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

Abstract

The air dose rate in an environment contaminated with 134 Cs and 137 Cs depends on the amount, depth profile and horizontal distribution of these contaminants within the ground. This paper introduces and verifies a tool that models these variables and calculates ambient dose equivalent rates at 1 m above the ground. Good correlation is found between predicted dose rates and dose rates measured with survey meters in Fukushima Prefecture in areas contaminated with radiocesium from the Fukushima Dai-ichi Nuclear Power Plant accident. This finding is insensitive to the choice for modelling the activity depth distribution in the ground using activity measurements of collected soil layers, or by using exponential and hyperbolic secant fits to the measurement data. Better predictions are obtained by modelling the horizontal distribution of radioactive cesium across an area if multiple soil samples are available, as opposed to assuming a spatially homogeneous contamination distribution. Reductions seen in air dose rates above flat, undisturbed fields in Fukushima Prefecture are consistent with decrement by radioactive decay and downward migration of cesium into soil. Analysis of remediation strategies for farmland soils confirmed that topsoil removal and interchanging a topsoil layer with a subsoil layer result in similar reductions in the air dose rate. These two strategies are more effective than reverse tillage to invert and mix the topsoil.

Effect of Remediation Parameters on in-Air Ambient Dose Equivalent Rates When Remediating Open Sites with Radiocesium-contaminated Soil

“…Ḣ*(10) at 1 m provides a conservative estimate of the effective dose rate to persons within environmental fields (Satoh et al 2016), hence it can be used to limit the yearly effective doses to people living or working within contaminated areas (ICRP 2009). Previously Malins et al (2016) developed a tool to calculate ambient dose equivalent rates in areas where soil is contaminated with radiocesium. The tool was validated by using soil activity samples from Japan contaminated by the 2011 Fukushima Daiichi Nuclear accident to predict Ḣ*(10) values, then comparing the predictions to environmental measurements taken with hand-held survey meters.…”

Characterizing regional-scale temporal evolution of air dose rates after the Fukushima Daiichi Nuclear Power Plant accident

“…Based on their results, we use simple averaging for car survey data within the 100-m radius. We use weighted averaging to represent the large footprint of airborne survey, the weight of which is computed by the radiation transport simulations (Malins et al, 2016). The third distribution p(y | z W ) represents the process model (i.e., geostatistical model) to describe the spatial pattern given the measured dose rates in the walk surveys.…”

Assessment of the calibration of gamma spectrometry systems in forest environments

“…in open field and forested environments (Clovas et al, 1999;Malins et al, 2016). These methods have also been used to simulate the spectral response of NaI(Tl) detectors, at ground level and airborne survey heights, for open field geometries (Allyson, 1994;Allyson & Sanderson, 1998;Cresswell et al, 2001;Cresswell & Sanderson, 2012).…”