include both low- and high-level (e.g., spent nuclear fuels) radioactive materials combined with chemical hazardous waste. Common laboratory waste management methods for radioactive constituents in waste include storage for decay and indefinite on-site storage, burial at a LLRW site, incineration, and sanitary sewer disposal. Disposal options for mixed waste are usually very expensive, and for many types of mixed waste, there are no management options other than indefinite storage on-site.
8.C.1.1 Minimization of Mixed Waste
Rigorous application of waste minimization principles can often solve the problems of managing mixed waste. Such efforts are most successful when scientists and EHS staff work together to evaluate laboratory processes. A successful collaborative minimization initiative undertaken by the NIH Mixed Waste Minimization Program demonstrated that the ultraviolet peroxidation treatment of aqueous mixed waste could reduce or eliminate a large portion of the mixed waste generated in the NIH research laboratories. This treatment method degrades hazardous organic compounds in high-volume aqueous mixed waste streams. The removal efficiency for a number of volatile and semi-volatile compounds is in excess of 99.99%. The treated waste can be discharged to the sanitary sewer (Rau, 1997).
Modifying laboratory processes, improving operations, or using substitute materials are approaches that can achieve minimization of mixed waste. Examples of these approaches include the following:
• Use 2.5-mL scintillation vials (“minivials”) rather than 10-mL vials. Adapters are available for scintillation counters with 10-mL vial racks.
• Count phosphorus-32 (32P) without scintillation fluid by the Cerenkov method on the tritium (3H) setting of a liquid scintillation counter (approximately 40% efficiency); iodine-125 (125I) can be counted without scintillation fluid in a gamma counter.
• Use microscale chemistry techniques.
• Eliminate the methanol/acetic acid (chemical) and radioactive mixed hazards in gel electrophoresis work by skipping the gel-fixing step if it is not required.
• Line lead containers with disposable plastic or use alternative shielding materials to prevent lead contamination by radioactivity.
• Reduce the volume of dry waste by compaction of contaminated waste gloves, absorbent pads, and glassware.
Some simple operational improvements can help minimize mixed waste. Purchase chemicals and radioactive materials in quantities necessary for a planned experiment to avoid creating surplus materials that may end up as waste. Establish procedures that will prevent commingling radioactive waste with noncontaminated materials and trash.
Consider substituting a less-hazardous constituent for either the chemical or the radioactive source of the mixed waste. The experimenter should use the minimum activity necessary and select the radionuclide with the most appropriate decay characteristics. Examples include the following:
• Use nonignitable scintillation fluid (e.g., phenyl-xylylethane, linear alkylbenzenes, and diisopro-pylnaphthalene) instead of flammable scintillation fluid (e.g., toluene, xylene, and pseudocumene). LSF that is sold as being “biodegradable” or “sewer disposable” is more appropriately labeled as “nonignitable” because biodegradability in the sanitary sewer can vary considerably with the local treatment facility.
• Use nonradioactive substitutes such as scintillation proximity assays for phosphorus-32 (32P) or sulfur-35 (35S) sequencing studies or 3H cation assays, and enhanced chemiluminescence as a substitute for 32P and 35S DNA probe labeling and Southern blot analysis.
• Substitute enriched stable isotopes for radionuclides in some cases. Mass spectrometry (MS) techniques, such as inductively coupled plasma-MS, are beginning to rival the sensitivity of some counting methods. Examples include use of oxy-gen-18 (18O) and deuterium (2H) with mass spectrometry detection as substitutes for oxygen-19 (19O) and 3H.
• Substitution of shorter-half-life radionuclides such as 32P (t1/2 = 14 days) for phosphorus-33 (33P) (t1/2 = 25 days) in orthophosphate studies, or 33P or 32P for 35S (t1/2 = 87 days) in nucleotides and deoxynucleotides. In many uses, iodine-131 (131I) (t1/2 = 8 days) can be substituted for 125I (t1/2 = 60 days). Additional exposure precautions may be required.
8.C.1.2 Safe Storage of Mixed Waste
Store waste containing short-half-life radionuclides for decay prior to subsequent waste management processing and disposal. On-site decay-in-storage of LLW is very efficient and minimizes handling and transportation risks. Most institutions designate a room or facility equipped with good ventilation, effluent trapping, and fire suppression to contain and manage on-site