Dispersion and transport of radioactive materials

24.1.1 Regional dispersion

According to the May 24, 2012 press release from TEPCO, radioactivity levels of noble gases, iodine-131, cesium-134, and cesium-137 released into the air as a result of the Fukushima Daiichi NPP accident from March 12-31,2011 were ~5 x 1017,5 x 1017,1 x 1016 and 1 x 1016Bq, respectively [5]. Since the devices capable of directly measuring the levels of radioactive materials (such as the exhaust stack monitor) were damaged in the accident, the above-mentioned release amounts were primarily estimated through computer simulation, based on the observed air dose rates, wind direction and wind speed. These observed data were either acquired by the monitor­ing cars near the power station or provided by the Japan Meteorological Agency, and an over-simplified assumption of constant release rates of radioactive nuclides was often made in these simulations.

Since the beginning of the accident, radiation level measurements for a variety of environmental matrices at inland and coastal locations near Fukushima have been monitored by MEXT [10]. This comprehensive data­base has been serving as an important source for studies attempting to simulate the dispersion and transport of the released radionuclides. Starting on March 11, 2011, IRSN participated in the analysis of developments and probable radiological consequences of the Fukushima Daiichi NPP acci­dent. With the calibration by local monitoring data, IRSN used the three­dimensional IdX model of its C3X platform to model the atmospheric dispersion of released radionuclides at the regional level (several hundred to several thousand km) and reconstructed the release history of radioac­tive materials as follows [11] . The first releases, occurring between March 12 and 14, 2011, spread mainly northward, then northeast and east, over the Pacific Ocean. On March 15 and 16, the radioactive releases from unit 2 spread over Japan, but the weather conditions were changing rapidly. On March 16 and the following days, the releases spread eastward, moving over the Pacific and sparing most of Japan. Between the afternoon of March 20 and 23, the radioactive releases again spread over Japan. After March 23, the contaminated air masses moved toward the Pacific. The subsequent releases have been too low to cause a significant increase in the radioactiv­ity in Japan’s terrestrial environment.

During the spread of the contaminated air, a portion of the airborne radionuclides in the form of very fine particles (aerosols) or soluble gases (such as the portion of radioactive iodines) were deposited on the ground either in the form of dry deposition or wet deposition. Dry deposition occurs when the radioactive plume directly or indirectly (by first incorporating into dust or smoke) falls to the ground, while wet deposition means that the radioactive plume combines with water droplets or snow first before falling to the ground. Both types of deposition contributed to the spread of radio­nuclides over the terrestrial environment of Japan. Similar deposition mech­anisms also occurred in areas outside of Japan when the contaminated air dispersed over the world. A unique situation presented by the Fukushima accident is the overlap of a release phase with a fallout phase beginning on March 16 [11] . The release phase lasted for 12 days starting on March 12 with a threat of new releases lasting for at least several more weeks. In addi­tion to the immediate risk of exposure to the radioactive plume in the release phase, the fallout phase also became significant after March 16 owing to the fallout from the first atmospheric contamination event.

To understand the widespread effects of contamination by radioactive material, and to assess doses and the deposition of radioactive materials for future evacuation zones, the MEXT and US Department of Energy (US DOE) jointly performed airborne monitoring, checking the air dose rate 1 m above the ground surface within 80 km of Fukushima Daiichi NPP and the deposition of radioactive materials on the ground surface [12]. The map of air dose rates at 1 m from the ground surface measured between April 6 and 29, 2011, is shown in Plate X (a) (between pages 448 and 449), which represents the radioactive dispersion around Fukushima. After both dry and wet deposition of contaminated air, the total deposition of radioactive cesium-137 and cesium-134 in the soil surface (Plate X (b)) were found to be similar to the radioactive dispersion.

The radioactive materials in the air not only polluted the terrestrial ground surface, but also contaminated the surface waters dozens of kilome­tres from the NPP. This is probably the main source of the observed radio­active pollution in seawater (cesium-137 and iodine-131 concentrations of 2-27 Bq/L and 3-57 Bq/L, respectively) 30 km offshore from the damaged power plant before March 30, 2011 [11] . However, in addition to the air­borne spreading mechanism, more serious pollution was caused by the leaking of water which was used to cool the damaged reactor. In particular, a crack in the pit adjacent to the unit 2 turbine hall led to the release of heavily polluted water directly into the sea. On April 6, 2011, at approxi­mately 6 a. m. local time, TEPCO successfully stopped this release by plug­ging the leak with an injection of sodium silicate. Current estimates of direct marine release are usually based on the quantification of this leakage incident.

Since the half-life of cesium-137 is much longer than that of iodine-131, after the serious radiation pollution of the seawater, the concentration of cesium-137 in the seawater was measured, and its spatial distribution between April 11 and July 11, 2011 was modeled by IRSN, as shown in Plate XI (between pages 448 and 449). The concentration of cesium-137 fell sharply with time. Between April 11 and 18, the seawater outlet point of the damaged nuclear plant had a cesium-137 concentration around 900 Bq/L, which by the following week was significantly reduced to around 200 Bq/L. At the same time, the area with concentrations above the detection limit (around 5 Bq/L), the coloured zones shown in Plate XI, also decreased.

Добавить комментарий

Ваш e-mail не будет опубликован. Обязательные поля помечены *