Properties and Behavior of Radionuclides
For the purposes of this document, radionuclides should be considered to have properties similar to those of other heavy metals.This does not imply that all radionuclides are heavy metals, but that the majority of sites requiring remediation of radioactively contaminated materials are contaminated with radionuclides that have similar properties. Like metals, the contaminants of concern are typically nonvolatile and less soluble in water than some other contaminants. However, the solubility and volatility of individual radionuclides will vary and should be evaluated for each wastestream being remediated. For example, cesium-137 is more volatile than uranium-238 and some cesium may volatilize, requiring off-gas treatment, when treated with processes at elevated temperatures (e.g., vitrification). Similarly, the mobility of radium-226, which is generally soluble in water, will be greater than that of thorium-230, which is much less soluble and poses a significant challenge for separation technologies.
Unlike organic contaminants (and similar to metals), radionuclides cannot be destroyed or degraded; therefore, remediation technologies applicable to radionuclides involve separation, concentration/volume reduction, and/or immobilization. Some special considerations when remediating sites contaminated with radionuclides include the following:
Implementation of remediation technologies should consider the potential for radiological exposure to workers (internal and external) and the technologies themselves. The degree of hazard is based on the radionuclide(s) present and the type and energy of radiation emitted (i.e., alpha particles, beta particles, gamma radiation, and neutron radiation). The design should take into account exposure considerations and the principles of keeping exposures as low as reasonably achievable (ALARA).
Because radionuclides are not destroyed, ex situ techniques will require eventual disposal of residual radioactive wastes. These waste forms must meet disposal site waste acceptance criteria.
There are different disposal requirements associated with different types of radioactive waste. Remediation technologies addressed in this document are generally for low-level radioactive waste (LLW), transuranic waste (TRU), and/or uranium mill tailings. The technologies are not applicable to spent nuclear fuel and, for the most part, are not applicable for high-level radioactive waste.
Some remediation technologies result in the concentration of radionuclides. By concentrating radionuclides, it is possible to change the classification of the waste, which impacts requirements for disposal. For example, concentrating radionuclides could result in LLW becoming TRU waste (if TRU radionuclides were concentrated to greater than 100 nanocuries/gm with half-lives greater than 20 years per gram of waste). Also, LLW classifications (e.g., Class A, B, or C for commercial LLW) could change due to the concentration of radionuclides. Waste classification requirements, for disposal of residual waste (if applicable), should be considered when evaluating remediation technologies.
Disposal capacity for radioactive and mixed waste is limited; in addition, there is no storage currently available for mixed TRU. For example, commercial LLW disposal capacity is no longer available for many out-of-compact (regions without a licensed LLW disposal facility) generators because the disposal facility in Barnwell, SC, closed (to out-of-compact generators) on 30 June 1994. Currently there is only one disposal facility (Envirocare of Utah, Inc.) licensed to accept mixed waste (i.e., low-activity mixed LLW and hazardous waste) for disposal. Mixed waste can be treated to address the hazardous characteristics of the soil, thereby allowing the waste to be addressed as solely a radioactive waste.