Characterization of Microbial Survivall in the Presence of Radioactive 14C Spiked Salts in Order to Minimize W,aste from Graphite Moderated Nuclear Reactors

Abstract

The Pebble Bed Modular Reactor (PBMR) is a high temperature, Gas (HTG) (helium) cooled reactor, which uses graphite as a moderator and a neutrons reflector. During the reactors operation, graphite is irradiated and contains radionuclides, such as actinides, fission and activation products. This study investigated the removal of 14C in irradiated graphite. Two experiments were performed. In the first experiment, non-irradiated graphite was exposed to a Mixed Culture (MC) of bacteria for 120 hours. The objective was to determine if bacteria thrives in the presence of inert graphite. Techniques used for graphite analysis were Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). SEM was used to verify the presence of bio-film on the graphite surface, which would indicate bacteria survival. XRD was used to determine whether there was a change in the graphite crystallographic structure, indicating possible metabolism of graphite carbon by the bacteria. A second experiment was conducted with 14C-containing salts chosen to represent the chemical form of 14C on the surface of irradiated graphite. The MC bacteria were exposed to these salts for 120 hours as well. The bacteria samples were analysed using Denaturing Gradient Gel Electrophoresis (DGGE). The objective was to determine the diversity of the bacteria thriving in radioactive carbon species. The results obtained from the analyses were as follows; SEM results indicated that bio-film is formed on the surface of the graphite, which indicated that bacteria can survive in the presence of graphite. The XRD results indicated that there was no change in the crystallographic structure of graphite whether or not there is bio-film formation. The DOGE results indicated that there were bacterial species that survive in 14C containing salts. When comparing the DOGE results of both salts, sodium bicarbonate was more dominant than sodium acetate in bacterial survival.