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Management and disposal of waste from sites contaminated by radioactivity
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Radiat. Phys. Chem. Vol. 51, No. 4±6, pp. 579±587, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0969-806X(97)00205-3 0969-806X/98 $19.00 + 0.00
MANAGEMENT AND DISPOSAL OF WASTE FROM SITES CONTAMINATED BY RADIOACTIVITY CARLYLE J. ROBERTS Dames & Moore, Principal, 3065 Southwestern Blvd., Suite 202, Orchard Park, Bualo, New York, NY 14127-1286, U.S.A. AbstractÐVarious methods of managing and disposing of wastes generated by decontamination and decommissioning (D & D) activities are described. This review of current waste management practices includes a description of waste minimization and volume reduction techniques and their applicability to various categories of radwaste. The importance of the physical properties of the radiation and radioactivity in determining the methodology of choice throughout the D & D process is stressed. The subject is introduced by a survey of the common types of radioactive contamination that must be managed and the more important hazards associated with each type. Comparisons are made among high level, transuranic, low level, and radioactive mixed waste, and technologically-enhanced, naturallyoccurring radioactive material (TENORM). The development of appropriate clean-up criteria for each category of contaminated waste is described with the aid of examples drawn from actual practice. This includes a discussion of the application of pathway analysis to the derivation of residual radioactive material guidelines. The choice between interim storage and permanent disposal of radioactive wastes is addressed. Approaches to permanent disposal of each category of radioactive waste are described and illustrated with examples of facilities that have been constructed or are planned for implementation in the near future. Actual experience at older, existing, low-level waste disposal facilities is discussed brie¯y. # 1998 Elsevier Science Ltd. All rights reserved
INTRODUCTION
implemented in order to reduce these risks to acceptable levels. It is intended for the non-specialist and does not assume more than a basic understanding of radiation and radioactivity.
In many countries of the world, bene®cial uses of nuclear technology, including radiation and radioactivity, have been discouraged by widespread concern among private citizens and governmental agencies about potential, uncontrolled releases of radioactive waste materials into the environment. Regardless of how well intentioned, peaceful applications of nuclear technology inevitably produce by-product materials containing radioactivity at concentrations that exceed those that occur naturally. This is true for radioisotope applications in research, medicine, and industrial processing, for nuclear power production, and for the operation of certain types of radiation-producing machines. In addition, commercial processing of naturally radioactive minerals can produce waste that requires isolation from the environment. As a result, there is a world-wide eort to develop acceptable methods to manage and ultimately and safely dispose of all forms of radioactive waste (radwaste). To some extent this is a technical challenge, but it also requires a substantial communications eort to explain to a sceptical public what the potential risks of placing radioactive waste in the environment actually are and what is being done to ensure that the risks remain acceptably low. This paper presents an overview of the risks from disposal of radioactive waste and the various approaches being considered and, in some cases,
ORIGIN OF RADIOACTIVE CONTAMINANTS
Radioactive materials can be classi®ed very broadly as either manmade or natural. NaturallyOccurring Radioactive Material (referred to as NORM) is broadly distributed throughout the environment. It is present on earth either because the radionuclides have extremely long half-lives (relative to the age of the earth) or they are constantly being produced by natural processes. These natural processes include cosmic ray interactions with the atmosphere and transformation or ``decay'' of radionuclides in a series or chain which is maintained by a long-lived ``parent.'' Examples of the ®rst category (``primordial'' radioisotopes) are uranium-238 (4.5 billion year half-life) and thorium232 (14 billion year half-life). Examples of the second category include tritium (H-3) and radiocarbon (C-14), produced continuously by cosmic rays, and radium-226 and radon-222, which occur in a chain of fourteen decay events beginning with uranium238 and ending with stable (non-radioactive) lead206. When industrial processes such as mining and milling signi®cantly increase the concentration of NORM in a bulk material or its availability for 579
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dispersion in the human environment, it may then require special precautions in handling and disposal. Manmade radioactive material is generated by a variety of human activities. By far the largest quantities of radioactivity have been created by nuclear reactors operated either for electric power generation or weapons production (UNSCEAR, 1993). Radioactivity from reactors includes ®ssion products, activation products, and transuranic nuclides. Other radioisotopes for use in research and medicine are produced by particle accelerators. To some extent, all of these manmade radioisotopes contribute to the generation of radioactive waste, often as a result of contaminating other materials, e.g., soil, building materials, protective clothing, body ¯uids. TYPES OF RADIOACTIVE WASTE
The term ``radioactive waste'' (or ``radwaste'') covers a wide range of physical and chemical forms and concentrations of radioactivity. While de®nitions of various waste classes vary from nation to nation, radioactive waste is often, if not universally, categorized as either high-level waste (HLW), lowlevel waste (LLW) or transuranic (TRU) waste (LWV, 1993). In general, a designation as high-level waste is applied to the highly radioactive waste generated from reprocessing spent nuclear reactor fuel to separate re-usable uranium and plutonium from the ®ssion products and the spent fuel itself, if it is not to be reprocessed. Both forms of HLW require massive shielding, because of the intense gamma radiation they emitÐ as well as some form of cooling to remove the heat of radioactive decay. These requirements become less stringent within 100 to 200 years because of the relatively short half-life of several important nuclides (notably, strontium-90 and cesium-137). Reprocessing plant waste is generated in liquid form and, therefore, must be solidi®ed before it can be shipped safely or disposed of permanently with con®dence that it will remain isolated from the environment (primarily the groundwater) for a very long time. Several countries have advanced solidi®cation programs and some already have begun to vitrify reprocessing waste in the form of borosilicate glass. Spent fuel will require special ``packaging'' before disposal in the same facilities that will accept solidi®ed reprocessing waste. At this time, permanent disposal in a deep geologic repository is the option being pursued most aggressively; however, the opening of a permanent repository to accept HLW still appears to be many years in the future. Currently, the only alternative HLW disposal method being considered seriously is emplacement beneath deep sea sediments. Although this approach appears technically feasible, there is a moratorium on its use until more information on
the potential risks has been gathered and a regulatory system is implemented by the international community (NEA/OECD, 1996). Transuranic (TRU) waste is a result primarily of nuclear fuel fabrication, reprocessing, and nuclear weapons production. In the United States, TRU is de®ned as waste containing more than 3,700 Bq gÿ1 of alpha-emitting nuclides with atomic numbers greater than 92 and half-lives greater than 5 (or 20) years. Most TRU waste emits much less penetrating radiation and generates less heat than the ®ssion products in HLW. However, as a result of the extremely long half-life and high radiotoxicity of many of the transuranic nuclides, TRU waste usually requires isolation from the environment comparable to HLW. On the other hand, TRU usually is contact-handleable, meaning that it can be handled without heavy shielding. Low-level waste is produced by a broader spectrum of human activities than is the case for HLW or TRU and it contains a much wider variety of radioisotopes. In most cases low-level waste does not require shielding, although a 55-gallon drum containing cesium-137 close to the U. S. concentration limit for LLW (the Class C limit) would have a surface exposure rate of approximately 6 Sv hÿ1. Containers having dose rates greater than about 1 mSv hÿ1 at contact would be shielded or handled remotely. Most containers of LLW have surface dose rates of less than 0.1 mSv hÿ1 and require no special precautions during the relatively short time they must actually be handled. About one-half of the commercial LLW volume and a much larger fraction of the total radioactivity currently being generated in many developed countries originates in commercial nuclear power plants. It consists of activation products such as iron-55, cobalt-60, nickel-63, and ®ssion products such as antimony-125 and cesium-137. Most of this is in the form of compacted trash (paper, rags, plastic gloves, anti-contamination clothing), and contaminated bulk materials (concrete, steel, wood). There also are smaller volumes of more concentrated radioactive wastes such as spent ion-exchange resins and evaporator ``bottoms.'' Roughly 10% of the LLW volume is from institutional sources such as hospitals, medical schools, research laboratories, and universities. This LLW is in the form of laboratory glassware, trash, and animal carcasses, and a large fraction is in scintillation ¯uids. With the notable exceptions of tritium (H-3) and carbon-14, many of the radionuclides used in medicine and research have half-lives measured in days or less. The balance of the LLW volume, about 40%, is produced by industrial users. The relatively low speci®c activity, very large volume residuals produced by processing NORM represent yet another category of radioactive waste. An example is uranium mill ``tailings.'' In the U.S. alone this activity has produced about 150 million
Management and disposal of waste
tons of tailings containing 10 to 100 Bq gÿ1 of Th230 and Ra-226. Although the process of extracting uranium does not concentrate the remaining radionuclides in the tailings, the radioactivity is more easily dispersed to the environment than when it was associated with the original ore body beneath the surface of the ground. A second example of NORM-containing waste is the phosphogypsum byproduct of fertilizer production from sedimentary phosphate rock containing about 1 Bq gÿ1 Ra-226. The annual production of gypsum in Florida (USA) is 30 million tons, about one-third of the total worldwide production. Processing of by-product heavy minerals (amang) from tin mining in Southeast Asia and Australia also results in large volumes of residues and contaminated waste. The amang feed materials, derived from tin ore, contain concentrations of uranium and thorium up to four orders of magnitude greater than average background levels. If a radioactive waste also contains constituents that are considered chemically toxic and/or hazardous, the waste is classi®ed as a ``mixed'' waste. Liquid HLW may be mixed waste because of heavy metal content or extreme pH. Slightly contaminated lead shielding also would be mixed waste because of the toxicity of lead. Mixed wastes are even more dicult than radioactive wastes to de®ne precisely because there are many dierent de®nitions of hazardous materials. The treatment and disposal of hazardous waste is regulated under entirely dierent statutes than those that apply to radioactive wastes, and the requirements are sometimes contradictory and often not optimal for each type. On the brighter side, it may be possible to treat mixed waste so that the hazardous characteristics are eliminated, after which it may be managed solely as a radioactive waste. WASTE TREATMENT AND MINIMIZATION
During the past decade the cost of radioactive waste disposal has risen sharply, largely because of increased requirements for isolating the waste from the environment and, in some cases, to maintain the option of retrieving the waste should the integrity of the disposal facility appear to be compromised. This has placed greater emphasis on the development of methods to minimize waste volume. At least one major LLW disposal site in the U.S. now requires, as one of its waste acceptance criteria, that the waste be volume-reduced to the maximum extent feasible. The simplest approach, of course, is decay-in-storage. For short-lived nuclides this results in an industrial waste that can be disposed of relatively easily and inexpensively. Waste volume-reduction methods in common use for solids include compaction, incineration, dewatering of resins, metal sectioning, and smelting, as well as a variety of decontamination methods that
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simply remove the surface layers on which radioactivity has been deposited (Murray, 1994). Compactable wastes such as plastic sheeting (herculite), metal pipe, and glassware typically can be volume-reduced by a factor of 6. Incineration can reduce waste volume by a factor of 80 or more. Materials including wood, plastics, anti-contamination clothing, oil, and organic solvents can be reduced to a small volume of ash. However, in some cases the incinerator operator will require the waste generator to take back the residues for ®nal disposition. Smelting, which concentrates the radioactivity in the slag, is the least ecient method in common use today: volume is typically reduced at a ratio of about 2:1. The extent to which metal from the smelting operation can be recycled depends on many factors, including the concentration of the original contaminants, the degree of separation achieved, and the applicable regulations. A pilot program is now under way in the U.S. to evaluate recycling of mildly contaminated steel by fabricating it into radioactive waste containers (barrels and boxes). To reduce the volume requiring disposal, bulky radioactive waste with low speci®c activity, such as contaminated soil, often can be treated eectively by soil-washing techniques. In some cases, simple wet screening will separate the ®ne particles containing most of the radionuclides from the larger particles that meet the clean-up criteria and constitute most of the waste volume. Liquid wastes often are volume-reduced by evaporation or decontaminated by sorption (ion exchange) methods. Although such treatment may not reduce their volume, liquid and slurry wastes often are solidi®ed by cementation or vitri®cation. As a solid, the waste can be transported with greater safety and disposed of with greater assurance that the integrity of the facility will not be jeopardized by waste slumping or consolidation after emplacement. In some cases, the hazardous characteristics of mixed high- and low-level liquid wastes can be removed by solidi®cation techniques, e.g., by reducing the leachability of heavy metal constituents. RADIOACTIVE WASTE CHARACTERISTICS THAT INFLUENCE DISPOSAL METHODS
The primary considerations in evaluating the potential impacts on the public and the environment from disposing of radioactive wastes derive from the presence of the longer-lived radionuclides. Immediately following closure of a permanent disposal facility and during the period of institutional control, which often is assumed to continue after closure for up to 100 years, environmental surveillance and active maintenance, if required, would occur. For this reason, a signi®cant o-site release of radioactivity is very unlikely. Therefore, the
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nuclides of primary concern to the designer of a permanent disposal facility are those that would retain appreciable activity more than 100 years after disposal. Among the longer-lived species, the relative toxicity varies over a wide range depending on such factors as chemical form and solubility in body ¯uids, physical state (e.g., particle size if in aerosol form), and the type and energy of emitted radiation. This variability is demonstrated in Table 1 which lists the eective dose equivalent resulting from an intake of one becquerel via ingestion and by inhalation. The table also shows the external dose rate produced by uniform ground surface contamination by each of the listed radionuclides. Note that Sr-90 and Cs-137 will decay to innocuous levels within about 300 years, but the remaining nuclides listed in Table 1 will persist in the environment for thousands of years. As one would predict because they emit alpha radiation, radium, uranium, and transuranics, Pu239 and Am-241 deliver higher dose equivalents relative to beta/gamma emitters per becquerel taken into the body, whether the route of entry is inhalation or ingestion. In contrast, the gamma emitters, represented in Table 1 by Cs-137 and Ra-226 progeny, deliver a much higher external whole-body dose than pure beta emitters such as Sr-90 or the alpha emitters for a given concentration on the ground or dispersed in air or water in which a person is immersed. In addition to the parameters of half-life and radiotoxicity listed in Table 1, there are other properties of radionuclides that are important in determining their potential impact if released from a disposal site. As noted below, an important potential pathway for dispersal is via ground or surface water. The rate at which a radioactive species dissolves in ground water is measured by its leachability or leaching index. The rate of dissolution is a complicated function of many parameters, including the chemical form of the nuclide, the nature of the
waste form (cement, bitumen, activated metal), the composition of the ground water, and whether contact with water is constant or intermittent. Because of the large number of important variables that determine leach rates, generally applicable values are not readily available and reliable estimation of these quantities is one of the more challenging aspects of predicting the performance of disposal facilities. Fortunately, there are data in the literature that enable modelers to choose conservative leach rates and place reasonable upper bounds on the potential for ground water contamination. Another important parameter that determines the rate at which radioactivity may migrate away from a disposal site is the distribution coecient (Kd). The Kd is a measure of the various sorption interactions between water and a geologic media that cause the radionuclides to migrate at a slower rate than the ground water itself. Numerically, the Kd is the ratio of the concentration of a nuclide in the solid (soil) phase to the concentration in water. Typical values for long-lived nuclides also are listed in Table 1. It is clear that the long-lived alpha emitters interact strongly with soil (i.e., they have large Kd's) and, therefore, their migration in ground water is much slower than elements such as carbon, iodine, and technetium, which are subject to essentially no retardation. POTENTIAL ENVIRONMENTAL PATHWAYS TO MAN
Releases of radioactivity from disposal sites conceivably could occur at any stage in the life cycle of a facility, beginning with the operational phase when waste is being received, stored, processed (e.g., volume reduced), and emplaced, through closure of the site, and then through the institutional control period and beyond. There are a variety of possible scenarios involving accidents, equipment failures, and human errors in judgment that could occur during the operational phase and could result
Table 1. Relative radiotoxicity and related characterizations of common nuclides a
EDE per unit intake relative to Pu-239*
Nuclide H-3 C-14 Sr-90 Tc-99 I-129 Cs-137 Ra-226 Np-237 U-238 Pu-239 Am±241
Half life (years) 12.3 5730 29.1 2.13 1.57 30.0 1600 2.14 4.47 2.41 432
E+5 E+7 E+6 E+9 E+4
Inhalation
Ingestion
0.00% 0.00% 0.30% 0.00% 0.04% 0.01% 2.00% 125.86% 27.59% 100.00% 103.45%
0.00% 0.06% 4.03% 0.04% 7.80% 1.41% 37.45% 125.52% 7.20% 100.00% 102.93%
External dose rate Distribution coecient relative to Ra-226** or Kd (ml g-1)*** 0.00% 0.00% 0.00% 0.00% 0.20% 32.36% 100.00% 10.40% 0.81% 0.00% 0.30%
0±1 1±5 5 ± 30 0.1 ± 1 1 ± 60 40 ± 500 70 ± 220 0.1 ± 50 35 ± 50 500±2000 300±1900
*For Pu-239 unit dose factors (Sv/Bq)are: inhalation - 1.16 E ÿ 04; ingestion Ð 9.56 E ÿ 07Ref: Appendix E, NUREG/CR-5512, 1992, October. **For Ra-226, unit dose factor (mSv/y per Bq/cc) is 2.31 for an in®nite volume source and a density of 1.8 g/ccRef: A.1, ANL/EAD/LD2, 1993, September. ***Typical, average distribution coecients for soils and clay, see NUREG/CR-5512.
Management and disposal of waste
in acute or short-term releases. Such incidents would not involve any unique pathways, however. Of greater concern are the potential causes of what, in the absence of eective monitoring and surveillance, could become chronic, long-term releases. Regardless of the cause, the radioactivity that escapes the con®nes of a disposal facility will be subjected to a complex variety of physical, chemical, and biological processes. Most of these processes will tend to disperse or dilute the activity, but some will concentrate it. Environmental pathways may be broadly categorized as airborne or liquid (i.e., water). The critical pathway for mobilization of radioactivity from disposal facilities usually is surface or ground water. All LLW disposal facilities are designed to prevent or at least minimize contact between the waste mass and water. Therefore, in order to introduce radioactivity into water, there must be a failure of the protective barriers. Should this occur, there are two obvious routes for the contaminated water to take, depending on the geohydrologic characteristics of the site. Flow may be into the ground, with contamination of an aquifer as a result. This could provide a direct pathway to man by ingestion of drinking water from a well or by an indirect path through ingestion of the water by cows or beef cattle or by root uptake into edible plants. If the radioactivity is introduced into surface water, additional pathways are opened. Direct external exposure from immersion (i.e., swimming) or simply standing by a stream bank is possible although these are not likely to be signi®cant pathways. Of greater concern are pathways from irrigation water to food crops, from direct ingestion by man, or from use in watering cattle. Another potentially critical pathway, because of the possibility of biological concentration, is uptake by ®sh and other edible aquatic forms. Overland ¯ow of contaminated water also is a concern because it would contaminate surface soil which then could be resuspended in air and inhaled. Gases from decaying organic matter also may provide a route for the escape of certain radionuclides (e.g., H-3 and C-14) from LLW disposal facilities. This pathway may be precluded by prohibiting the disposal of putrescible wastes. The relative importance of the many possible pathways depends upon the actual or assumed failure modes of a given disposal facility. Contamination of groundwater ®rst requires a mechanism to breach the con®nement barriers. In the case of conventional shallow land burial, the trench cap might be breached as a result of subsidence of the waste mass, by cracking as a result of climatic cycling, by being penetrated by an intruder, or a variety of other causes. If precipitation enters through a failed cap and leaches activity from the waste mass, the activity could follow groundwater pathways described above. If the underlying geolo-
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gic media has very low hydraulic conductivity, the result might be ``bathtubbing'' or over¯owing of leachate onto the ground surface rather than migration into an aquifer. ROLE OF INTRUDERS
In addition to protecting the public from o-site releases of radioactivity, a second design objective for a disposal facility is to protect individuals from inadvertently intruding into the waste site after institutional controls are removed. This objective provides the basis for the concentration limits that de®ne the three classes of LLW that are recognized in the U. S.. These concentration limits, in turn, are designed to ensure that an inadvertent intruder would not receive a dose greater then 5 mSv in one year. During the design process, a variety of socalled ``intruder scenarios'' are examined and a site speci®c analysis is conducted to predict possible impacts that could occur during the post-institutional control period. Usually the critical or controlling intruder scenario is the ``resident farmer'' whose family potentially could be exposed for extended periods of time. This agricultural scenario could include the full range of exposure pathways, assuming that credible events (e.g., failure of a trench cap) are identi®ed that can lead to ground and surface water contamination. For example, trench over¯ow could contaminate fertile soil in which crops were grown. Cultivation of this soil would resuspend the radioactivity and allow it to be inhaled. Contamination of a well water supply and/or a farm pond opens up additional direct and indirect pathways mentioned above. Current disposal practices should meet speci®c technical requirements that account for natural site characteristics, facility design (including closure), waste properties and packaging, and institutional controls such as monitoring and control of site access. The objective of these controls is to prevent the radioactivity contained in the waste from entering any of the pathways to man as a result of natural or spontaneous events or human intervention. RADIOACTIVE WASTE DISPOSAL METHODS
There are two basic methods for disposing of radioactive wasteÐdispersion into the environment or isolation from the environment. In general, the former alternative is not used except in the case of liquid waste euent from hospitals and clinics. These facilities, and other facilities that house patients who have received radioisotopes for diagnostic or therapeutic treatment, discharge their sanitary waste directly to the local sewer system. Other situations that might justify choosing this option could arise if the radionuclide were easily dispersable, such as a noble gas (e.g., krypton-85), tritiated
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water vapor (containing hydrogen-3), or carbon-14 dioxide. In accordance with the general principle that no radiation doses should be permitted unless there is a concomitant gain, any proposal for disposal by dispersal should receive a careful cost/bene®t analysis before a decision is made. A decision to proceed would require a demonstration of a clear, net, societal gain. A net bene®t might occur because, for example, the potential collective dose to workers engaged in a conventional waste packaging operation exceeded the collective dose to the nearby population from releasing the waste directly to the environment. Because radioactivity can be measured at very low concentrations in media such as air and water, it is possible to con®rm the acceptability of disposal by dispersal by appropriate environmental monitoring, albeit only after this method of disposal has been initiated. The ®nal stage in disposing of a low-level liquid waste by conventional methods such as evaporation and ion exchange removes most radionuclides from the aqueous phase. Following this, the radioactivity is solidi®ed further, if necessary, and then disposed of by shallow-land burial or a more sophisticated method that provided the required isolation from the environment. However, if the decontaminated euent from such treatment contained tritium, conventional methods would not lower its concentration. While methods for removing tritium from water do exist, their application is quite expensive. Therefore, if the tritium concentration were suciently low, the aqueous waste might become a candidate for disposal by slow release to a diluting body of water. The disposal of radioactive krypton-85 (10.7 year half-life) by release to the atmosphere for dispersion and dilution oers another illustration of this approach to disposal. Krypton is used routinely to test hermetically sealed systems to ensure that they are leak tight. Because krypton is a noble gas with a half-life that is too long for decay-in-storage to be practical, the preferred option is to release the gas through a vent stack at an intermittent, carefully controlled rate. Conservative air dispersion calculations are noted for two manufacturing sites in the northeastern U.S. using measured annual average meteorological parameters. The results demonstrated that 10 to 100 curies of Kr-85 could be released safely over a period of several weeks without exceeding regulatory limits. Eective dose equivalents to a maximally exposed o-site individual were on the order of 0.001 microSv, with skin doses of about 0.01 microSv. The more common general method of disposing of radioactive waste, and the approach more likely to be accepted by the general public, is to con®ne it in such a way that its isolation from the environment is assured with an acceptable degree of con®dence (Berlin and Stanton, 1989). This is seldom an issue for radionuclides with half-lives of 30 years or
less. This includes most of the most of the higher yield ®ssion products such as strontium-90 and cesium-137. While these nuclides are present in relatively high concentrations in nuclear reactor wastes, they will be reduced in quantity by a factor of 10 in 100 years and by 100 in 200 years. This fact reduces the importance of maintaining the integrity of the disposal facility for long periods of time. In contrast, facilities that are designed to isolate NORM wastes containing U-238 or Th-232 or reactor wastes with long-lived transuranic constituents must include features that ensure stability for 1,000 to 10,000 years or more. Although numerous nuclides of concern have half-lives that are orders of magnitude longer than 10,000 years, pathway analyses often demonstrate that the peak dose to the public would occur within 10,000 or, at most, 100,000 years. This provides a degree of comfort with very long-term predictions of radiological impacts even though it is generally conceded that beyond about 10,000 years it is essentially impossible to predict geological or climatic events with any reasonable degree of certainty. Also, it is reassuring to note that in less than 10,000 years the radioactivity in HLW will decay to a smaller quantity than is present in uranium ore equivalent to the amount that was required to produce the reprocessed nuclear fuel in the ®rst place (NEA/OECD, 1996). High-level waste disposal For high-level waste (HLW) from nuclear fuel reprocessing and for spent reactor fuel, the only option currently being pursued is con®nement in a deep geologic repository. While considerable attention is paid to the design and performance of the waste encapsulation, this is primarily to provide protection during shipment and emplacement, and for a period of several hundred years until most of the radioactivity has decayed away. Until then, all operations must be carried out by remote handling behind heavy shielding. The HLW will not be placed in the repositories until the shorter-lived radionuclides have decayed and the rate of heat generation has dropped to a level that does not require highly ecient, guaranteed cooling capability. The more problematic concerns about HLW disposal arise because of the extremely long half-life of some components of the waste (for example, plutonium-239 at 2.4E + 4 years, neptunium-237 at 2.1E + 6 years, and iodine-129 at 1.6E + 7 years). In this time frame, continued isolation can be provided only by the geologic media itself. Thus, the primary concern is to ®nd a deep, extremely stable formation where the waste is very unlikely to encounter groundwater. This requirement is linked to the related needs to site the repository in a geologic setting free of seismic activity and removed from the threat of serious erosion. Because leaching by intruding water is the primary potential mechanism for transporting waste
Management and disposal of waste
radioactivity into the human environment, the leachability of the waste form also is an issue. In several nations, liquid HLW currently is being vitri®ed to produce a very stable solid form suitable for shipment and disposal in a geologic repository. HLW repositories are in various stages of development in several dierent media. Media being investigated for suitability include clay (Belgium), granite (Canada and Sweden), salt (Germany and U.S.), sedimentary rock (Japan), and tu (U.K. and U.S.). (NEA/OECD, 1996) In many cases, the depths being studied range from 400 meters to more than 1,000 meters. Great depth oers the obvious advantage of a very long pathway through which radioactivity must be transported before it can expose the public. Less apparent is the protection these depths oer to a potential inadvertent intruder who is exploring for mineral resources. TRU and medium-level waste disposal TRU and medium-level waste do not require the very high degree of isolation that high-level waste does but, in the case of TRU waste, con®nement must be maintained for an equally long period. Much of this waste, particularly alpha-emitting waste, does not require remote handling. Also, somewhat less importance is placed on waste form integrity and resistance to leaching. Many of these solid wastes receive no pre-treatment other than volume reduction. For shorter lived, medium-level wastes, engineered, near-surface facilities have been developed in several countries, most notably in France. This concept involves placing the stabilized waste containers in shallow, concrete lined vaults. The packages are then grouted in place and covered with additional concrete to prevent human intrusion and in®ltration by precipitation. In some cases, additional low-level waste containers may be placed on top, and the entire mound is enclosed in a clay cap with provisions to divert any water that does in®ltrate through the cap. In Germany, short-lived medium-level wastes have been placed in salt mines, and in Sweden and Finland cavities in bedrock have been used for this purpose. In general, repositories for medium-level wastes are not as deep as those being considered for HLW disposal. The choice of disposal methodology involves obvious economic considerations. Where suitable mined cavities already exist and mineral resources have been exhausted, underground disposal may easily be justi®ed, even if the radiological waste characteristics do not require such a high degree of isolation. A requirement for retrievability will have a major impact on the design of the facility and add substantially to the cost, whether a near-surface or deep geologic repository is chosen.
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Low-level waste disposal With the exception of low-level waste (LLW) containing naturally occurring radioactive material (NORM), most LLW is relatively short-lived whether it is generated by power reactor operation, medical and research applications, or industrial uses. As a result, the requirements for prolonged and assured con®nement are not nearly as stringent as they are for HLW disposal. The method of choice usually is a form of near-surface, or shallowland, disposal. Experience has demonstrated that simply digging a trench and haphazardly dumping the waste into the cavity and then covering it with soil does not adequately protect the public. This experience has shown that it is important to consider waste form stability and emplacement technique in order to avoid excessive voids from developing after closure. The danger is that compaction of the waste mass in just a few years can result in signi®cant failure of the cap and allow precipitation to gain access to the waste mass. This, in turn, can result in excessive leaching to groundwater if the underlying strata are suciently porous or, if the soil is highly impermeable, leachate can over¯ow to the surface and run overland (the so-called ``bathtub eect''). A variety of designs have been developed in recent years to decrease the possibility of either of these two events happening. The requirements for waste form stability have been increased relative to the amount of radioactivity to be placed in the disposal facility. When justi®ed, concrete vaults have been used to reduce the likelihood of in®ltration in the near term and/or to facilitate retrievability, if this is required. For waste that contains substantial quantities of long-lived activity, standard packaging such as steel boxes and drums and concrete pads and vaults oer no advantages. If isolation is required over a time frame reaching beyond a few hundred years, it usually is assumed that the integrity of steel containers and concrete will be lost. In such cases, geologic media such as clays often are preferred for constructing impermeable covers. Assuming that proper precautions have been taken to avoid settlement of the waste mass, the geologic cover is less prone than concrete to failure that would allow signi®cant amounts of water to penetrate. Disposal of low speci®c activity NORM waste Mineral residues from large-scale industrial processes such as uranium mining and milling, fertilizer production from phosphate rock, and reprocessing of tin byproduct (amang) present special problems because of their very large volumes. Many of these wastes are of suciently low speci®c activity that they are not a serious occupational hazard because of external radiation. At U-238 or Th-232 concentrations above approximately 1 Bq gÿ1, external gamma irradiation is
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of concern for workers if the radioactive progeny in the decay series are approximately in equilibrium. A more serious threat to workers who are required to handle large quantities of these materials under dry conditions is inhalation of airborne particulates. To a lesser extent, inhalation of radon daughter products may also be a signi®cant source of internal radiation. However, this is seldom a problem when work is performed outdoors so that radon progeny do not accumulate as they would indoors. Because of the very long half-life of the decay series' parents, U-238 and Th-232, the cost of a concrete structure for disposing of low speci®c activity, NORM waste is seldom, if ever, justi®ed. The concrete cannot be assumed to maintain its integrity for as long as the parent nuclides will support the radioactivity of the daughter products in the waste (literally billions of years). Therefore, a more stable earthen structure is preferred. Moreover, superior shielding properties are not generally required for these wastes. The o-site public can be protected more than adequately from direct radiation by a relatively thin covering of uncontaminated soil above the waste. A thin clean covering also will prevent the dispersal of radioactive particulates from the surface of the waste. A more critical need is to attenuate the exhalation of radon222, the isotope derived from uranium-238 via radium-226. In many cases, the dominant pathway for exposure of o-site individuals is from radon. The radon-220 from the thorium-232 series has such a short half-life (56 seconds) that it is not a concern o-site (or on-site). If properly installed, a clay cap of approximately 1 meter in thickness will provide ample protection against radon for persons residing near the disposal facility. A major pathway that could result in exposure of the population in the environs of a NORM disposal site is migration through the groundwater. This is of special signi®cance in humid climates with abundant rainfall. In such an environment, precautions must be taken when designing and constructing the facility to minimize in®ltration of precipitation into the waste. In addition, the site should provide relatively impermeable strata beneath the waste mass so that vertical and horizontal movement of any water that does contact the waste is restricted. In addition, the geologic media underlying the site should, ideally, be capable of retarding the movement of solutes in the leachate by various mechanisms (e.g., adsorption, ion exchange). In many cases, the most troublesome nuclides are the radium isotopes, primarily Ra-226 and Ra-228. If the geologic environment does not provide sucient retardation (as measured by the distribution coecient or Kd), it can be augmented by adding suitable materials to the waste mass itself or by placing a geochemical barrier at the bottom of the waste but above the native soil. This barrier could be simply another layer of clay with a high Kd.
A typical engineered cell for disposal of NORMcontaining waste in a humid environment would include the following layers: 1) a topmost covering of erosion-resistant material, perhaps granitic rock rip-rap to dissipate erosional forces and discourage human and animal intruders; 2) a drainage layer of granular stone to route precipitation away from the underlying cover; 3) a compacted, clay barrier layer with low hydraulic conductivity (<1 10ÿ6 cm sÿ1) to prevent penetration of any water that had not been diverted by the overlying granular layer; 4) a ®ll layer over the waste to provide an even surface for placement of the clay barrier layer. This layer could include a permeable geochemical material to modify the properties of in®ltrating water and thereby reduce the solubility of radionuclides in the waste mass below; 5) the waste mass itself; and 6) a granular base layer to collect and divert water that percolated through the waste, particularly during construction when the cell is open. The function of the bottom layer is to increase the groundwater ¯ow away from the cell, thus reducing contact time between in®ltrating water and the waste mass. The concept of minimizing contact time between radionuclides in the waste and in®ltrating water can be at odds with regulatory requirements if chemically hazardous or toxic materials also are present in the waste, i.e., if it is a mixed waste. In the U.S., a shallow-land disposal facility is required to have a leachate collection system and two impermeable liners to separate and isolate the waste mass from the underlying strata. When water percolates into the waste, this design tends to maximize rather than minimize the time that leachate remains in contact with the waste. During the time the liner serves its intended purpose, this con®guration minimizes the transport of radioactive and other harmful materials. However, within a few tens of yearsÐwhen the liner failsÐthe radiological impacts are likely to exceed the impacts from an unlined facility that allowed leachate with lower concentrations of radionuclides to escape. PAST EXPERIENCE WITH LLW DISPOSAL
With few exceptions, actual experience with management and performance of facilities for radioactive waste disposal has been limited to sites which accept only LLW. Past experience at some sites that practiced shallow-land burial has revealed potential problems. Maintenance costs at several sites have been higher than anticipated and additional resources will be required in the future in order to ensure that the neighboring population continues to be protected. In the U.S., seven commercial shallow land disposal facilities have operated and three (at Barnwell, South Carolina; Clive, Utah; and Richland, Washington) continue to receive waste. At three of the closed sites there has been move-
Management and disposal of waste
ment of radioactivity from the waste mass. At Maxey Flats, Kentucky and West Valley, New York a combination of cap failure due primarily to subsidence, heavy precipitation, and tight clayey soil caused water to accumulate in the trenches. The potential pathway at both sites, therefore, is over¯ow and contamination of surface soil. Migration into the ground water is not considered a potential pathway of signi®cance at West Valley. At Maxey Flats, the fractured subsurface rock beneath the site caused concern about potential releases into groundwater in the future, but the more immediate concern was surface runo. The third closed commercial site at Sheeld, Illinois has more varied topography and looser soils. Because of this, surface runo carrying contaminated soil away from the immediate disposal area was a potential pathway. As was the case at the other two closed sites, subsidence of trench caps increased in®ltration and the potential for ground water migration; however, because of the higher soil permeability, the potential for bathtubbing is less. CONCLUSION
Experience, primarily at low-level waste disposal facilities (closed and open), has provided valuable lessons at a signi®cant economic cost. As a result of these lessons learned, operating practices have been modi®ed substantially and guidance is available to ensure public health and safety as new LLW disposal facilities are opened. Potential pathways through
587
which radioactivity could be dispersed into the human environment have been studied extensively and are well understood. Analysis of these pathways has provided valuable input to the siting, design, operation, and closure of low-level radioactive waste disposal facilities. Continued application of pathway analysis can provide additional assurance that future waste management practices will protect the public from adverse radiological health impacts.
REFERENCES
ANL/EAD/LD-2 (1993) ``Manual for Implementing Residual Radioactive Material Guidelines Using RESRAD, Version 5.0'', Argonne National Laboratory, September, 1993. Berlin, R. E. and Stanton, C. C. (1989) ``Radioactive Waste Management'', John Wiley and Sons, 1989. LWV, 1993. (1993) ``The Nuclear Waste Primer'', League of Women Voters Education Fund, Lyons and Burford, Publishers, 1993. Murray, R. L. (1994) ``Understanding Radioactive Waste'', Battelle Press, Fourth Edition, 1994. NEA/OECD, (1996) ``Radioactive Waste Management in Perspective'', Nuclear Energy Agency/Organization for Economic Co-operation and Development, 1996. NUREG/CR-5512, (1992) ``Residual Radioactive Contamination from Decommissioning, Technical Basis for Translating Contamination Levels to Annual Total Eective Dose Equivalent, Final Report'', Paci®c Northwest Laboratory, October, 1992. UNSCEAR, (1993) ``Sources and Eects of Ionizing Radiation'', United Nations Scienti®c Committee on the Eects of Atomic Radiation, UNSCEAR 1993 Report to the General Assembly, United Nations, 1993.
Radiat. Phys. Chem. Vol. 51, No. 4±6, pp. 579±587, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0969-806X(97)00205-3 0969-806X/98 $19.00 + 0.00
MANAGEMENT AND DISPOSAL OF WASTE FROM SITES CONTAMINATED BY RADIOACTIVITY CARLYLE J. ROBERTS Dames & Moore, Principal, 3065 Southwestern Blvd., Suite 202, Orchard Park, Bualo, New York, NY 14127-1286, U.S.A. AbstractÐVarious methods of managing and disposing of wastes generated by decontamination and decommissioning (D & D) activities are described. This review of current waste management practices includes a description of waste minimization and volume reduction techniques and their applicability to various categories of radwaste. The importance of the physical properties of the radiation and radioactivity in determining the methodology of choice throughout the D & D process is stressed. The subject is introduced by a survey of the common types of radioactive contamination that must be managed and the more important hazards associated with each type. Comparisons are made among high level, transuranic, low level, and radioactive mixed waste, and technologically-enhanced, naturallyoccurring radioactive material (TENORM). The development of appropriate clean-up criteria for each category of contaminated waste is described with the aid of examples drawn from actual practice. This includes a discussion of the application of pathway analysis to the derivation of residual radioactive material guidelines. The choice between interim storage and permanent disposal of radioactive wastes is addressed. Approaches to permanent disposal of each category of radioactive waste are described and illustrated with examples of facilities that have been constructed or are planned for implementation in the near future. Actual experience at older, existing, low-level waste disposal facilities is discussed brie¯y. # 1998 Elsevier Science Ltd. All rights reserved
INTRODUCTION
implemented in order to reduce these risks to acceptable levels. It is intended for the non-specialist and does not assume more than a basic understanding of radiation and radioactivity.
In many countries of the world, bene®cial uses of nuclear technology, including radiation and radioactivity, have been discouraged by widespread concern among private citizens and governmental agencies about potential, uncontrolled releases of radioactive waste materials into the environment. Regardless of how well intentioned, peaceful applications of nuclear technology inevitably produce by-product materials containing radioactivity at concentrations that exceed those that occur naturally. This is true for radioisotope applications in research, medicine, and industrial processing, for nuclear power production, and for the operation of certain types of radiation-producing machines. In addition, commercial processing of naturally radioactive minerals can produce waste that requires isolation from the environment. As a result, there is a world-wide eort to develop acceptable methods to manage and ultimately and safely dispose of all forms of radioactive waste (radwaste). To some extent this is a technical challenge, but it also requires a substantial communications eort to explain to a sceptical public what the potential risks of placing radioactive waste in the environment actually are and what is being done to ensure that the risks remain acceptably low. This paper presents an overview of the risks from disposal of radioactive waste and the various approaches being considered and, in some cases,
ORIGIN OF RADIOACTIVE CONTAMINANTS
Radioactive materials can be classi®ed very broadly as either manmade or natural. NaturallyOccurring Radioactive Material (referred to as NORM) is broadly distributed throughout the environment. It is present on earth either because the radionuclides have extremely long half-lives (relative to the age of the earth) or they are constantly being produced by natural processes. These natural processes include cosmic ray interactions with the atmosphere and transformation or ``decay'' of radionuclides in a series or chain which is maintained by a long-lived ``parent.'' Examples of the ®rst category (``primordial'' radioisotopes) are uranium-238 (4.5 billion year half-life) and thorium232 (14 billion year half-life). Examples of the second category include tritium (H-3) and radiocarbon (C-14), produced continuously by cosmic rays, and radium-226 and radon-222, which occur in a chain of fourteen decay events beginning with uranium238 and ending with stable (non-radioactive) lead206. When industrial processes such as mining and milling signi®cantly increase the concentration of NORM in a bulk material or its availability for 579
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dispersion in the human environment, it may then require special precautions in handling and disposal. Manmade radioactive material is generated by a variety of human activities. By far the largest quantities of radioactivity have been created by nuclear reactors operated either for electric power generation or weapons production (UNSCEAR, 1993). Radioactivity from reactors includes ®ssion products, activation products, and transuranic nuclides. Other radioisotopes for use in research and medicine are produced by particle accelerators. To some extent, all of these manmade radioisotopes contribute to the generation of radioactive waste, often as a result of contaminating other materials, e.g., soil, building materials, protective clothing, body ¯uids. TYPES OF RADIOACTIVE WASTE
The term ``radioactive waste'' (or ``radwaste'') covers a wide range of physical and chemical forms and concentrations of radioactivity. While de®nitions of various waste classes vary from nation to nation, radioactive waste is often, if not universally, categorized as either high-level waste (HLW), lowlevel waste (LLW) or transuranic (TRU) waste (LWV, 1993). In general, a designation as high-level waste is applied to the highly radioactive waste generated from reprocessing spent nuclear reactor fuel to separate re-usable uranium and plutonium from the ®ssion products and the spent fuel itself, if it is not to be reprocessed. Both forms of HLW require massive shielding, because of the intense gamma radiation they emitÐ as well as some form of cooling to remove the heat of radioactive decay. These requirements become less stringent within 100 to 200 years because of the relatively short half-life of several important nuclides (notably, strontium-90 and cesium-137). Reprocessing plant waste is generated in liquid form and, therefore, must be solidi®ed before it can be shipped safely or disposed of permanently with con®dence that it will remain isolated from the environment (primarily the groundwater) for a very long time. Several countries have advanced solidi®cation programs and some already have begun to vitrify reprocessing waste in the form of borosilicate glass. Spent fuel will require special ``packaging'' before disposal in the same facilities that will accept solidi®ed reprocessing waste. At this time, permanent disposal in a deep geologic repository is the option being pursued most aggressively; however, the opening of a permanent repository to accept HLW still appears to be many years in the future. Currently, the only alternative HLW disposal method being considered seriously is emplacement beneath deep sea sediments. Although this approach appears technically feasible, there is a moratorium on its use until more information on
the potential risks has been gathered and a regulatory system is implemented by the international community (NEA/OECD, 1996). Transuranic (TRU) waste is a result primarily of nuclear fuel fabrication, reprocessing, and nuclear weapons production. In the United States, TRU is de®ned as waste containing more than 3,700 Bq gÿ1 of alpha-emitting nuclides with atomic numbers greater than 92 and half-lives greater than 5 (or 20) years. Most TRU waste emits much less penetrating radiation and generates less heat than the ®ssion products in HLW. However, as a result of the extremely long half-life and high radiotoxicity of many of the transuranic nuclides, TRU waste usually requires isolation from the environment comparable to HLW. On the other hand, TRU usually is contact-handleable, meaning that it can be handled without heavy shielding. Low-level waste is produced by a broader spectrum of human activities than is the case for HLW or TRU and it contains a much wider variety of radioisotopes. In most cases low-level waste does not require shielding, although a 55-gallon drum containing cesium-137 close to the U. S. concentration limit for LLW (the Class C limit) would have a surface exposure rate of approximately 6 Sv hÿ1. Containers having dose rates greater than about 1 mSv hÿ1 at contact would be shielded or handled remotely. Most containers of LLW have surface dose rates of less than 0.1 mSv hÿ1 and require no special precautions during the relatively short time they must actually be handled. About one-half of the commercial LLW volume and a much larger fraction of the total radioactivity currently being generated in many developed countries originates in commercial nuclear power plants. It consists of activation products such as iron-55, cobalt-60, nickel-63, and ®ssion products such as antimony-125 and cesium-137. Most of this is in the form of compacted trash (paper, rags, plastic gloves, anti-contamination clothing), and contaminated bulk materials (concrete, steel, wood). There also are smaller volumes of more concentrated radioactive wastes such as spent ion-exchange resins and evaporator ``bottoms.'' Roughly 10% of the LLW volume is from institutional sources such as hospitals, medical schools, research laboratories, and universities. This LLW is in the form of laboratory glassware, trash, and animal carcasses, and a large fraction is in scintillation ¯uids. With the notable exceptions of tritium (H-3) and carbon-14, many of the radionuclides used in medicine and research have half-lives measured in days or less. The balance of the LLW volume, about 40%, is produced by industrial users. The relatively low speci®c activity, very large volume residuals produced by processing NORM represent yet another category of radioactive waste. An example is uranium mill ``tailings.'' In the U.S. alone this activity has produced about 150 million
Management and disposal of waste
tons of tailings containing 10 to 100 Bq gÿ1 of Th230 and Ra-226. Although the process of extracting uranium does not concentrate the remaining radionuclides in the tailings, the radioactivity is more easily dispersed to the environment than when it was associated with the original ore body beneath the surface of the ground. A second example of NORM-containing waste is the phosphogypsum byproduct of fertilizer production from sedimentary phosphate rock containing about 1 Bq gÿ1 Ra-226. The annual production of gypsum in Florida (USA) is 30 million tons, about one-third of the total worldwide production. Processing of by-product heavy minerals (amang) from tin mining in Southeast Asia and Australia also results in large volumes of residues and contaminated waste. The amang feed materials, derived from tin ore, contain concentrations of uranium and thorium up to four orders of magnitude greater than average background levels. If a radioactive waste also contains constituents that are considered chemically toxic and/or hazardous, the waste is classi®ed as a ``mixed'' waste. Liquid HLW may be mixed waste because of heavy metal content or extreme pH. Slightly contaminated lead shielding also would be mixed waste because of the toxicity of lead. Mixed wastes are even more dicult than radioactive wastes to de®ne precisely because there are many dierent de®nitions of hazardous materials. The treatment and disposal of hazardous waste is regulated under entirely dierent statutes than those that apply to radioactive wastes, and the requirements are sometimes contradictory and often not optimal for each type. On the brighter side, it may be possible to treat mixed waste so that the hazardous characteristics are eliminated, after which it may be managed solely as a radioactive waste. WASTE TREATMENT AND MINIMIZATION
During the past decade the cost of radioactive waste disposal has risen sharply, largely because of increased requirements for isolating the waste from the environment and, in some cases, to maintain the option of retrieving the waste should the integrity of the disposal facility appear to be compromised. This has placed greater emphasis on the development of methods to minimize waste volume. At least one major LLW disposal site in the U.S. now requires, as one of its waste acceptance criteria, that the waste be volume-reduced to the maximum extent feasible. The simplest approach, of course, is decay-in-storage. For short-lived nuclides this results in an industrial waste that can be disposed of relatively easily and inexpensively. Waste volume-reduction methods in common use for solids include compaction, incineration, dewatering of resins, metal sectioning, and smelting, as well as a variety of decontamination methods that
581
simply remove the surface layers on which radioactivity has been deposited (Murray, 1994). Compactable wastes such as plastic sheeting (herculite), metal pipe, and glassware typically can be volume-reduced by a factor of 6. Incineration can reduce waste volume by a factor of 80 or more. Materials including wood, plastics, anti-contamination clothing, oil, and organic solvents can be reduced to a small volume of ash. However, in some cases the incinerator operator will require the waste generator to take back the residues for ®nal disposition. Smelting, which concentrates the radioactivity in the slag, is the least ecient method in common use today: volume is typically reduced at a ratio of about 2:1. The extent to which metal from the smelting operation can be recycled depends on many factors, including the concentration of the original contaminants, the degree of separation achieved, and the applicable regulations. A pilot program is now under way in the U.S. to evaluate recycling of mildly contaminated steel by fabricating it into radioactive waste containers (barrels and boxes). To reduce the volume requiring disposal, bulky radioactive waste with low speci®c activity, such as contaminated soil, often can be treated eectively by soil-washing techniques. In some cases, simple wet screening will separate the ®ne particles containing most of the radionuclides from the larger particles that meet the clean-up criteria and constitute most of the waste volume. Liquid wastes often are volume-reduced by evaporation or decontaminated by sorption (ion exchange) methods. Although such treatment may not reduce their volume, liquid and slurry wastes often are solidi®ed by cementation or vitri®cation. As a solid, the waste can be transported with greater safety and disposed of with greater assurance that the integrity of the facility will not be jeopardized by waste slumping or consolidation after emplacement. In some cases, the hazardous characteristics of mixed high- and low-level liquid wastes can be removed by solidi®cation techniques, e.g., by reducing the leachability of heavy metal constituents. RADIOACTIVE WASTE CHARACTERISTICS THAT INFLUENCE DISPOSAL METHODS
The primary considerations in evaluating the potential impacts on the public and the environment from disposing of radioactive wastes derive from the presence of the longer-lived radionuclides. Immediately following closure of a permanent disposal facility and during the period of institutional control, which often is assumed to continue after closure for up to 100 years, environmental surveillance and active maintenance, if required, would occur. For this reason, a signi®cant o-site release of radioactivity is very unlikely. Therefore, the
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nuclides of primary concern to the designer of a permanent disposal facility are those that would retain appreciable activity more than 100 years after disposal. Among the longer-lived species, the relative toxicity varies over a wide range depending on such factors as chemical form and solubility in body ¯uids, physical state (e.g., particle size if in aerosol form), and the type and energy of emitted radiation. This variability is demonstrated in Table 1 which lists the eective dose equivalent resulting from an intake of one becquerel via ingestion and by inhalation. The table also shows the external dose rate produced by uniform ground surface contamination by each of the listed radionuclides. Note that Sr-90 and Cs-137 will decay to innocuous levels within about 300 years, but the remaining nuclides listed in Table 1 will persist in the environment for thousands of years. As one would predict because they emit alpha radiation, radium, uranium, and transuranics, Pu239 and Am-241 deliver higher dose equivalents relative to beta/gamma emitters per becquerel taken into the body, whether the route of entry is inhalation or ingestion. In contrast, the gamma emitters, represented in Table 1 by Cs-137 and Ra-226 progeny, deliver a much higher external whole-body dose than pure beta emitters such as Sr-90 or the alpha emitters for a given concentration on the ground or dispersed in air or water in which a person is immersed. In addition to the parameters of half-life and radiotoxicity listed in Table 1, there are other properties of radionuclides that are important in determining their potential impact if released from a disposal site. As noted below, an important potential pathway for dispersal is via ground or surface water. The rate at which a radioactive species dissolves in ground water is measured by its leachability or leaching index. The rate of dissolution is a complicated function of many parameters, including the chemical form of the nuclide, the nature of the
waste form (cement, bitumen, activated metal), the composition of the ground water, and whether contact with water is constant or intermittent. Because of the large number of important variables that determine leach rates, generally applicable values are not readily available and reliable estimation of these quantities is one of the more challenging aspects of predicting the performance of disposal facilities. Fortunately, there are data in the literature that enable modelers to choose conservative leach rates and place reasonable upper bounds on the potential for ground water contamination. Another important parameter that determines the rate at which radioactivity may migrate away from a disposal site is the distribution coecient (Kd). The Kd is a measure of the various sorption interactions between water and a geologic media that cause the radionuclides to migrate at a slower rate than the ground water itself. Numerically, the Kd is the ratio of the concentration of a nuclide in the solid (soil) phase to the concentration in water. Typical values for long-lived nuclides also are listed in Table 1. It is clear that the long-lived alpha emitters interact strongly with soil (i.e., they have large Kd's) and, therefore, their migration in ground water is much slower than elements such as carbon, iodine, and technetium, which are subject to essentially no retardation. POTENTIAL ENVIRONMENTAL PATHWAYS TO MAN
Releases of radioactivity from disposal sites conceivably could occur at any stage in the life cycle of a facility, beginning with the operational phase when waste is being received, stored, processed (e.g., volume reduced), and emplaced, through closure of the site, and then through the institutional control period and beyond. There are a variety of possible scenarios involving accidents, equipment failures, and human errors in judgment that could occur during the operational phase and could result
Table 1. Relative radiotoxicity and related characterizations of common nuclides a
EDE per unit intake relative to Pu-239*
Nuclide H-3 C-14 Sr-90 Tc-99 I-129 Cs-137 Ra-226 Np-237 U-238 Pu-239 Am±241
Half life (years) 12.3 5730 29.1 2.13 1.57 30.0 1600 2.14 4.47 2.41 432
E+5 E+7 E+6 E+9 E+4
Inhalation
Ingestion
0.00% 0.00% 0.30% 0.00% 0.04% 0.01% 2.00% 125.86% 27.59% 100.00% 103.45%
0.00% 0.06% 4.03% 0.04% 7.80% 1.41% 37.45% 125.52% 7.20% 100.00% 102.93%
External dose rate Distribution coecient relative to Ra-226** or Kd (ml g-1)*** 0.00% 0.00% 0.00% 0.00% 0.20% 32.36% 100.00% 10.40% 0.81% 0.00% 0.30%
0±1 1±5 5 ± 30 0.1 ± 1 1 ± 60 40 ± 500 70 ± 220 0.1 ± 50 35 ± 50 500±2000 300±1900
*For Pu-239 unit dose factors (Sv/Bq)are: inhalation - 1.16 E ÿ 04; ingestion Ð 9.56 E ÿ 07Ref: Appendix E, NUREG/CR-5512, 1992, October. **For Ra-226, unit dose factor (mSv/y per Bq/cc) is 2.31 for an in®nite volume source and a density of 1.8 g/ccRef: A.1, ANL/EAD/LD2, 1993, September. ***Typical, average distribution coecients for soils and clay, see NUREG/CR-5512.
Management and disposal of waste
in acute or short-term releases. Such incidents would not involve any unique pathways, however. Of greater concern are the potential causes of what, in the absence of eective monitoring and surveillance, could become chronic, long-term releases. Regardless of the cause, the radioactivity that escapes the con®nes of a disposal facility will be subjected to a complex variety of physical, chemical, and biological processes. Most of these processes will tend to disperse or dilute the activity, but some will concentrate it. Environmental pathways may be broadly categorized as airborne or liquid (i.e., water). The critical pathway for mobilization of radioactivity from disposal facilities usually is surface or ground water. All LLW disposal facilities are designed to prevent or at least minimize contact between the waste mass and water. Therefore, in order to introduce radioactivity into water, there must be a failure of the protective barriers. Should this occur, there are two obvious routes for the contaminated water to take, depending on the geohydrologic characteristics of the site. Flow may be into the ground, with contamination of an aquifer as a result. This could provide a direct pathway to man by ingestion of drinking water from a well or by an indirect path through ingestion of the water by cows or beef cattle or by root uptake into edible plants. If the radioactivity is introduced into surface water, additional pathways are opened. Direct external exposure from immersion (i.e., swimming) or simply standing by a stream bank is possible although these are not likely to be signi®cant pathways. Of greater concern are pathways from irrigation water to food crops, from direct ingestion by man, or from use in watering cattle. Another potentially critical pathway, because of the possibility of biological concentration, is uptake by ®sh and other edible aquatic forms. Overland ¯ow of contaminated water also is a concern because it would contaminate surface soil which then could be resuspended in air and inhaled. Gases from decaying organic matter also may provide a route for the escape of certain radionuclides (e.g., H-3 and C-14) from LLW disposal facilities. This pathway may be precluded by prohibiting the disposal of putrescible wastes. The relative importance of the many possible pathways depends upon the actual or assumed failure modes of a given disposal facility. Contamination of groundwater ®rst requires a mechanism to breach the con®nement barriers. In the case of conventional shallow land burial, the trench cap might be breached as a result of subsidence of the waste mass, by cracking as a result of climatic cycling, by being penetrated by an intruder, or a variety of other causes. If precipitation enters through a failed cap and leaches activity from the waste mass, the activity could follow groundwater pathways described above. If the underlying geolo-
583
gic media has very low hydraulic conductivity, the result might be ``bathtubbing'' or over¯owing of leachate onto the ground surface rather than migration into an aquifer. ROLE OF INTRUDERS
In addition to protecting the public from o-site releases of radioactivity, a second design objective for a disposal facility is to protect individuals from inadvertently intruding into the waste site after institutional controls are removed. This objective provides the basis for the concentration limits that de®ne the three classes of LLW that are recognized in the U. S.. These concentration limits, in turn, are designed to ensure that an inadvertent intruder would not receive a dose greater then 5 mSv in one year. During the design process, a variety of socalled ``intruder scenarios'' are examined and a site speci®c analysis is conducted to predict possible impacts that could occur during the post-institutional control period. Usually the critical or controlling intruder scenario is the ``resident farmer'' whose family potentially could be exposed for extended periods of time. This agricultural scenario could include the full range of exposure pathways, assuming that credible events (e.g., failure of a trench cap) are identi®ed that can lead to ground and surface water contamination. For example, trench over¯ow could contaminate fertile soil in which crops were grown. Cultivation of this soil would resuspend the radioactivity and allow it to be inhaled. Contamination of a well water supply and/or a farm pond opens up additional direct and indirect pathways mentioned above. Current disposal practices should meet speci®c technical requirements that account for natural site characteristics, facility design (including closure), waste properties and packaging, and institutional controls such as monitoring and control of site access. The objective of these controls is to prevent the radioactivity contained in the waste from entering any of the pathways to man as a result of natural or spontaneous events or human intervention. RADIOACTIVE WASTE DISPOSAL METHODS
There are two basic methods for disposing of radioactive wasteÐdispersion into the environment or isolation from the environment. In general, the former alternative is not used except in the case of liquid waste euent from hospitals and clinics. These facilities, and other facilities that house patients who have received radioisotopes for diagnostic or therapeutic treatment, discharge their sanitary waste directly to the local sewer system. Other situations that might justify choosing this option could arise if the radionuclide were easily dispersable, such as a noble gas (e.g., krypton-85), tritiated
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water vapor (containing hydrogen-3), or carbon-14 dioxide. In accordance with the general principle that no radiation doses should be permitted unless there is a concomitant gain, any proposal for disposal by dispersal should receive a careful cost/bene®t analysis before a decision is made. A decision to proceed would require a demonstration of a clear, net, societal gain. A net bene®t might occur because, for example, the potential collective dose to workers engaged in a conventional waste packaging operation exceeded the collective dose to the nearby population from releasing the waste directly to the environment. Because radioactivity can be measured at very low concentrations in media such as air and water, it is possible to con®rm the acceptability of disposal by dispersal by appropriate environmental monitoring, albeit only after this method of disposal has been initiated. The ®nal stage in disposing of a low-level liquid waste by conventional methods such as evaporation and ion exchange removes most radionuclides from the aqueous phase. Following this, the radioactivity is solidi®ed further, if necessary, and then disposed of by shallow-land burial or a more sophisticated method that provided the required isolation from the environment. However, if the decontaminated euent from such treatment contained tritium, conventional methods would not lower its concentration. While methods for removing tritium from water do exist, their application is quite expensive. Therefore, if the tritium concentration were suciently low, the aqueous waste might become a candidate for disposal by slow release to a diluting body of water. The disposal of radioactive krypton-85 (10.7 year half-life) by release to the atmosphere for dispersion and dilution oers another illustration of this approach to disposal. Krypton is used routinely to test hermetically sealed systems to ensure that they are leak tight. Because krypton is a noble gas with a half-life that is too long for decay-in-storage to be practical, the preferred option is to release the gas through a vent stack at an intermittent, carefully controlled rate. Conservative air dispersion calculations are noted for two manufacturing sites in the northeastern U.S. using measured annual average meteorological parameters. The results demonstrated that 10 to 100 curies of Kr-85 could be released safely over a period of several weeks without exceeding regulatory limits. Eective dose equivalents to a maximally exposed o-site individual were on the order of 0.001 microSv, with skin doses of about 0.01 microSv. The more common general method of disposing of radioactive waste, and the approach more likely to be accepted by the general public, is to con®ne it in such a way that its isolation from the environment is assured with an acceptable degree of con®dence (Berlin and Stanton, 1989). This is seldom an issue for radionuclides with half-lives of 30 years or
less. This includes most of the most of the higher yield ®ssion products such as strontium-90 and cesium-137. While these nuclides are present in relatively high concentrations in nuclear reactor wastes, they will be reduced in quantity by a factor of 10 in 100 years and by 100 in 200 years. This fact reduces the importance of maintaining the integrity of the disposal facility for long periods of time. In contrast, facilities that are designed to isolate NORM wastes containing U-238 or Th-232 or reactor wastes with long-lived transuranic constituents must include features that ensure stability for 1,000 to 10,000 years or more. Although numerous nuclides of concern have half-lives that are orders of magnitude longer than 10,000 years, pathway analyses often demonstrate that the peak dose to the public would occur within 10,000 or, at most, 100,000 years. This provides a degree of comfort with very long-term predictions of radiological impacts even though it is generally conceded that beyond about 10,000 years it is essentially impossible to predict geological or climatic events with any reasonable degree of certainty. Also, it is reassuring to note that in less than 10,000 years the radioactivity in HLW will decay to a smaller quantity than is present in uranium ore equivalent to the amount that was required to produce the reprocessed nuclear fuel in the ®rst place (NEA/OECD, 1996). High-level waste disposal For high-level waste (HLW) from nuclear fuel reprocessing and for spent reactor fuel, the only option currently being pursued is con®nement in a deep geologic repository. While considerable attention is paid to the design and performance of the waste encapsulation, this is primarily to provide protection during shipment and emplacement, and for a period of several hundred years until most of the radioactivity has decayed away. Until then, all operations must be carried out by remote handling behind heavy shielding. The HLW will not be placed in the repositories until the shorter-lived radionuclides have decayed and the rate of heat generation has dropped to a level that does not require highly ecient, guaranteed cooling capability. The more problematic concerns about HLW disposal arise because of the extremely long half-life of some components of the waste (for example, plutonium-239 at 2.4E + 4 years, neptunium-237 at 2.1E + 6 years, and iodine-129 at 1.6E + 7 years). In this time frame, continued isolation can be provided only by the geologic media itself. Thus, the primary concern is to ®nd a deep, extremely stable formation where the waste is very unlikely to encounter groundwater. This requirement is linked to the related needs to site the repository in a geologic setting free of seismic activity and removed from the threat of serious erosion. Because leaching by intruding water is the primary potential mechanism for transporting waste
Management and disposal of waste
radioactivity into the human environment, the leachability of the waste form also is an issue. In several nations, liquid HLW currently is being vitri®ed to produce a very stable solid form suitable for shipment and disposal in a geologic repository. HLW repositories are in various stages of development in several dierent media. Media being investigated for suitability include clay (Belgium), granite (Canada and Sweden), salt (Germany and U.S.), sedimentary rock (Japan), and tu (U.K. and U.S.). (NEA/OECD, 1996) In many cases, the depths being studied range from 400 meters to more than 1,000 meters. Great depth oers the obvious advantage of a very long pathway through which radioactivity must be transported before it can expose the public. Less apparent is the protection these depths oer to a potential inadvertent intruder who is exploring for mineral resources. TRU and medium-level waste disposal TRU and medium-level waste do not require the very high degree of isolation that high-level waste does but, in the case of TRU waste, con®nement must be maintained for an equally long period. Much of this waste, particularly alpha-emitting waste, does not require remote handling. Also, somewhat less importance is placed on waste form integrity and resistance to leaching. Many of these solid wastes receive no pre-treatment other than volume reduction. For shorter lived, medium-level wastes, engineered, near-surface facilities have been developed in several countries, most notably in France. This concept involves placing the stabilized waste containers in shallow, concrete lined vaults. The packages are then grouted in place and covered with additional concrete to prevent human intrusion and in®ltration by precipitation. In some cases, additional low-level waste containers may be placed on top, and the entire mound is enclosed in a clay cap with provisions to divert any water that does in®ltrate through the cap. In Germany, short-lived medium-level wastes have been placed in salt mines, and in Sweden and Finland cavities in bedrock have been used for this purpose. In general, repositories for medium-level wastes are not as deep as those being considered for HLW disposal. The choice of disposal methodology involves obvious economic considerations. Where suitable mined cavities already exist and mineral resources have been exhausted, underground disposal may easily be justi®ed, even if the radiological waste characteristics do not require such a high degree of isolation. A requirement for retrievability will have a major impact on the design of the facility and add substantially to the cost, whether a near-surface or deep geologic repository is chosen.
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Low-level waste disposal With the exception of low-level waste (LLW) containing naturally occurring radioactive material (NORM), most LLW is relatively short-lived whether it is generated by power reactor operation, medical and research applications, or industrial uses. As a result, the requirements for prolonged and assured con®nement are not nearly as stringent as they are for HLW disposal. The method of choice usually is a form of near-surface, or shallowland, disposal. Experience has demonstrated that simply digging a trench and haphazardly dumping the waste into the cavity and then covering it with soil does not adequately protect the public. This experience has shown that it is important to consider waste form stability and emplacement technique in order to avoid excessive voids from developing after closure. The danger is that compaction of the waste mass in just a few years can result in signi®cant failure of the cap and allow precipitation to gain access to the waste mass. This, in turn, can result in excessive leaching to groundwater if the underlying strata are suciently porous or, if the soil is highly impermeable, leachate can over¯ow to the surface and run overland (the so-called ``bathtub eect''). A variety of designs have been developed in recent years to decrease the possibility of either of these two events happening. The requirements for waste form stability have been increased relative to the amount of radioactivity to be placed in the disposal facility. When justi®ed, concrete vaults have been used to reduce the likelihood of in®ltration in the near term and/or to facilitate retrievability, if this is required. For waste that contains substantial quantities of long-lived activity, standard packaging such as steel boxes and drums and concrete pads and vaults oer no advantages. If isolation is required over a time frame reaching beyond a few hundred years, it usually is assumed that the integrity of steel containers and concrete will be lost. In such cases, geologic media such as clays often are preferred for constructing impermeable covers. Assuming that proper precautions have been taken to avoid settlement of the waste mass, the geologic cover is less prone than concrete to failure that would allow signi®cant amounts of water to penetrate. Disposal of low speci®c activity NORM waste Mineral residues from large-scale industrial processes such as uranium mining and milling, fertilizer production from phosphate rock, and reprocessing of tin byproduct (amang) present special problems because of their very large volumes. Many of these wastes are of suciently low speci®c activity that they are not a serious occupational hazard because of external radiation. At U-238 or Th-232 concentrations above approximately 1 Bq gÿ1, external gamma irradiation is
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of concern for workers if the radioactive progeny in the decay series are approximately in equilibrium. A more serious threat to workers who are required to handle large quantities of these materials under dry conditions is inhalation of airborne particulates. To a lesser extent, inhalation of radon daughter products may also be a signi®cant source of internal radiation. However, this is seldom a problem when work is performed outdoors so that radon progeny do not accumulate as they would indoors. Because of the very long half-life of the decay series' parents, U-238 and Th-232, the cost of a concrete structure for disposing of low speci®c activity, NORM waste is seldom, if ever, justi®ed. The concrete cannot be assumed to maintain its integrity for as long as the parent nuclides will support the radioactivity of the daughter products in the waste (literally billions of years). Therefore, a more stable earthen structure is preferred. Moreover, superior shielding properties are not generally required for these wastes. The o-site public can be protected more than adequately from direct radiation by a relatively thin covering of uncontaminated soil above the waste. A thin clean covering also will prevent the dispersal of radioactive particulates from the surface of the waste. A more critical need is to attenuate the exhalation of radon222, the isotope derived from uranium-238 via radium-226. In many cases, the dominant pathway for exposure of o-site individuals is from radon. The radon-220 from the thorium-232 series has such a short half-life (56 seconds) that it is not a concern o-site (or on-site). If properly installed, a clay cap of approximately 1 meter in thickness will provide ample protection against radon for persons residing near the disposal facility. A major pathway that could result in exposure of the population in the environs of a NORM disposal site is migration through the groundwater. This is of special signi®cance in humid climates with abundant rainfall. In such an environment, precautions must be taken when designing and constructing the facility to minimize in®ltration of precipitation into the waste. In addition, the site should provide relatively impermeable strata beneath the waste mass so that vertical and horizontal movement of any water that does contact the waste is restricted. In addition, the geologic media underlying the site should, ideally, be capable of retarding the movement of solutes in the leachate by various mechanisms (e.g., adsorption, ion exchange). In many cases, the most troublesome nuclides are the radium isotopes, primarily Ra-226 and Ra-228. If the geologic environment does not provide sucient retardation (as measured by the distribution coecient or Kd), it can be augmented by adding suitable materials to the waste mass itself or by placing a geochemical barrier at the bottom of the waste but above the native soil. This barrier could be simply another layer of clay with a high Kd.
A typical engineered cell for disposal of NORMcontaining waste in a humid environment would include the following layers: 1) a topmost covering of erosion-resistant material, perhaps granitic rock rip-rap to dissipate erosional forces and discourage human and animal intruders; 2) a drainage layer of granular stone to route precipitation away from the underlying cover; 3) a compacted, clay barrier layer with low hydraulic conductivity (<1 10ÿ6 cm sÿ1) to prevent penetration of any water that had not been diverted by the overlying granular layer; 4) a ®ll layer over the waste to provide an even surface for placement of the clay barrier layer. This layer could include a permeable geochemical material to modify the properties of in®ltrating water and thereby reduce the solubility of radionuclides in the waste mass below; 5) the waste mass itself; and 6) a granular base layer to collect and divert water that percolated through the waste, particularly during construction when the cell is open. The function of the bottom layer is to increase the groundwater ¯ow away from the cell, thus reducing contact time between in®ltrating water and the waste mass. The concept of minimizing contact time between radionuclides in the waste and in®ltrating water can be at odds with regulatory requirements if chemically hazardous or toxic materials also are present in the waste, i.e., if it is a mixed waste. In the U.S., a shallow-land disposal facility is required to have a leachate collection system and two impermeable liners to separate and isolate the waste mass from the underlying strata. When water percolates into the waste, this design tends to maximize rather than minimize the time that leachate remains in contact with the waste. During the time the liner serves its intended purpose, this con®guration minimizes the transport of radioactive and other harmful materials. However, within a few tens of yearsÐwhen the liner failsÐthe radiological impacts are likely to exceed the impacts from an unlined facility that allowed leachate with lower concentrations of radionuclides to escape. PAST EXPERIENCE WITH LLW DISPOSAL
With few exceptions, actual experience with management and performance of facilities for radioactive waste disposal has been limited to sites which accept only LLW. Past experience at some sites that practiced shallow-land burial has revealed potential problems. Maintenance costs at several sites have been higher than anticipated and additional resources will be required in the future in order to ensure that the neighboring population continues to be protected. In the U.S., seven commercial shallow land disposal facilities have operated and three (at Barnwell, South Carolina; Clive, Utah; and Richland, Washington) continue to receive waste. At three of the closed sites there has been move-
Management and disposal of waste
ment of radioactivity from the waste mass. At Maxey Flats, Kentucky and West Valley, New York a combination of cap failure due primarily to subsidence, heavy precipitation, and tight clayey soil caused water to accumulate in the trenches. The potential pathway at both sites, therefore, is over¯ow and contamination of surface soil. Migration into the ground water is not considered a potential pathway of signi®cance at West Valley. At Maxey Flats, the fractured subsurface rock beneath the site caused concern about potential releases into groundwater in the future, but the more immediate concern was surface runo. The third closed commercial site at Sheeld, Illinois has more varied topography and looser soils. Because of this, surface runo carrying contaminated soil away from the immediate disposal area was a potential pathway. As was the case at the other two closed sites, subsidence of trench caps increased in®ltration and the potential for ground water migration; however, because of the higher soil permeability, the potential for bathtubbing is less. CONCLUSION
Experience, primarily at low-level waste disposal facilities (closed and open), has provided valuable lessons at a signi®cant economic cost. As a result of these lessons learned, operating practices have been modi®ed substantially and guidance is available to ensure public health and safety as new LLW disposal facilities are opened. Potential pathways through
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which radioactivity could be dispersed into the human environment have been studied extensively and are well understood. Analysis of these pathways has provided valuable input to the siting, design, operation, and closure of low-level radioactive waste disposal facilities. Continued application of pathway analysis can provide additional assurance that future waste management practices will protect the public from adverse radiological health impacts.
REFERENCES
ANL/EAD/LD-2 (1993) ``Manual for Implementing Residual Radioactive Material Guidelines Using RESRAD, Version 5.0'', Argonne National Laboratory, September, 1993. Berlin, R. E. and Stanton, C. C. (1989) ``Radioactive Waste Management'', John Wiley and Sons, 1989. LWV, 1993. (1993) ``The Nuclear Waste Primer'', League of Women Voters Education Fund, Lyons and Burford, Publishers, 1993. Murray, R. L. (1994) ``Understanding Radioactive Waste'', Battelle Press, Fourth Edition, 1994. NEA/OECD, (1996) ``Radioactive Waste Management in Perspective'', Nuclear Energy Agency/Organization for Economic Co-operation and Development, 1996. NUREG/CR-5512, (1992) ``Residual Radioactive Contamination from Decommissioning, Technical Basis for Translating Contamination Levels to Annual Total Eective Dose Equivalent, Final Report'', Paci®c Northwest Laboratory, October, 1992. UNSCEAR, (1993) ``Sources and Eects of Ionizing Radiation'', United Nations Scienti®c Committee on the Eects of Atomic Radiation, UNSCEAR 1993 Report to the General Assembly, United Nations, 1993.