Chemical treatment methods show potential for stabilizing contaminants
PNNL study identifies three treatment methods
PNNL researchers have identified three promising treatment methods for cost-effectively stabilizing residual waste in Hanford waste storage tanks and reducing the release of technetium, chromium and uranium. This image shows where tank waste resides and examples of typical tank waste particulates. Enlarged view
Researchers from Pacific Northwest National Laboratory recently completed a study that reveals the optimal chemical treatment methods for reducing the release of three significant contaminants from residual waste in underground storage tanks at the Hanford Site. The Site’s underground tanks contain a large amount of waste associated with the production of nuclear weapons.
Why it matters:
Chemical treatment approaches that reduce contaminant release could allow larger volumes of residual waste to remain in the tanks. These inexpensive methods could reduce the cost and effort associated with waste retrieval operations while minimizing the overall level of risk to the environment and human health.
During the Cold War era, the Hanford Site in southeastern Washington State produced two-thirds of the plutonium generated in the United States for nuclear weapons. Today, 177 underground storage tanks contain spent fuel reprocessing wastes that must be stabilized for final disposal. Although most of the waste stored in these tanks is expected to be recovered and treated, current retrieval methods are expensive and time-consuming. Moreover, these methods would leave a one-inch thick layer of contaminated slurry at the bottom of each tank.
To address this problem, PNNL researchers evaluated three different chemical treatment methods for stabilizing residual waste in a Hanford tank and reducing the release of technetium, chromium and uranium. These treatment methods, which reduce the release of contaminants by converting them into insoluble forms, consisted of the addition of lime on top of tank residual waste, the addition of ferrous iron and goethite (an iron oxide mineral), and the use of Ceramicrete technology, which is based on chemical reactions that immobilize contaminants within concrete-like waste forms.
Researchers found that all three stabilization methods reduced the leachable concentrations of uranium and technetium to values well below their respective maximum contaminant levels set by the Environmental Protection Agency. The addition of lime was the most effective method for limiting uranium release, allowing only 0.001% of the total mass of uranium to leach from the tank. The goethite and Ceramicrete methods were also effective, limiting uranium release to less than 0.004-0.05% of the total mass.
The goethite and Ceramicrete methods were the most effective approaches for limiting technetium release, allowing about 6% of the total mass of technetium to leach from the tank. By contrast, about 10% of the technetium was leached after the addition of lime and nearly 30% was leached from the untreated tank sample. Ceramicrete was the most effective method for chromium, but all three methods reduced chromium release to less than 5% of the total mass, compared to 23% that was released from the untreated waste. The findings suggest that the optimal treatment method depends on the type of contaminant that is most problematic for a particular tank waste, although Ceramicrete may have the broadest applicability.
Further testing with other tank waste samples is needed to evaluate the applicability of these approaches to a more broad range of tank waste compositions.
This research was supported by the Laboratory Directed Research and Development program within PNNL.
Cantrell, K.J., Um, W., Williams, B.D., Bowden, M.E., Gartman, B., Lukens, W.W., Buck, E.C., and Mausolf, E.J. “Chemical stabilization of Hanford tank residual waste.” Journal of Nuclear Materials. (2013). [DOI: 10.1016/j.jnucmat.2013.10.060].