Japan Atomic Energy Agency, 11601-13 Nishi-jusanbugyo, Hitachinaka-city, Ibaraki 319-1206, Japan.
Japan Atomic Energy Agency, 11601-13 Nishi-jusanbugyo, Hitachinaka-city, Ibaraki 319-1206, Japan.
J Environ Radioact. 2019 Dec;210:105813. doi: 10.1016/j.jenvrad.2018.09.011. Epub 2018 Oct 19.
As part of the investigation of the distribution of ambient dose equivalent rates around the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), car-borne surveys using Kyoto University RAdiation MApping (KURAMA) systems have been conducted over a wide area in eastern Japan since 2011. The enormous volume of measurement data collected until 2016, including those until 2012 which were reported in the previous paper, was analyzed, and dependencies of the decreasing trend of the dose rates in regions within 80 km of the FDNPP on land-use categories, evacuation order areas and magnitude of the dose rates were examined. The air dose rates within 80 km of the FDNPP tended to decrease considerably with respect to the physical decay of radiocaesium. The decrease of the dose rate in the "forest" was slower than its decrease in other regions, while that in "urban area" was the fastest. The decrease in the air dose rate from 2011 was the fastest outside the evacuation order area until 2015, and it was the slowest in the "difficult-to-return zone". However, the decreasing trend starting from 2013 showed that the decrease in the "zone in preparation for the lifting of the evacuation order" and in the "residence restriction area" was the fastest. It was found that the air dose rates decreased depending on the magnitude of the dose rates and elapsed time from the FDNPP accident, i.e. the decrease in air dose rates in areas with relatively low dose ranges (such as 0.2-0.5 μSv/h) was the largest during a period relatively early after the accident, and the decreasing rate in the dose rate ranges of 1.9-3.8 and 3.8-9.5 μSv/h were the fastest after 2013. The averaged ratios were analyzed to obtain the ecological half-lives of the fast and slow decay components, and those in whole area within 80 km of FDNPP were estimated to be 0.44 ± 0.05 y and 6.7 ± 1 y, respectively. The ecological half-lives with respect to the land use categories, evacuation order areas and magnitude of the dose rates were also evaluated. The decrease in the dose rates obtained by the car-borne survey was larger than that obtained on flat ground with few disturbances using the NaI(Tl) survey meter during approximately 1.5 y after the FDNPP accident. Thereafter, the difference of decreasing tendencies in the air dose rates between both the measurements was negligibly small, with the ratio of dose rates by the car-borne survey to those by the fixed-point measurement of 0.72-0.77.
作为对福岛第一核电站（FDNPP）周围环境剂量当量率分布调查的一部分，自 2011 年以来，使用京都大学辐射测绘系统（KURAMA）在日本东部的广大地区进行了车载调查。对截至 2016 年收集的大量测量数据进行了分析，包括 2012 年之前报告的那些数据，这些数据检验了在距离 FDNPP 80km 范围内的剂量率降低趋势与土地利用类别、疏散令区域和剂量率大小的相关性。FDNPP 周围 80km 范围内的空气剂量率相对于放射性铯的物理衰减明显降低。“森林”中的剂量率下降速度比其他地区慢，而“城市地区”的下降速度最快。2011 年以来，疏散令区域以外的空气剂量率下降速度最快，“难返区”最慢。然而，从 2013 年开始的下降趋势表明，“准备解除疏散令区”和“居住限制区”的下降速度最快。结果表明，空气剂量率的下降取决于剂量率的大小和距 FDNPP 事故的时间，即事故发生后相对较早的时期，剂量率范围相对较低（如 0.2-0.5μSv/h）的区域空气剂量率下降幅度最大，2013 年后 1.9-3.8μSv/h 和 3.8-9.5μSv/h 剂量率范围的下降速度最快。对平均比值进行了分析，以获得快速和缓慢衰减成分的生态半衰期，估计 FDNPP 周围 80km 范围内的所有区域的生态半衰期分别为 0.44±0.05y 和 6.7±1y。还评估了与土地利用类别、疏散令区域和剂量率大小有关的生态半衰期。FDNPP 事故发生后约 1.5 年，使用车载调查获得的剂量率下降幅度大于使用 NaI(Tl) 调查仪在平坦地面上获得的剂量率下降幅度，干扰较少。此后，两种测量方法的空气剂量率下降趋势差异可以忽略不计，车载调查的剂量率与定点测量的剂量率之比为 0.72-0.77。
