Differences in the Behavior of 233Pa, 237Np and 239Pu in Bentonite Contaminated by Sulfate Reducing Bacteria

Abstract

The behaviors of 233Pa, 237Np and 239Pu in high level radioactive wastes from nuclear fuel reprocessing were investigated by a laboratory experiment. Radioactive wastes are glassified and disposed of in geological repositories encased in bentonite as an additional artificial barrier to protect the environment. There is, however, the possibility that some anaerobic bacteria, especially sulfate-reducing bacteria, may flourish within the bentonite during the long disposal period (more than a century). The effects of sulfate-reducing bacteria on the behavior of the radionuclides within bentonite were investigated using the distribution coefficient (Kd) of 233Pa, 237Np and 239Pu. The Kd was obtained with a 0.22 m membrane filter separating radionuclide contents in solid and liquid phases. The anaerobic bacteria, including sulfate-reducing bacteria, used for this investigation originated from the anaerobic treatment of pulp and paper waste and operated for more than one year at Eh around -85 mV. The bentonite used for this study was produced in Japan. The active anaerobic bacteria clearly accumulates considerable amounts of 233Pa and 239Pu by producing high Kd values of nearly 100,000, while Kds of 233Pa and 239Pu for the sterilized anaerobic bacteria were less than 10,000. In other words, live anaerobic bacteria can hold considerably higher amounts of the radionuclides compared to dead bacteria. Furthermore, high Kd values were obtained for anaerobic bacteria at pH 5-9. In contrast, Kd values for the radionuclide 237Np were not influenced by the anaerobic bacteria but were controlled by chemical environmental conditions such as like pH. Another comparison was conducted for the radionuclides for mixtures of non-sterilized bacteria with bentonite.