7.C.1.2 Safe Storage of Mixed Waste

Waste containing short-half-life radionuclides should be stored for decay prior to subsequent waste management. The U.S. NRC refers to storage for decay as "decay-in-storage." Because on-site storage of low-level waste is very efficient and minimizes handling and transportation risks, laboratories and institutions should provide the space and safe storage facilities for decay-in-storage. In most cases, safe storage requires a designated room or facility that has been modified to contain the waste, protect workers, minimize the risk of a fire or spill, and minimize radiation levels outside the area. Proper ventilation and effluent trapping are critical needs for such a facility. Storage of mixed waste in the laboratory is not recommended because the required level of protection is difficult to achieve in a working area. Storage of such waste is not recommended when the waste may become more difficult to handle with age, such as with certain biological, putrescible, or reactive materials.

The specific U.S. NRC requirements for decay-in-storage of radioactive waste are usually detailed in the institution's license. Decay-in-storage is usually limited to half-lives of less than 65 days (although half-lives of up to 120 days are routinely approved by the U.S. NRC), but includes many of the radionuclides used in biomedical research. When the short-half-life radionuclides have decayed to background levels (the length of time depending on the initial radioactivity level but typically defined as a storage period of at least 10 half-lives), the chemical-radioactive waste can be managed as a chemical waste. After the decay period, U.S. NRC licenses usually require that the mixed waste be surveyed for external radiation prior to releasing it to the chemical waste stream.

EPA requirements for decay-in-storage of mixed waste have varied over time and by state and EPA region. Storage of mixed waste for more than 90 days, the period of time usually allowed for chemically hazardous waste, may require the approval of the state or regional EPA hazardous chemical waste authority. In permitted storage facilities, storage may be limited to 1 year for some types of mixed waste. Workers should contact their institution's environmental health and safety staff or local hazardous waste agency to determine their regulatory status and requirements for storing mixed waste for decay.

7.C.1.3 Hazard Reduction of Mixed Waste

Chemical hazards can be reduced by carrying out various common chemical reactions with the waste in the laboratory (see also section 7.B.5 and 7.D). However, "treatment" of chemically hazardous waste has regulatory implications that must be considered. Many of the same considerations apply to treatment of mixed waste.

Nevertheless, there are still justifiable and legal reasons to carry out such operations in the laboratory when hazards can be reduced safely. Neutralization, oxidation, reduction, and various other chemical conversions as well as physical methods of separation and concentration can be applied prudently to many laboratory-scale mixed wastes. However, the dual character of the hazard, chemical and radioactive, requires that additional precautions be exercised. Treatment for the chemical hazard must not create a radioactivity risk for personnel or the environment. For example, vapors or aerosols from a reaction, distillation, or evaporation must not lead to escape of unsafe levels of radioactive materials into the atmosphere. Fume hoods appropriate for such operations should be designed to trap any radioactive effluent. When mixed waste is made chemically safe for disposal into the sanitary sewer, the laboratory must ensure that the radioactivity hazard is below the standards set by the publicly owned treatment works (POTW). Several examples for reducing the hazard of mixed waste are described below:

  • The worker can reduce the chemical hazard to a safe level and then handle the material as only a radioactive hazardous waste. Many low-level radiation materials can then be allowed to decay to a safe level, following which simple disposal is allowable.

  • Some trichloroacetic acid (TCA) solutions contain chloroform in excess of 6 ppm. Such a solution is considered a hazardous chemical waste because it fails the TCLP test. If the neutralized solution is not acceptable to the sewage treatment plant because of the presence of chloroform, it may be possible to remove that component from the solution by filtration through activated charcoal. The resulting radioactive filtrate can usually be disposed of in the sanitary sewer, and the contaminated charcoal can usually be disposed of as a chemical waste.

  • Some radioactive methanol-acetic acid solutions from gel electrophoresis can be recycled via distillation and the methanol reused. The solution is neutralized prior to distillation to protect the distillation equipment from corrosion and to reduce the level of methyl acetate formed during the process.

  • The volume of waste phenol, chloroform, methanol, and water containing radionuclides can be reduced by separating the nonaqueous portion using a separatory funnel. After separation, the organic phase can be distilled to produce chloroform waste,

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