Radiocesium in Jeju Island

Mun Sek Kim
March 25, 2021

Submitted as coursework for PH241, Stanford University, Winter 2021


Fig. 1: Jeju Island in South Korea. (Source: Wikimedia Commons)

The Fukushima Daiichi nuclear power plant accident happened on March 11, 2011, in the northeastern region of Japan. [1] The tsunami caused by a 9.0 magnitude earthquake damaged the power plant, which then led to an explosion that released large amounts of radioactivity into the environment that spread worldwide. [2] It has been reported that from March 11 to March 16 in 2011, amounts of 1.6 × 1017 Bq for I-131, 1.8 × 1016 Bq for Cs-134, and 1.5 × 1016 Bq for Cs-137 (all of which are all solids at room temperature) were discharged as dust particles into the Fukushima atmosphere. [2] The radioacitive discharge was 10~15% that of the Chernobyl nuclear plant accident. [2,3] The released radionuclides then propagated across the Pacific, where they were measured and reported. [4-14]

Radiocesium in Aerosols and Rainwater

Fig. 1 shows Jeju Island in South Korea, the province of South Korea located closest to Japan. Fukushima-derived radionuclides were detected in the island in 2011.

Aerosol samples around the island were collected and measured between April 6th and 7th in 2011. The measured concentrations of I-131, Cs-134, and Cs-137 were 0.887 mBq m-3, 0.541 mBq m-3, and 0.550 mBq m-3, respectively. [2] To put this number in perspective, a Bq is 1 nuclear decay per second, so radioactivity of 1 mBq m-3 translates to 1 γ ray emitted every 100 seconds (17 minutes) from a cubic meter of sampled air. Also, rainwater samples showed I-131, Cs-134, and Cs-137 concentrations of 2.81 Bq L-1, 1.67 Bq L-1, and 2.02 Bq L-1, respectively. These concentrations were about 5 times higher than the detection level for the entire Korean region. [15]

Radiocesium in Soils

Fukushima-derived radiocesium was detected in the surface soil. The deposited radiocesium was measured by sampling the soil using the stainless steel sampler. [2] The radionuclides get into the soil by raining out onto the soils, so the radionuclides only penetrates the soil as frar as the rainwater does. Hence, all the radiocesium exists in the surface soil. The results showed that the Fukushima-derived radiocesium was evident for Cs-134, whereas the Cs-137 appeared to originate from a number of sources - the Fukushima accident, past atmospheric nuclear testing, and the Chernobyl nuclear accident. [2,16]

Based on five sampling sites on Jeju Island, the mean activity concentrations of Cs-134 and Cs-137 were 11.8±2.0 Bq m-2 and 13.6±2.3 Bq m-2. [2] The contributions to the Cs-134 deposition from rainwater and fallout were 16.0 Bq m-2 and 18.8 Bq m-2, respectively. [2] Also, the total amounts of radiocesium including both Cs-134 and Cs-137 were 25.4 Bq m-2 from the soils and 34.8 Bq m-2 from the rainwater and fallout. [2] As it is difficult to exactly distinguish the radioactivity of Cs-134 between the rainwater and fallout, as some could be contained in dust particles and some in dissolved ions in the surface ground water, the measurements were estimated combining the results from each measurements of the rainwater and fallout from March to December, 2011. [2]

Other Radionuclides in Rainwater

Gamma-emitting radionuclides in a 10.66 L rainwater sample in Jeju Island were collected and observed on April 7th in 2011 after the Fukushima accident. [2] From the measurements, both short-lived γ-emitting radionuclides (La-140, Ag-110m, Nb-95, Sn-113, Te-129, Te-129m, Te-132, and I-132) and long-lived γ-emitting radionuclides (I-131, Cs-134, and Cs-137) were detected. [2] The presence of both kinds of radionuclide indicates that the source of the radioactivity was Fukushima.

© Mun Sek Kim. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


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