128 www.jrpr.org ABSTRACT Invited Paper Received October 10, 2019 Accepted October 28, 2019 Corresponding author: Kimiaki Saito 178-4-4 Wakashiba, Kashiwa, Chiba 227-0871, Japan Tel: +80-9771-0741 E-mail: Saito.kimiaki@jaea.go.jp This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non- Commercial License (http://creativecommons.org/ licenses/by-nc/3.0) which permits unrestricted non- commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Copyright © 2019 The Korean Association for Radiation Protection Temporal Change in Radiological Environments on Land afer the Fukushima Daiichi Nuclear Power Plant Accident Kimiaki Saito 1, *, Satoshi Mikami 1 , Masaki Andoh 1 , Norihiro Matsuda 1 , Sakae Kinase 1 , Shuichi Tsuda 2 , Tetsuro Sato 3 , Akiyuki Seki 1 , Yukihisa Sanada 1 , Haruko Wainwright-Murakami 4 , Kazuya Yoshimura 1 , Hiroshi Takemiya 1 , Junko Takahashi 5 , Hiroaki Kato 5 , Yuichi Onda 5 1 Japan Atomic Energy Agency; 2 OECD Nuclear Energy Agency; 3 Hitachi Solutions East Japan Ltd.; 4 Lawrence Berkeley National Laboratory; 5 University of Tsukuba, Japan Massive environmental monitoring has been conducted continuously since the Fukushima Dai- ichi Nuclear Power accident in March of 2011 by di ferent monitoring methods that have difer- ent features together with migration studies of radiocesium in diverse environments. Tese re- sults have clarifed the characteristics of radiological environments and their temporal change around the Fukushima site. At three months afer the accident, multiple radionuclides includ- ing radiostrontium and plutonium were detected in many locations; and it was confrmed that radiocesium was most important from the viewpoint of long-term exposure. Radiation levels around the Fukushima site have decreased greatly over time. Te decreasing trend was found to change variously according to local conditions. Te air dose rates in environments related to hu- man living have decreased faster than expected from radioactive decay by a factor of 2–3 on av- erage; those in pure forest have decreased more closely to physical decay. Te main causes of air dose rate reduction were judged to be radioactive decay, movement of radiocesium in vertical and horizontal directions, and decontamination. Land-use categories and human activities have signifcantly a fected the reduction tendency. Diference in the air dose rate reduction trends can be explained qualitatively according to the knowledge obtained in radiocesium migration studies; whereas, the quantitative explanation for individual sites is an important future chal- lenge. Te ecological half-lives of air dose rates have been evaluated by several researchers, and a short-term half-life within 1 year was commonly observed in the studies. An empirical model for predicting air dose rate distribution was developed based on statistical analysis of an exten- sive car-borne survey dataset, which enabled the prediction with confdence intervals. Di ferent types of contamination maps were integrated to better quantify the spatial data. Te obtained data were used for extended studies such as for identifying the main reactor that caused the con- tamination of arbitrary regions and developing standard procedures for environmental mea- surement and sampling. Annual external exposure doses for residents who intended to return to their homes were estimated as within a few millisieverts. Di ferent forms of environmental data and knowledge have been provided for wide spectrum of people. Diverse aspects of lessons learned from the Fukushima accident, including practical ones, must be passed on to future generations. Keywords: Fukushima Accident, Radiological Environment, Temporal Change, Large-scale Environmental Monitoring, Radiocesium Migration, Exposure Dose pISSN 2508-1888 | eISSN 2466-2461 Journal of Radiation Protection and Research 2019;44(4):128–148 https://doi.org/10.14407/jrpr.2019.44.4.128 JRP R