Philip E. Hamrick, Ph.D. National Institute of Environmental Health Sciences Research Triangle Park, NC

Transcription

1 MANAGEMENT OF LOW-LEVEL RADIOACTIVE WASTES Philip E. Hamrick, Ph.D. National Institute of Environmental Health Sciences Research Triangle Park, NC The procedures for the management of low-level radioactive waste have changed greatly over the past several years due to changes in regulations, availability of disposal sites, and cost. Previously, management was often thought of only in terms of land disposal. Now, those who have responsibility for the management of waste are faced with a wide and sometimes complex variety of choices of disposal methods and practical procedures. This paper will present and compare some of the options available for management of low-level radioactive waste generated in institutions. No discussion will be made of low-level wastes generated by the nuclear power industry. Management of low-level radioactive waste implies involvement in the total process of the use of radioactive material. Responsible management will be concerned with the proposed use of radioactive material and whether amounts that are proposed are the least that can effectively accomplish the proposed objectives of the project. Are there modifications that can be made in the proposed procedures that will result in more effective and efficient use of the radioactive material? Are there any radioisotopes that can be substituted for the proposed isotopes that are less hazardous or which can be prepared for disposal more easily? Many institutions require that protocols be submitted for any proposed use of radioisotopes. These protocols are reviewed by the Radiation Safety Committee not only for safety but for efficient use of radioactive materials and minimization of generated waste.

2 I a3 Low-level radioactive waste is defined as all radioactive waste other than spent nuclear fuel, waste from nuclear fuel processing, transuranic waste, or waste from uranium mines or mill tailings. The definition therefore includes most of the radioactive waste that is generated in research institutions unless a nuclear reactor is also located at the research facility. Isotopes generally encountered are those used as tracers in biological research such as H3, C14, P32, S35, and Table 1 lists some of the more commonly used tracers, halflives, and particle type and energy. Additional isotopes such as Tc99m. Au198, Co58m, and Ca47 are encountered in the diagnosis and treatment of humans. These will generally have short half-lives which simplifies management. As seen above the definition for low-level radioactive waste is unrelated to the amount of activity that is to be disposed. However, most institutional waste is low in activity when compared to the activities generated in nuclear reactors. Activities for most biological experiments will range from a few microcuries to several millicuries. The same is true for most medical diagnostic procedures. However, the activity may be considerably greater for medical treatment with radioisotopes. Greater amounts may also be used when labeling chemicals for later use as tracers. Low-level radioactive waste may be and is generated in a wide variety of forms. Paper, glass, and other solid materials are often contaminated including large pieces of equipment that may be too costly to decontaminate. Liquids are generated containing radioisotopes and may contain organic solvents or other chemicals that will affect their

3 84 Table 1. Commonly Used Radioisotopes. Half Maximum Type of Maxim Nuclide Life Specific Activity Radiation Energy (WV). ( days 1 (Curies/gm) x lo x lo lo lo x io lo lo x 6.02 x lo lo lo lo lo lo lo x lo lo lo x 10 - Abstra-ted from: Radiological Health Handbook- U. S. Departmant of Health, Education and Welfare - Public Health Sewice Publication No. 2016, Rockville, Maryland, 1970

4 I 85 disposal. Animals are injected with radioactive material and will require special disposal procedures. The following sections will discuss specific pickup and handling procedures, incineration, packing, and preparing waste for shipment off-site. Waste Disposal Options: Before specific procedures can be developed for handling and preparing waste for disposal, the final disposal methods and waste destinations must be selected. The primary options for disposal are listed in Table 2. It is unlikely that any one of the listed options will be sufficient to handle all the waste generated by an institution. Rather a combination of disposal methods will most likely be required. Only 10 years ago almost all of the waste generated was disposed of by land burial. As land burial regulations have changed and costs have risen the percentage of waste being buried is decreasing. At present there are only three commercial sites for the burial of radioactive waste. These are located at Richland, Washington; Betty, Nevada; and Barnwell, South Carolina. However, all three sites are not equally available for use. The Congress, in an effort to share the burden of waste burial among states using radioactive material, passed legislation in 1980 allowing states to form compacts in which all radioactive waste generated outside the compact of states could be excluded after January. - 1, Since the states have been slow to move in setting up the compacts and in providing for burial sites within the compact, Congress passed additional legislation which allows waste that is generated

5 86 Table 2. Disposal Options. 1. Land Burial 2. Incineration 3. Decay 4. Sewer 5. Recycling

6 87 outside a compact region to be buried within the region on payment of a penalty or surcharge on the waste. This penalty increases with each year. The compacts also have the authority to limit shipment of waste out of the compact region. In the Southeast compact region no waste can be shipped out of the region for disposal if "(1) it meets the licensing requirements of the regional facility (Barnwell) of (2) it could meet such requirements through processing by available methods." The other two sites will not accept any waste generated in the Southeast Region unless it is accompanied by written certification that the waste is not acceptable at the Barnwell facility. The Southeast Compact is made up of the following states: Alabama, Florida, Mississippi, North Carolina, South Carolina, Tennessee, Virginia, and Georgia. Under the compact plan, the operation of the Barnwell site is due to cease as a regional facility by the end of By that time another host state should have been chosen and have an operating burial facility. North Carolina is the most likely choice to serve as the next host state. Surcharges at the Barnwell site for 1986 are $10 per cubic foot for waste that is generated outside the compact. Recent South Carolina legislation provides for a surcharge of $20 per cubic foot if the waste originates in states which have not entered into a compact or have not certified they will develop their own disposal site. It is clear that the incentive to enter into compacts and develop sites is strong and will become even stronger as surcharges increase each year.

7 I 88 The Barnwell site will not accept any liquid waste (must be < 0.5 % free standing liquid). Liquid waste must be solidified prior to shipment in either vinyl ester styrene, cement, or bitumen. However, cement is the solidification media that has most often been used. Liquid in absorbents is not acceptable. No organic liquids or containers that have contained organic liquids are acceptable even if solidified. Incinerator ash must be solidified before being acceptable. There are a number of other restrictions that may impact on the way waste is managed, and the operating site license should be consulted.. Another disposal option that is becoming more popular is incineration. This option is particularly attractive for disposing of waste such as liquiid scintillation fluid which may not be acceptable at any of the burial sites. Waste scintillation fluid or animal tissues which contain less than 0.05 microcuries per gram of C14 or H3 may be disposed of as if it were not radioactive. Depending on the type of scintillation fluid it may not be possible to dispose of the fluid via the sewer and incineration may be the only practical disposal option. It is usually possible to get an on-site incinerator licensed to burn solvents. If the use of radioactive material is great enough at an institution it may be feasible to amend the radioactive materials license to permit incineration of a wide variety of isotopes. However, much more stringent controls must be placed on monitoring the amount of radioactivity incinerated and in monitoring the ash and stack effluents. A simpler solution for many small institutions is to send the material -to another location to be incinerated. Because of the wide variety of

8 I 89 isotopes incinerated, these commercial incinerators w ill normally treat the ash as radioactive waste. The ash may be solidified or treated in some other way to make it appropriate for burial. Decay is the option of choice for disposal of isotopes with short half-lives. Determination of what constitutes a "short" half-life will depend to a large extent on how much storage capacity is available. It may be necessary to keep some materials as long as 10 half-lives or more in order to reduce the activity below the limits given in appendix B of 10CFR20. Many institutions will find it feasible to store isotopes with half-lives less than 30 days but may encounter problems finding enough storage space for half-lives much greater than this. Storage of radioactive material presents some problems other than those related to space. The waste must be adequately shielded and protected from the effects of weather or other events that could lead to leakage of the containers. Secondary containment may be required to prevent the release of any radioactivity in the case of leakage. Special fire protection and air filtering systems may be needed depending on the amount of activity stored. Another option for disposal of some radioactive material is via the sewer. The NRC, in the January 9, 1986, draft 10CFR20, allows the release of some material through the sanitary sewer system. A synopsis of the provisions is as follows: The material must be readily soluble in water. The average monthly released activity must be within the limits specified in Table 3 appendix B of draft locfr20.

9 90 3. If more than one isotope is released the limits in (2) must be adjusted so that the sum of the fractional limits is less than one. 4. Total activity released per year must be less than 5 Ci of H3, 1 Ci of C14, and 1 Ci of all other radioactive materials. Another highly desirable option of disposing of waste is to recycle the waste. However, for most of the waste streams this is not economically feasible. The foremost recycling efforts have been made with liquid scintillation fluid. For this to be most effective the pcocess should be designed for specific scintillation fluids. It may then be possible to clean and recover certain solvents that can then be re-used. More often mixtures of liquid scintillation fluid are processed to remove the radioactive material and then the solvents are used as auxiliary fuel in incineration. The removed radioactive material and other process residue will likely have to be incinerated in an incinerator licensed to burn radioactive material. Any process that produces residues containing radioactive material that is destined for land burial should be approved by the burial facility. For most sites it will be necessary to show that the residue is completely free of any solvents. Application for disposal processes not specifically covered by the license regulations may be made to the NRC or appropriate state agency. The application would have to include such things as a description of the waste including physical and chemical properties important to risk evaluation, an evaluation of potential effects on the local environment and facilities, and procedures to insure that doses are within the applicable limits and as low as reasonably achievable.

10 I waste 91 Pickup and Handling Procedures Routine pickup and handling procedures will vary with each institution depending on such factors as: the amount and variety of radioisotopes used, the size of the radiation safety staff, the particular policies developed by the radiation safety committee, and the particular methods that are used for ultimate disposal. For small institutions where only one or two investigators use radioisotopes or where only one or two isotopes are used within the same *laboratory, the researcher may sene as the radiation safety officer as well as be in charge of all aspects of procurement and disposal of radioactive materials. This can be a very time consuming task since, even for a small amount of waste, the researcher must be familiar with a large number of regulations. If the waste stream remains constant once procedures are developed, less time will be needed for reviewing old and new regulations. Larger institutions will normally have a person or persons assigned full time to oversee all aspects of radiation safety and disposal of radioactive material. Some institutions may require each investigator to prepare their own waste for disposal. However, it is more usual for the waste to be picked up from the laboratory by radiation safety personnel and then prepared for a centralized waste disposal station or collection point. The primary objection to this arrangement is that may be delivered that is improperly prepared or labeled and may be difficult to trace.

11 92 Regardless of the size of the institution or the method of pickup, there are several procedures that should be followed: 1. Separate waste by isotope as much as possible since disposal procedures will vary depending on the characteristics of the particular isotope. 2. Separate waste by half-life if separation by isotope is not possible. 3. Separate waste by physical form - solid, liquid, gaseous, and animal. 4. Separate waste by disposal method - land burial, incineration, sewer, etc. 5. Transport waste to central waste preparation area using secondary containment or absorbents for liquid waste and thick or double plastic bags for solid waste Shield all waste where there is an external hazard. Label all waste with the following information: researcher, date, activity, isotope, chemical and physical form, and any associated unlabeled hazardous chemicals. Item 7 above is very important since all this information will be needed to properly prepare the waste for treatment or disposal. It may be more convenient to use a pickup request form which includes all of this information and perhaps other information such as room numbers, amounts of unlabeled hazardous chemicals, radiation levels, etc. However, the waste should always be marked with isotope, activity, and name of researcher in case the waste is separated from the waste pickup form.

12 I 93 Personnel that are employed to pickup and package waste must be well trained and alert to any discrepancies between labels and actual waste materials. They must be thoroughly familiar with standard radiation safety principles and be able to apply these to specific situations as they arise. Attention to detail is essential especially with regard to maintaining complete records of all waste collected and packed. Centralized Waste Handling and Volume Reduction After waste is picked up from the laboratories, it is usually carried to a central collection point for separation and preliminary treatment prior to final treatment. In some institutions each reseracher prepares his waste, takes it to a barrel or some other container that is the final container for the waste, and lists what he has placed in the container on a sheet attached to the container. The safety office then seals the container and prepares it for land burial or incineration. This procedure may work well for a small institution but would be subject to abuse by unscrupulous investigators in larger institutions. The procedure does have the advantage that little manpower is needed by the radiation safety office to handle the waste and little space is needed for centralized storage and packing. For larger institutions a centralized storage facility is required. The facility should be designed specifically for handling and storing hazardous materials. A hood, preferably a walk-in hood, should be available for pouring liquids such as organic solvents. Space should be available to store material for decay. Shielding may be required for

13 I 94 storing some isotopes. Secondary containment should be provided for all liquids. Explosion proof fixtures may be required if very much flammable material is handled. Consideration should be given to the purchase of a compactor for solid waste destined for land burial. As land burial prices continue to increase this will become even more attractive. Volume may be reduced by a factor of two or three depending on the waste. An alternative is to send the waste to a vendor who will do the compacting. It is even possible to have the waste compacted by a super compactor that compacts the entire drum of waste into a "puck" which is then packed into another drum. Motors and other items usually not considered as good material for compaction can be compacted with these super compactors. Volume reduction factors of five or more are possible. If on-site compaction is chosen as a management procedure, it will be necessary to design the system to collect any liquid that may be in the waste and to prevent release of aerosols containing either radioactive or other hazardous materials. It may be necessary to place the compaction process under negative pressure and use HEPA and charcoal filters. Another volume reduction technique is to use a glass crusher. If all glass items are crushed, it may not be necessary to use compaction. This is particularly true where paper and plastic waste is incinerated and the primary solid waste is glass. Glass liquid scintillation vials may be crushed and the liquid scintillation fluid collected for -incineration or some other treatment. Liquid scintillation vials containing C14 and H3 may be crushed and the crushed glass disposed of in sanitary landfills. It may be necessary to rinse the crushed glass

14 95 to insure there is no solvent remaining on the glass. Machines are also available to slit plastic vials if an incinerator is not available to burn the intact vial. A possible volume reduction technique is evaporation. This may be particularly useful for aqueous waste. The primary problem is finding either a heat source to aid in evaporation or a space for large surface area evaporation. Monitoring must be done to detect volatile radioactive material that is released and procedures developed to trap the waste. The toxic and flammable nature of the solvents must be considered. Perhaps the best volume reduction technique is incineration. Volume reduction factors of 10 or more are common. Liquid scintillation fluid and animal carcasses and tissues containing less that 0.05 microcuries per gram of C14 or H3 may be incinerated without regard to radioactivity. In an efficient incinerator, all the C14 and H3 will be converted to carbon dioxide and water and released to the atmosphere. Calculations have shown that the amount of C14 and H3 released to the environment in this way is small compared to the amounts of naturally occurring C14 and H3 and to the amounts that are continually being created by cosmic radiation incident on the upper atmosphere. It is not necessary to monitor the stack or ash for these materials. The liquid auxiliary fuel or the vials may be incinerated intact. The vials will tend to melt and stick to each other fonning a glass slag which may complicate the ash removal process. By having a bed of ashes in the incinerator before burning the vials it is possible to keep the glass from sticking to the refractory. Care must be taken not to introduce

15 96 too much high energy containing solvent into the incinerator at one time since it is possible to starve the incinerator for air resulting in incomplete solvent burn. These conditions will lead to visible emissions and possible particulate emissions violations. If other radioactive materials or isotopes are incinerated it will be necessary to monitor the ash and monitor or calculate the concentrations of radionuclides in the stack effluent. NCRP Commentary No. 3, "Screening Techniques for Determining Compliance with Environmental Standards - Releases of Radionuclides to the Atmosphere" 11986), contains an excellent screening method. The method is simple and requires only a few calculations and reference to some tables. The procedures in the manual meet ALARA requirements that annual releases should be less than 10% of the concentrations in 10CFR20 appendix B. If isotopes such as P32 are incinerated much of the activity will remain in the ash and the ash will have to be treated as radioactive. For P32 it is usually therefore better to let it decay before incineration. Other isotopes may or may not be converted to a gaseous form on incineration. For example, in compounds containing S35 most of the sulfur will be converted to sulfur dioxide, however, some salts of sulfur may remain in the ash. If incineration is chosen as a method of treatment, the incinerator should be chosen with care. If an existing incinerator is to be used it may have to be modified. Many institutions have incinerators designed to burn pathological waste but these will normally have to be modified or adjusted to burn high energy content waste such as liquid scintillation fluid. The incinerator should be capable of operation at

16 97 about 2,OOOF and have sufficient residence time to insure the destruction of toxic compounds. Waste should be chosen carefully for incineration. It will not be economically feasible to incinerate all waste. Sealed sources, foils, stock solutions, etc. may be better treated by other methods. Aqueous waste will be expensive to incinerate since it will have little energy value and should be minimized. Non- combustible waste should also be minimized. Animal carcasses and waste, even though containing high moisture, should be incinerated. Solidification processes must be chosen that are acceptable for the $articular burial site. As noted previously, the Barnwell site will not accept any absorbed liquid waste whereas some of the other sites will. However, each site specifies what absorbents are acceptable. If only a few drums need to be solidified in something like cement, it is probably better to seek a vendor who will solidify the waste than to invest in an expensive solidification process. Disposal of material via the sewer can be done by the individual researcher or by the radiation safety office. For other than small amounts in rinse water it is probably easier to maintain records of release to the sewer if disposal is done by the radiation safety office. Also, the safety office is usually better equipped to retain those isotopes with short half-lives for a period before release. Conclusions The efficient management of radioactive waste is a complex and time consuming process which is made even more difficult by the changing

17 I 98 regulations and restrictions on land burial. Each institution has a unique waste stream and its waste management program must be designed specifically for that institution and its facilities. With the increasing cost in land disposal it is clear that investment in some waste processing equipment such as a compactor, glass crusher, or incinerator will be economically justified. Even small to intermediate size institutions should consider having a well-trained, full-time individual handling waste pickup and packaging. It is possible to contract for these services to be done by outside groups but someone within the institution should be quite familiar with the entire waste management program since the institution generating the waste will be held responsible for the proper handling and disposal of the waste. In order to make the waste handling procedures more efficent it is possible to combine the radioactive waste management with waste management of RCRA controlled and other hazardous materials. The final disposition of the waste must be kept separate but many of the collection and handling procedures are common to both waste streams.

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