity. This reduces the amount of air to be filtered and also cuts down the “plating-out” of radionuclides on the duct-work, which can be a source of radiation fields within the plant.
Filters must be suitable for the job they are supposed to do. They should be made of nonflammable materials such as glass or asbestos fibre and should be tested before and after installation. If fine (e.g., “Absolute”) filters are used it is often necessary to precede them with a coarse filter to avoid rapid clogging with dust.
Filters must be very efficient to be adequate for fuel processing plants and incinerators burning highly active waste. For example, a sand filter capable of passing 10,000m3/min had an efficiency of more than 99.5%, but this was inadequate. The necessary efficiency of 99.99% attained with a bed of glass fibres 100 cm thick.
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2. Electrostatic Precipitators:
Small airborne particles are usually electrically charged. The charge can be increased by passing the air through a corona discharge, or through a charged fabric screen. The particles are attracted to a surface carrying the opposite charge, from which they can be removed mechanically. It is possible to use the same principle by imposing a charge on filters.
3. Steam Ejector Nozzles:
The most efficient air-cleaning device other than “Absolute” filters consists of a nozzle in which the air is mixed with steam and expelled into an expansion chamber where the steam condenses on the particles. After passing through a second constriction into another expansion chamber, where the air is scrubbed with water jets, the removal efficiency for 0.3 micron particles is 99.9%.
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4. Incinerator Off-Gases:
The hot gas from an incinerator carries with it fly ash, tars and water vapour as well as particles. Tars may be removed and the gases cooled by water scrubbing devices. Water droplets must then be eliminated by re-heating or passage through a “cyclone”. This is a cylinder with a conical bottom. Gas injected tangentially at the top sets up a vortex, which causes deposition of particles on the sides.
In smaller incinerators the gases are cooled and some fly ash is removed by passage through a cooling chamber fitted with baffles. After this stage a roughing or “bag” filter is used, followed if necessary by Absolute or charcoal filters.
5. Processing Plant Gases:
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The devices required for cleaning gaseous effluent depend on the nature of the process. Off-gas from boiling high level wastes must be passed through condensers and scrubbers to recover nitric acid as well as to remove volatile radionuclides. However, these and other air-cleaning equipment will not remove gases such as 85Kr, nor hold back all of the radioactive halogens.
Radioactive iodine in molecular form is fairly easily absorbed by alkaline scrubbers and copper or silver mesh filters, but in the form of methyl iodine it can only be arrested by an activated charcoal filter. These filters have to be kept cool, not only to remove the decay-heat of adsorbed halogens but also because 85Kr is absorbed much more powerfully by cold charcoal. This is the only practical means we have for removal of radioactive noble gases.
The very large dispersive capacity of a high stack usually makes it unnecessary to remove 14C (as 14CO2) or tritium (mainly 3H 1HO) because their toxicity is very low. However, the coolant CO2 in a gas- graphite reactor does contain enough 14C to require alkaline scrubbing, which removes radioiodine as well.
Disposal Practices for Liquids:
1. Storage:
The necessity for long-term storage of very large quantities (many millions of gallons) of high level, strongly acid waste has led to the development of tankage and pipeline systems which have stood up to severe conditions for many years. Failures have occurred, but good design and carefully selected materials have prevented environmental contamination.
Tanks are constructed from material, often stainless steel, which will not be corroded by the solutions to be stored. Secondary containment is provided by catch tanks or drip trays and sufficient spare tankage is kept available for rapid emptying of a ruptured tank. Leakage is detected by a monitoring system which alarms immediately if radioactive liquid appears in the catch tank (Fig. 7.4) Movement of active liquid is effected by pumping rather than by gravity to ensure that it is the result of deliberate action rather than accident.
2. Evaporation:
The most straightforward and apparently the simplest method of treatment for radioactive liquid wastes is evaporation. In a carefully designed evaporator with an efficient droplet de-entertainment system the radionuclide content of the distillate can be about one millionth of that in the pot. There is little about the design that is specifically related to radioactivity except that shielding may have to be provided for the operator, and off-gases must be monitored and possibly treated in some way.
Unfortunately, evaporation is expensive because it consumes a large amount of energy and the end product—the concentrate — is still a radioactive liquid waste. Evaporation to dryness or to the point of crystallisation has been practised, but the residue is so soluble in water that without further processing it is not suitable for disposal.
Where the discharge of a large volume of low-level waste into the environment is unacceptable, the cost of evaporation may be justified by its many advantages. Practically all liquid wastes are treated by evaporation in Denmark and Sweden, and it is also widely used in Japan. Residues from evaporation may be mixed with cement, fused with glass frit or various ceramic mixtures, or incorporated with melted bitumen. The product is then handled as a solid waste.
3. Flocculation and Precipitation:
The cheapest and simplest process for treatment of radioactive liquids is removal of the activity on some kind of precipitate, either as an integral part of the precipitated material, or adsorbed on its surface.
In most waste tanks sludge settles out which may contain upto 90% of the activity, and a copious precipitate of metallic hydroxides is formed on neutralisation, which may carry down upto 90% of the remainder. Further purification of the clear effluent after separation of these sludges can be achieved by addition of lime and sodium carbonate. Upto 99% of the remaining activity can sometimes be removed by this treatment. Treatment with lime and sodium phosphate is also very effective (Fig. 7.5).
The treatment used depends upon the particular radionuclides present in the waste, and also on its gross composition, e.g., the pH and salt content of the solution. In some cases ferric chloride, clay or other additives are introduced at carefully chosen points in the process. The selection of the process, and modifications introduced as the composition of the waste changes, require constant analysis and control by specialised chemists.
One problem common to all flocculation processes is how to deal with the sludge. The floc settles very slowly and after it has been drained through filters or separated by centrifugation, it is in the form of a thick cheese-like solid, which, in spite of its appearance, still contains 80 to 90% of water.
In a successful process the sludge is repeatedly frozen and thawed. The separation of pure ice crystals leaves behind a concentrated salt solution, which coagulates the small particles of floc into a form which settles more rapidly and is less likely to clog vacuum filters.
4. Ion Exchange:
The effluent from a flocculation process may still contain too much activity for discharge to public waters. It can then be passed through ion exchangers, which are expensive but very efficient. They cannot be used economically on a solution with a high salt content because their ion- exchange capacity would rapidly be exhausted by adsorbing the dissolved salts.
The effluent from a well-controlled flocculation process has a low total-solids content and after filtration to remove traces of floc, it can be passed through a cation exchanger or mixed-bed resin suitable for removal of the radioactive contaminants.
If properly chosen, such a resin will remove 99.9% of most radionuclides. Certain minerals—clinoptilolite, greensand and vermiculite are examples—are also efficient ion-exchangers (Fig. 7.6). They are much cheaper than synthetic resins but they require longer contact times for maximum effectiveness.
5. Glass:
The very high-level “self-heating” wastes—the primary wastes held in stainless steel tanks—are too active to be treated by flocculation or ion exchange. Storage in liquid form is seldom regarded as a permanent solution—somehow these wastes must be fixed in a non-leachable solid form which can be stored safely without danger of leakage or constant maintenance costs. One of the most promising ways to fix high level waste is to incorporate it into a glass.
Glass is a leach-resistant material which can be made from simple ingredients. Its quality varies with composition but is not usually sensitive to changes in minor constituents. Its slow melting point makes it convenient for casting in various shapes and sizes for different disposal procedures. Glasses are super-cooled, very viscous, liquid solutions of silicates. Soda glass is made by melting together silica, calcium carbonate and sodium carbonate.
Other varieties contain potassium or potassium plus lead instead of sodium, and phosphate or borate in place of part of the carbonate. Metallic oxides are incorporated to form coloured glasses. Such a mixture might well be suitable for fixing the radioactive metallic oxides which form the major proportion of “mixed fission products”, after the nitric acid has been removed and the residue ignited.
Successful fixation of radionuclides in glass has been reported from the USA, UK and Canada. British and American practice has concentrated on borate and silicate glasses, or fusing the waste oxides with glass frit, whereas the Canadians have used a natural silicate, nepheline syenite, instead of a glass mix (Fig. 7.7).
Glass fixation is now being done on quite a large scale. Careful studies of leaching, by sampling of the soil and ground water down-stream from the disposal, have shown that fission products equivalent to the dissolving of 10–10g of glass per cm2 per day are being removed from the disposal. Less than 1 mCi has been dissolved in ten years from 1100 Ci. This suggests that burial of active glass in dry soil, or even disposal into a big body of water, would be acceptable for quite large quantities of wastes.
6. Calcination:
Several methods have been developed for evaporation and subsequent calcination of wastes. Oxides are often soluble in water, so materials are usually added that will bind the oxides into soluble complexes. The calcination process is done in a heated steel container, a fluidised bed or a spray calciner. The pot calciner is essentially an expendable piece of steel pipe heated in an electric furnace. The waste, mixed with flass-forming fluxes such as borax or lead oxide, is heated to about 900°C.
The spray calciner is a heated steel cylinder with a nozzle at the top through which the waste is sprayed. At a temperature of 875°C a fine powder is produced which must be stored in a dry place as it is leachable by water.
It is characteristic of all waste fixation methods involving evaporation, sintering and fusion that elaborate off-gas treatment systems are required to prevent environmental contamination by dust and volatile radionuclides. The concentrating equipment itself is essentially simple and often not expensive to build, but the gas purification plant is always sophisticated, complex and expensive. However, it is also very effective.
7. Rock Fracturing:
The oil industry has developed methods for creating fissures in rock in order to encourage movement of oil or gas through a formation towards a well. This process has been adapted to disposal of medium-level wastes.
A horizontally bedded formation—shale has been used upto now—is drilled to several thousand feet. A high pressure jet of sand and water cuts through the well casing and penetrates between the strata near the bottom of the hole.
The well is then sealed and water forced down under very high pressure, splitting the rock between the bedding planes. The water is followed up by the waste, mixed with cement, sugar and other additives. The mixture spreads out in a thin horizontal sheet, which solidifies after several hours. Typically the sheet is about a half-inch thick.
The method has been used for disposal of a very large volume of waste. The equipment, including large bins for ingredients of the cement mix, mixing apparatus, a drilling rig and a very powerful pump is expensive, but the method is suitable for large-scale operation because successive sheets can be injected at intervals of a few feet through the depth of the bedded rock formation.
8. Salt Mines:
The hazard that must be met by most radioactive waste management systems is contamination of public waters leading directly or indirectly to intake of radionuclide by man. An ideal situation for disposal would, therefore, be one where public access was impossible and contact with water incredible. The nearest approach to these conditions is found in a deep salt mine.
The presence of the salt guarantees that water bas been absent for millions of years, and geological study can produce assurance that water is not rapidly penetrating into the salt bed. The excavated galleries of salt mines are large and stable tunnels, suitable for storage and roomy enough for safe work with active loads.
Disposal Practices for Solids:
As with liquid wastes, the most intractable problem is the safe management of the high- volume, low activity waste. The high activity waste is at first sight more dangerous, but although safe custody may be expensive it is not technically difficult.
Low-level waste consists mainly of “garbage”—contaminated clothing, equipment and structural material; broken glassware, clean-up materials such as cloths and mops; and a large amount of “potentially contaminated” material such as packing and paper which must be treated as active simply because it originates in an active area.
Much of this material can be reduced in volume by incineration or baling under high pressure. Fumes and smoke from incinerators and the dusty air from balling plants are cleaned up by methods dealt with under gases, but the ash and baled waste remain to be dealt with.
In some countries geographical or legal circumstances restrict the possibility of burial of radioactive material in the ground. Elsewhere, ground burial is regarded favourably. In the latter case bales and non-combustible waste are likely to be buried in sparsely populated regions. Where land is cheap, low-level wastes may be buried without any volume reducing process.
1. Conditioning:
Pre-treatment of waste before final disposal is called “conditioning”. The aim is usually immobilisation of radionuclides together with, if possible, volume reduction. There is very wide variation in practice from one country to another.
A very effective conditioning process is fixation in bitumen or asphalt. Bitumen is very resistant to radiation, has a low melting point, is impermeable to water and has some mechanical flexibility. Radionuclides are enclosed in, or even mixed with, bitumen leach very slowly into water.
Sludges are dewatered when mixed with melted bitumen, which helps considerably in restricting the volume of the disposals. In general, bitumen is beginning to be favoured over concrete as the method of choice for “fixing” otherwise mobile waste radionuclides.
2. Ground Disposal:
In some countries direct burial of contaminated material in the ground is forbidden at any level, whereas in others the amount and nature of ground disposals is left to the discretion of the operator.
Nearly all cations move through soil more slowly than the ground water although some anions— ruthenate and iodide, for example—are retarded very little. In the case of the average “mixed fission products” usually of concern in waste management, the fastest moving radionuclide is ruthenium, usually followed by Sr, Cs and Ce in that order.
Relative rates of movement are affected by the nature and pH of the soil and the ground water, but even in acidic sandy soil Sr moves through the soil at only 1/25 to 1/100 of the rate of movement of the ground water.
If the site of the waste management area is selected with care in relation to potable water supplies, so that the time of transit between the point of disposal and the point of human consumption is prolonged in relation to the half-life of the critical radionuclides, direct ground disposal of low level waste is effective and safe.
There are a great many places where knowledge of the rate and direction of movement of the ground water, together with the distribution coefficients of radionuclides between water and soil, make it apparent that no significant discharge into the environment would be credible as a result of direct disposal into the ground.
When simple burial is unacceptable, disposal trenches and areas can be drained, with processing of the drain-water, or the area can be covered with asphalt and protected from encroachment of ground water by circumferential drainage.
A further step in the direction of safety is the “engineered enclosure”. This is a structure built like a concrete house basement. It usually takes the form of a long concrete-lined trench divided into sections by concrete cross-walls. The section in use is covered by a temporary roof.
The object of the structure is to prevent the ingress of water, so joints in the concrete are water-sealed and the base of the work is laid well above the maximum height of the water table. Since the facility will be used for reception of quite high-level waste the approaches must be suitable for trucks and mobile cranes.
Concrete trenches are unsuitable for reception of small, intensely radioactive objects such as spent teletherapy sources because of the inconvenience of scattered radiation fields. They can be accommodated in concrete-lined holes fitted with removable shielding plugs. Canadian practice is to construct these from sections of concrete drainpipe, painted on the outside with bitumen, which is also used to seal joints between sections (Fig. 7.8).
There are many different versions of the types of disposal facility just described. Some are in the open, some within buildings, but all are designed to prevent access of water to the contents. It is often convenient to delay the passage of radionuclides contained in high-volume low-level wastes before discharge into the environment in order to take advantage of radioactive decay. If the local soil and ground water regime are suitable this can often be done by discharge into seepage pits.
Disposable Methods for “Small Users”:
The International Atomic Energy Agency has issued a code of practice on management of radioactive wastes by hospitals, research institutes and industry when no special facilities are available on the site. It gives a review of the scope and nature of the necessary control, particularly in the establishment of permissible limits for discharge into the environment.
These institutions rely heavily upon the public sewers and garbage disposal systems, depending upon the fact that in practice there are levels of radioactivity below which things are not regarded as radioactive. Sometimes this is the level at which measurement becomes practicable and sometimes legal limits exist.
The ICRP and the IAEA both recognise 10–4 µCi/ml as the concentration in the sewer of an institution below which no restrictive action is required, irrespective of the nature of the radionuclide. This assumes that since large dilutions will occur before ingestion by the public, the individual at risk is the sewer worker. Plumbers can also encounter hazards in traps and filters, and they must be made aware of the situation.
Similarly, although very low-level discharges to public disposal areas are usually acceptable, the waste management authorities must know of the practice so that they can warn their staff or undertake special procedures such as tip-and-fill operations. It is particularly important that scavenging should be prevented, because a very small source, normally innocuous, can be hazardous if carried for a long time in a pocket.
Apart from the use of public facilities, radioactive waste disposal for the “small user” does not differ in principle from the methods available to the larger producer of waste. A good deal of common sense and sense of proportion are required in dealing with the problem, aided by technical advice such as that in the IAEA report. It is also useful to remember that an ordinary illuminated wristwatch gives a count of several thousand per minute on a Geiger counter from the face side and almost zero from the back.
Sea Disposal of Radioactive Wastes:
The public news media are convinced that vast quantities of radioactive wastes are dumped into the sea, particularly by the nuclear power industry. It is difficult to understand how this misunderstanding arose, because except in very special circumstances sea disposal is prohibitively expensive.
The British discharge fairly low-level liquids via long pipelines into the Irish Sea off the Wind scale and into the English Channel off Winfrith, and the European Nuclear Energy Agency has made two experimental disposals of packaged solid wastes by ship into the deep waters of the Atlantic. American disposal operations on the Pacific and Atlantic coasts, now abandoned, have been confined to low-level garbage-type wastes. The practice was given up because it was too expensive.
None of these operations has been concerned with more than a few tens of thousands of Curies in any one year, the monitoring has been very good and the potential hazards. Contrasting with this situation is the fact that tens of millions of Curies of strontium-90 and similar amounts of other fission products have fallen into the oceans of the world from nuclear weapons fall-out. Yet this man-made radioactivity in sea water is difficult to detect against the background of natural potassium – 40.