Radioecological Effects of Radionuclides after the Chernobyl Accident

The presence of radioactivity in the environment has been indirectly affected by nuclear weapons tests, but significant accidents from nuclear reactor plants, such as at Three Mile Island in the United States in 1979 and at Chernobyl in Ukraine in 1986, can also greatly increase radioactivity in the atmosphere. The Chernobyl accident is the most serious to have occurred in the history of nuclear reactor operation. The Chernobyl accident occurred at 01:23:48 hours local time on 26 April 1986 at Unit No. 4 of the plant. The reactor continued to burn for several days. After about 10 days, the fire was effectively smothered by the large quantities of sand and other materials dropped on the reactor. Various radionuclides were released to the environment in a period of 10 days, which resulted in a wide dispersion of radionuclides over the globe.

Right after the Chernobyl accident, the shorter-lived radionuclides settled on the ground in a 30-km zone through gravitational settling, whereas the longer-lived 90Sr, 137Cs, and plutonium isotopes remained in the atmosphere and were transported over wider areas through the turbulent transfer and the large-scale air flows. A variety of air sampling methods were used to measure air concentrations of radionuclides, and a network of air monitoring stations were maintained in many nations.

Many sites throughout the world had also been established to monitor the deposition of radionuclides on the Earth's surface. A wide range of atmospheric transport and deposition models as well as soil and biological models were used to investigate the various processes that distributed the radioactive plume released from the Chernobyl site.

The Chernobyl accident transported a significant fraction of the radioactivity that spread in the atmosphere of the whole globe after 10 days as shown in Figure 4. Clearly, the radioactivity was detected by many countries. Figure 5 shows the distinguished spikes of 137Cs and

Figure 4 Model-calculated spatial distribution of radioactivity over the Northern Hemisphere 10 days after the Chernobyl accident. From Warner F and Harrison RM (1993) Radioecology after Chernobyl: Biogeochemical Pathways of Artificial Radionuclides, SCOPE 50. Chichester: Wiley.

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

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Figure 5 Monthly variations of 90Sr and 137Cs depositions observed at Tokyo and Tsukuba areas in Japan. From Yasuhito I, Michio A, Katsumi H, etal. (2003) Resuspension: Decadal monitoring time series of the anthropogenic radioactivity deposition in Japan. Journal of Radiation Research 44: 319-328.

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Year

Figure 5 Monthly variations of 90Sr and 137Cs depositions observed at Tokyo and Tsukuba areas in Japan. From Yasuhito I, Michio A, Katsumi H, etal. (2003) Resuspension: Decadal monitoring time series of the anthropogenic radioactivity deposition in Japan. Journal of Radiation Research 44: 319-328.

90Sr depositions detected and measured at Japan in 1986 from the accident.

The accident has produced new data for us to examine the processes in detail. For example, the soil samples collected in 1993 at Pogonnoe, Belarus, a location 20 km north of the Chernobyl nuclear power plant, have been used for measuring the chemical properties of radionu-clides. The results have indicated that 129I, 137Cs, and 239 + 240Pu have migrated into soil with identical rates of distribution in depth of soil. It seems that the process of penetration into soil is physical, rather than chemical, for radionuclides. However, the penetration process is complicated if the radionuclides deposit over a long period of time, such as from the global fallout during nuclear weapons tests. The uptake of radionuclides by plants through roots has to be taken into account during the penetration process over a long term. Therefore, the studies at the atolls in the Marshall Islands mentioned earlier are unique, as far as the global fallout is concerned. The impacts of Chernobyl accident to the ecological environment still remain in some countries, for example in the upland of United Kingdom, after initial occurrence of accident 20 years ago.

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