Electrical utility TEPCO has begun large-scale spraying of the Fukushima Daiichi site with a resin to prevent the spread of radiologically-contaminated dust.
Fukushima Daiichi site dust spraying plan
It has been running tests for the past several weeks on bare ground, concrete and vegetation around the common spent fuel pond. Since the beginning of April, it has sprayed about 25,000 m3 of the synthetic resin, called Kuricoat, in tests.
Now, the facility will spray around the reactor buildings, on slopes and the fields upland to the facility.
To reduce the risk of radiation, the spraying will be remote-controlled. An operator will sit in a van up to 150 metres away from a remote-controlled dumper truck mounted with a spray gun and tank.
The need to isolate contaminated soil at the site has been reinforced by TEPCO's publication of site soil data sampled on 11 April.
First, the study found evidence for the first time that the incident has deposited plutonium at the site. Three isotopes were analysed: Pu-238 (half-life 88 years), Pu-239 (half-life 24,100 years) and Pu-240 (6563 years). Previous atmospheric nuclear tests have set a minimum value of plutonium in site soil. Government radiation testing data from 1978-2008 suggests an average Japanese soil plutonium contamination of 0.15 Bq/kg of dry soil Pu-238, and 4.5 Bq/kg of Pu-239, Pu-240 contamination. So background radiation is expressed as a ratio of Pu-238/Pu-239, Pu-240 (0.026). In two sites, in a playground upland of the reactors, and next to an industrial waste facility south of unit 4, the density of Pu-238 was higher than the others, and the ratio of Pu-238/Pu-239, Pu-240 was 100 times as high as the background radiation. This change leads TEPCO to suggest that the plutonium concentration was traceable back to the Fukushima Daiichi disaster. However, TEPCO also said that the amounts found at those two sites (playground: 0.12 Bq/kg Pu-238, 0.059 Bq/kg Pu-239, Pu-240; waste facility: 0.083 Bq/kg Pu-238, 0.032 Bq/kg Pu-239, Pu-240) have not changed drastically since March.
Second, high levels of contamination of other radioactive nuclides were found in three places on site in a 7 April survey by JAEA and in an 11 April survey by the Japan Chemical Analysis Centre. For example, a baseline survey in Fukushima province from 2009 found levels of caesium-137 (half-life 30 years) up to 21 Bq/kg (other radionuclides were not detected). In April, average caesium-137 values at each site were 240,000 Bq/kg (playground), 710 Bq/kg (forest) and 2,150,000 Bq/kg (waste facility). Iodine-131 (half-life 8 days) and caesium-134 (half-life 2 years) contamination levels were comparable at each site.
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