Fukushima Daiichi crisis: fallout
Contamination outlook20 September 2011
Air- and land-based radiological monitoring data since mid-March show rapid initial decreases in contamination. However, levels are now stabilising and are likely to remain high for years. By Tadahisa Nagata and Toshihiro Okajima
Radiological data collected by TEPCO and local governments spiked on 15 March, the day after the unit 3 explosion, and the day of the unit 2 explosion [Figure 1]. Dose rate at Iidate, Japan (red line) was particularly high, although the dose has declined since then. Since late April, other higher-radiation areas, including Namie, have been added to the graph.
Although radiation in the worst-affected areas, such as cities in Fukushima prefecture, is declining, the decrease of radioactivity in May has been very slow [Figure 2]. (For reference, the IAEA’s maximum public dose limit for radiation from a man-made source is 1 mSv/yr. This averages out at 0.114 µSv/hour over a year).
During the Chernobyl accident, many types of radionuclides were released and detected around the stations. They are classified into four types.
(1) Noble gas: Kr-85, Xe-133
(2) Volatile nuclides: I-131, Cs-134, Cs-137
(3) Moderately-volatile nuclides: Sr-90, Ru-106
(4) Non-melting nuclides: Zr-95, Mo-99, Ce-144, Pu-241.
At Chernobyl, the spread distance depended on the size of the released particles and properties of compounds. The size of particle may be classified into three classes as follows:
(a) Fuel which broke into pieces
(b) Sub-microscopic particles that condensed after evaporation
(c) Noble gases.
Item (a) includes (3) Sr-90 and (4) U/Pu. The diameter of these particles are more than 10 microns and their densities are very high: 8-10g/cm3. Therefore, the amount of fallout particles decreased markedly with increasing distance from the NPP. Almost all of them fell at the site. Items (b) and (c) included noble gas and the volatile element/compounds. The wind blew these particles long distances.
For the first two months, I-131 held the majority of influence on the radiation dose in areas far from the Chernobyl plant. After that, Cs-134 and Cs-137 had the most influence. More than 10 years later, Cs-137 and Sr-90 came to have the greatest influence.
At Fukushima, the Japanese government Ministry of Education, Culture, Sports, Science, and Technology (MEXT) measured soil radiation in areas including highly-affected areas such as Iidate and Namie. MEXT detected many nuclides, including
Ag-110m, Cs-134, Cs-136, Cs-137, I-131, La-140, Nb-95, Te-129m and Te-132. However, the half-lives of these elements are very short; only three are longer than a month: Ag-110m (250 days), Cs-134 (2 years), Cs-137 (30 years).
In addition, the proportion of Cs-134, Cs 137 and I-131 was very large (more than 80%). Table 1 shows the proportion of Cs-134/137 to I-131 in the high radiation areas (survey date: 8-19 May).
Based on the experience of Chernobyl, the main radioactivity at a distance of more than 10 km from Fukushima Daiichi NPP may be
I-131, Cs-134 and Cs-137. Other detected radionuclides have low concentrations and relatively short half-lives. Already, I-131 concentrations have declined. But the two caesium radionuclides, which have longer half-lives, have not declined. Therefore, a substantial decrease of radiation in the near term is not expected.
Tadahisa Nagata, general manager, strategic planning office, Japan Nuclear Technology Institute, 7F NOF Shiba Bldg., 4-2-3 Shiba, Minato-ku, Tokyo 108-0014, Japan; Toshihiro Okajima, project leader, information dept, Japan Atomic Industrial Forum, email@example.comTablesTable 1: Ratio of Cs-134/Cs-137 to I-131