Depth profiles of radioactive cesium in soil using a scraper plate over a wide area surrounding the Fukushima Dai-ichi Nuclear Power Plant, Japan

Abstract

During the Fukushima Dai-ichi Nuclear Power Plant (NPP) accident, radioactive cesium was released in the environment and deposited on the soils. Depth profiles of radioactive cesium in contaminated soils provide useful information not only for radiation protection and decontamination operations but also for geoscience and radioecology studies. Soil samples were collected using a scraper plate three times between December 2011 and December 2012 at 84 or 85 locations within a 100-km radius of the Fukushima Dai-ichi NPP. In most of the obtained radioactive cesium depth profiles, it was possible to fit the concentration to a function of mass depth as either an exponential or hyperbolic secant function. By using those functions, following three parameters were estimated: (i) relaxation mass depth β (g cm(-2)), (ii) effective relaxation mass depth βeff (g cm(-2)), which is defined for a hyperbolic secant function as the relaxation mass depth of an equivalent exponential function giving the same air kerma rate at 1 m above the ground as the inventory, and (iii) 1/10 depth L1/10 (cm), at which the soil contains 90% of the inventory. The average β value (wet weight) including ones by hyperbolic secant function in December 2012, was 1.29 times higher than that in December 2011. In fact, it was observed that depth profiles at some study sites deviated from the typical exponential distributions over time. These results indicate the gradual downward migration of radioactive cesium in the soils. The L1/10 values in December 2012 were summarized and presented on a map surrounding the Fukushima Dai-ichi NPP, and the average value of L1/10 was 3.01 cm (n = 82) at this time. It was found that radioactive cesium remained within 5 cm of the ground surface at most study sites (71 sites). The sech function can also be used to estimate the downward migration rate V (kg m(-2) y(-1)). The V values in December 2012 (n = 25) were in good agreement with those found by a realistic approach using a diffusion and migration model. Almost all values ranged between 1.7 and 9.6 kg m(-2) y(-1) in this study.

Evaluation of ambient dose equivalent rates influenced by vertical and horizontal distribution of radioactive cesium in soil in Fukushima Prefecture

“…Matsuda et al [19] characterized the activity depth profiles as a function of mass depth (A m (ζ)) by fitting exponential and hyperbolic secant functions. The exponential depth distribution is…”

Temporal Change of Environmental Contamination Conditions in Five Years after the Fukushima Accident

“…The other is a distribution having a concentration peak at a certain depth. The latter distribution was found to be well approximated by a hyperbolic secant (sech) function [10]. Further, in some locations, disturbed distributions were found.…”

“…Soil samples for different depth layers are collected using a scraper plate which can minimize cross contaminations among the samples [10]. The thickness of collected soil layer is changed from 5 mm to a few cm according to the condition.…”

Do forests represent a long-term source of contaminated particulate matter in the Fukushima Prefecture?

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