Distribution of local 137Cs anomalies on the seafloor near the Fukushima Dai-ichi Nuclear Power Plant

Abstract

An estimated 3.5±0.7×10(15) Bq of (137)Cs is thought to have been discharged into the ocean following the melt down at Fukushima Dai-ichi Nuclear Power Plant (F1NPP). While efforts have been made to monitor seafloor radiation levels, the sampling techniques used cannot capture the continuous distribution of radionuclides. In this work, we apply in situ measurement techniques using a towed gamma ray spectrometer to map the continuous distribution of (137)Cs on the seafloor within 20 km of the F1NPP. The results reveal the existence of local (137)Cs anomalies, with levels of (137)Cs an order of magnitude higher than the surrounding seafloors. The sizes of the anomalies mapped in this work range from a few meters to a few hundreds of meters in length, and it is demonstrated that the distribution of these anomalies is strongly influenced by meter scale features of the terrain.

Migration of radioactivity in multi-fraction sediments

“…Measurements on the shelf area around the FDNPP [3,39] showed strong heterogeneity in the distribution of the radiocesium on seafloor after the Fukushima accident. It can be the result of both local heterogeneities of sediment features [11] and the bottom topography [39].…”

“…Notice that the activity according to (39) is redistributed between the sediment fractions in the equilibrium state due to the particular choice of the clean initial state. Bq m À3 in the initial period t 1 ¼ 30 days.…”

“…It can be the result of both local heterogeneities of sediment features [11] and the bottom topography [39]. To separate the topography effect from the effects of transport, erosion and deposition of contaminated sediments on the exchange of activity between the seafloor and water, consider the idealized case of a straight channel with a bottom depression (Fig.…”

“…The radionuclide migration in sea deposits is governed by molecular diffusion and reworking of sediments by animals (bioturbation). Continuous measurement of the radiocesium distribution on the sea floor in the vicinity of FDNPP demonstrated strong patchiness of activity [15,37,39]. The potential reasons include heterogeneity in the physical and chemical features of the sediments [22] and local inhomogeneities of the bottom topography resulting in variations in erosion and deposition [39].…”

Decline in radiocesium in seafloor sediments off Fukushima and nearby prefectures

“…11b), but the cause of those differences is not clear. The data for the inventory calculation are based on samples obtained by 2-3 sampling casts, each consisting of 16-24 sampling tubes; thus, these data should be relatively less sensitive to local spatial changes such as those shown by Thornton et al (2013b). The temporal changes of F 0-3 may reflect dynamic features of the bottom sediment, such as deposition and removal of contaminated surface sediments.…”