Abstract
The purpose of this study is to evaluate the effect of microorganisms on the corrosion of engineering barriers for low-level radioactive waste (LLRW) and their ability to adsorb radionuclides in the warm and humid climate of Taiwan. Native microbial strains were sampled from outside and inside Lanyu repository and were isolated and identified to investigate their radiation tolerance, adsorption of radionuclides and effect on the corrosion of engineering barriers.The results thus obtained revealed that microbial activity is high both inside and outside Lanyu repository, where with microbial contents in water and soil samples range from 101~106 CFU/mL and 103~107 CFU/g, respectively. Additionally, the microbial contents on the surface of LLRW containers in Lanyu repository were in the range of 101 ~103 CFU/g. Of the microorganisms that were sampled inside and outside of Lanyu repository, 28 species, including bacteria, yeast, actinobacteria and fungi, were isolated for use in subsequent experiments.The isolated fungal strains can tolerate wider range of pH values than isolated bacteria or actinobacteria: even in a harsh medium at pH 10, their growth and metabolism reduces the pH of the medium to provide a suitable environment in which other strains can grow. Furthermore, a significant variation in the resistance of isolates to radiation was observed, with D10 values in the range 0.15 kGy~2.05 kGy for the isolated strains. All isolates could grow in the environment with the dose rate of 3.4x105 μSv/h. This dose rate greatly exceeded the dose rate on the surface of container. This demonstrated that the isolated local strains could survive under radiation in the LLRW repository. The results of the corrosion tests using the strains of interest that were cultured for 135 days and a galvanized carbon steel specimen revealed that all of tested strains produced notches of various sizes on the specimen. Interestingly, the PEN1 strain decomposed the galvanized surface layer and exposed the carbon steel substrate; the SEM-EDS results showed that the relative zinc content of the surface was 5.26±0.69 wt% and the iron content was 94.74±0.69 wt%. Therefore, the PEN1 strain released zinc from the galvanized carbon steel specimen into the culture medium, and the released zinc ions became accumulated in biomass. The electrochemical test results revealed that the rate of corrosion of the tested specimens varied with the microbial strains, and the specimen which cultured with PEN1 strain was corroded at the highest rate of 144.05 μm/year.The results of the corrosion test that involved 270 days of culturing of tested strains on solidified cement revealed growth of the strains, accompanied by a reduction of the pH of the medium below 10. The growth of the tested strains accelerated the release of calcium from the solidified cement. However, this release of calcium had no significant influence on compressive strength. All of the tested strains adsorbed cobalt, strontium and cesium. Their adsorption capacity depended strongly on the species of the strain and the metal. The CAN strain had the greatest capacity to adsorb cobalt, with an adsorption coefficient of 234.77±12.18 μg/g and a Kd value of 5186.42±174.40. This study demonstrated that local microbial strains can grow in the environment of LLRW disposal repository, and thier growth and metabolism may affect the function of engineering barriers of LRRW repository; moreover, isolated strains have adsorption capacity for radionuclides. Therefore, microbial effect must be considered as a safety assessment for the final disposal of LLRW in Taiwan. Keywords: Microorganism, Low-level radioactive waste, Bio-corrosion, Radionuclide sorption