Abstract: Freeze-drying was found to be an acceptable technique for converting radioactive animal carcasses into radioactive dry solid waste and radioactive aqueous by-product.  Health Phys. 69(1):115-116; 1995

Key words: waste management; waste disposal; waste storage; radioactivity

RADIOACTIVE animal tissue and carcasses constitute a major and sometimes troublesome radioactive waste stream in biomedical research facilities. Animal carcasses containing less than 1.85 X 10³ Bq g^-1 (0.05 µCi g^-1) of ¹⁴Cor ³H can be treated as nonradioactive animal carcasses (U.S. NRC 1989). For animal carcasses containing higher concentrations of ¹⁴C or ³H, or animal carcasses containing other radioactive materials, other methods of disposal must be used.

The most common methods used for the disposal of radioactive animal carcasses has been either packaging the animal carcasses in an absorbent medium, in drums, to be shipped for burial at a licensed commercial burial site, or by incineration.

Currently, commercial burial is not an option for many licensees (Vetter 1992). Incineration has its limitations and restrictions and in many locations is not permissible.

Animal carcasses containing large amounts of radioactivity might have to be frozen until the radioactivity has decayed (Emery et al. 1992) before incineration.

After incineration, the radioactive ashes must be collected, analyzed, and disposed of by an appropriate method of disposal.

One recently reported method for processing radioactive animal carcasses involves dry-distillation (Miyataka et al. 1989). In this process mice are dry-distilled at 800°C for 10 min and the evolved water is then distilled. Another method uses animal digestion vessels where animal carcasses are converted into digestate for disposal via the sink (Internal memorandum by Kaye, G. 1., 1994. Gordon 1. Kaye, Albany Medical College, 47 New Scotland Avenue, Albany, NY 12208).

The objective in this study was to investigate the potential application of freeze-drying for treating radioactive animal tissues and carcasses. Freeze-drying has the potential to convert tissues and carcasses into radioactive dry solid waste and radioactive contaminated water where both are easier to handle and process for disposal.

Many biomedical research laboratories use commercially available freeze-dryers in their research for the lyophilization of DNA solution. For this study, one of these available freeze-dryers (FREEZEMOBILE 24, The VirTis Co., Inc., Gardiner, NY 12525) was used. For testing purposes and so as not to contaminate the freeze-dryer, non-radioactive animal carcasses were used.

The operator's manual (VirTis Operator's Instruction Manual, The VirTis Co., Inc. Gardiner, NY 12525 (1981) for the instrument used describes the freeze-drying process as consisting of four basic steps:

1. Freezing: Rapid cooling of the product to a point below its eutectic. (A compound's eutectic is the common freezing point for all of its components.)
2. Vacuum: Removal of air and other noncondensable vapors from the chamber to facilitate vapor migration.
3. Heating: Carefully controlled heat input to the frozen product to speed the drying process.
4. Condensing: Trapping of water vapor molecules in the form of ice on the condenser surface.

The freeze-dryer that was used had 16 ports where 16 chambers could be connected and used simultaneously.

Freeze-drying of the animal tissues, parts, and carcasses converted them into dry solid waste and water. The dry solid waste that resulted from the freeze-drying process was reduced to 28% to 35% of the original animal weight and the moisture content of the solid material was estimated to be less than 3%. After several months of storage of the resulting dry solid material at room temperature, there were no detectable changes in texture or composition.

Freeze-drying is a viable option for the processing of animal tissues and carcasses containing radioactive materials. The animal carcasses are converted into solid dry waste and waste water. The solid dry waste is more convenient to handle and can be shipped for burial without the need for an absorbent material. If the processed tissues or carcasses lend themselves to decay-in-storage, they can be stored at room temperature, thus eliminating the need for a freezer.

Future studies will be performed to determine the optimum time required for obtaining the minimum residual moistures in the samples and to determine the distribution of the radioactive materials between the solid phase and the liquid phase for different isotopes and compounds. In this study, the size of the specimens used was limited by the size of the chamber. For larger animals, a larger size instrument can be used, or the animals can be dissected into smaller parts.