Earth Planets Space, Vol. 64 (No. 1), pp. e1-e4, 2012
Swedish Institute of Space Physics, Box 812, S-98128 Kiruna, Sweden
(Received July 11, 2011; Revised November 11, 2011; Accepted January 17, 2012; Online published February 27, 2012)
Data from the radiation monitoring network surrounding the Fukushima Dai-ichi Nuclear Power Plant (FNPP) revealed that the radiation levels generally decayed faster at a highly-contaminated area than at neighboring moderately-contaminated areas during the first month after the Fukushima nuclear accident in March, 2011. Two possible mechanisms are considered: secondary transport of radioactive dust by wind or rain, and non-uniform radionuclide ratio of contamination between radioiodine (131I) and radiocesium (134Cs and 137Cs). The composition data from soil does not favor the latter scenario, except for the local coastal region south of the FNPP, while inter-regional transport from the highly-contaminated area to the moderately-contaminated areas explains both the general difference in the decay rate in the entire area and the relatively slow decay at a high-dose rate anomaly 40 km northwest of the FNPP.
Key words: Radioactive contamination, iodine-cesium ratio, internal dose, Fukushima nuclear accident, re-suspension of radioactive material, dust transport.