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Time Trend Change of Air Dose Rate on Paved Areas in Fukushima City After the Fukushima Daiichi NPP Accident

Sin-ya Hohara, Masayo Inagaki, Hirokuni Yamanishi, Genichiro Wakabayashi, Wataru Sugiyama, and Tetsuo Itoh

Abstract The Kinki University Atomic Energy Research Institute investigated radioactive contamination resulting from the Fukushima Daiichi Nuclear Power Plant accident in urban areas of Fukushima City, Fukushima Prefecture, Japan. Activity measurement of the surface soil and a survey of the dose rate distribution in urban areas were performed. From the results of this research, dose rate changes in paved areas became clear, and gradients of the dose rate decrease for different paving materials were measured and analyzed.

Keywords Activities • Distribution • Dose rate

Introduction

On March 11, 2011, the Tohoku Region Pacific Coast Earthquake and the subsequent tsunami occurred, causing the Fukushima Daiichi Nuclear Power Plant (Fukushima Daiichi NPP) accident. As a result of the failure of the NPP's coolant systems, hydrogen explosions occurred that released radioactive materials to the general environment [1–6].

The released radioactive materials were dispersed as radioactive plumes over wide ranges of the Tohoku and Kanto regions. These radioactive plumes were deposited to grounds, building surfaces, and other substrates through several processes. Several surveys tried to measure the contamination in general environments [1–6].

The Kinki University Atomic Energy Research Institute (Kinki University AERI) has conducted research in the Fukushima area since the middle April of 2011 [6]. Two types of measurement were conducted: the measurement of the activity of radioactive materials in surface soils and the measurement of dose rate distributions in urban areas. From these measurements, it became clear that the concentration of radioactive materials that had dispersed from the Fukushima Daiichi NPP decreased faster than the physical half-lives of the radioactive materials themselves in the urban parts of the Fukushima Naka-Dori area.

In this chapter, the methods used and some of the results of the measurements are presented.

Methods

Kinki University AERI researched radioactive materials at Fukushima station (East Japan Railway Company) neighborhoods, which is located at the center of the downtown of Fukushima city. This place is 60 km from the Fukushima Daiichi NPP in a northwestern direction. The dose rate in the downtown area was more than ten times greater than the published environmental background dose rate [6, 9].

Two types of research were undertaken in this work: the first was a dose rate survey, and the other was an activity measurement for surface soils in the urban area. The measurement area of the dose rate survey and the surface soil sampling point for activity measurement are shown in Fig. 10.1. These places are in a 1-km neighborhood, because quantities of fallen radioactive materials in a unit square footage and ratios of radioactive nuclides were thought to be similar in these research locations. Both studies have continued since April 2011.

The methods of the two studies are described in the following sections.

 
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