제주의 토양 및 이끼에 존재하는 134Cs 및 137Cs 방사능 농도

Abstract

Some of the radionuclides, that had been released to the atmosphere from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, were transported to the Korean territory and began to be detected on March 28, 2011 at some regional environmental monitoring stations of Korea and continued to be detected until May 31, 2011 (Kim et al., 2012). The radioactivity of 131I, 134Cs and 137Cs in aerosols and rainwater reached peak values on April 6, 2011 at the Jeju monitoring station which is located in the northern region of Jeju Island. Jeju Island is the largest island of Korea located about 100 km distant in southwest direction from the southern end of the Koran peninsula. Radionuclides of the highly volatile elements I and Cs are released into the atmosphere in early stages of a nuclear reactor core melt-down accident and carried away long distance from the accident place, and thus they have been frequently used for tracing and evaluating the radiological impact from an nuclear power plant accident or an open air nuclear bomb test by which a large amount of radioactive fission products is released to the atmosphere (Aoyama et al., 1987, Molero et al., 1999, Pittauerova et al., 2011, Tagami et al., 2011). 131I has a very short half-life (? 8 d) and thus it can be traced only for a short period of time after an accident. The half-life of 134Cs is about 2 years so that it can be traced for several years after an accident. The half-life of 137Cs is long enough (? 30 a) to be traced for several tens of years. Tremendous amounts of 137 Cs were released into the atmosphere by open air nuclear bomb tests conducted during 50's and 60's and by the Chernobyl nuclear disaster occurred in 1986. Since the released 137Cs had spread out over the world and deposited on the ground, the most surface soils in the world generally have substantial inventory of 137Cs. Lee et al. (1998) reported the distribution of 137Cs in soils of many regions of Koran peninsula except for Jeju Island. As the time passes after the accident, radionuclides with short half-lives such as 131I are no longer measurable above minimum detectable limit (MDA). Instead 134Cs and 137Cs have become prime radionuclides used for assessing the radiological consequences of the Fukushima accident. Since both nuclides are chemically identical, the radioactivity ratio of 134Cs and 137Cs depends on the burnup of the damaged nuclear fuel. Hence it is specific to a particular accident and remains unchained if decay correction is properly made. The initial radioactivity ratio was 0.5 to 0.6 during the Chernobyl accident, and it was about 1.0 for the Fukushima accident. No attempt has been yet made to assess the radiological impact on the soil of Jeju Island from the FDNPP accident. About 7 months after the accident, we have selected 12 locations in Jeju Island and collected at each location a sample of surface soil and mosses growing on nearby stones and rocks. We measure specific activities of 134Cs and 137Cs in the surface soil and moss samples by gamma-ray spectrometry and estimate the amount of 137Cs in the soil originating from FDNPP accident. Activity profiles of 134Cs and 137Cs are obtained for a cored soil sample to see the migration behavior of both nuclides. Specific activities of 134Cs and 137Cs in the surface soil vary from less than MDA to 17 Bq/kg and from 12 Bq/kg to 109 Bq/kg, respectively. Specific activities of 134Cs and 137Cs in moss samples lie in the range 6 Bq/kg to 39 Bq/kg and 15 Bq/kg to 41 Bq/kg, respectively. The activity ratios 134Cs/137Cs in the soil samples are much less than the reference value of about 1.0, but they are close to 1.0 in the moss samples. Average amount of 137Cs added to the surface soil after the Fukushima accident is estimated to be 7.8 ± 1.7 Bq/kg. The depth profile of 137Cs specific activity has a lognormal shape with a peak between 5 cm and 7.5 cm below the ground. For the cored soil sample, 134Cs was detected up to 3 cm below the ground.