Menu
Home
Log in / Register
 
Home arrow Environment arrow Radiation Monitoring and Dose Estimation of the Fukushima Nuclear Accident

Results and Discussion

Calibration

The GM-tube detector measured the radiation fields regulated by the 137Cs standard radioactive source. Measurements were taken ten times every minute. The background air dose rate measured using a NaI scintillation counter was 0.09 μSv/h; this value was added to the air dose regulated from the standard source. The temporal change in the measurement is presented in Table 8.1. The measurement values barely approached a certain value correlated with the actual air dose rate, in contrast to the values obtained using the scintillation counter, and their variation coefficient decreased with the air dose rate. The GM-tube detector used here showed an instrument reading with a maximum error of ±20 %.

The GM-tube detector was calibrated with certified values from 0.09 to

3.69 μSv/h to obtain the following calibration equation:

Y = -0.0409X 2 + 0.9238X - 0.0264

Table 8.1 Temporal change in instrument reading Certified value

Fig. 8.1 Calibration of a GM-tube detector

where Y and X represent the calibrated value and the instrument reading, respectively. Figure 8.1 clearly shows the calibration equation as a quadratic rather than a linear function. The fact that the function is convex upward suggests that the GM-tube detector would underestimate the radiation fields at higher air dose rate conditions.

Air Dose Rate

8.3.2.1 Along Tohoku Expressway

During the screening activity, we traveled on the Tohoku Expressway in Fukushima Prefecture five times. The change of air dose rates is shown in Fig. 8.2. The dose rates obtained on March 19 were low, and they increased with time. The degree of increment, however, fluctuated greatly. It seemed that radioactive materials released still wafted over and were not yet strongly adsorbed into the ground. At the Adatara service area in Motomiya City around 7:00 p.m. on March 19, we observed the following dose rate change: the initial value was 2 μSv/h, it peaked at 4 μSv/h, and finally decreased to 2.7 μSv/h. This short-period change might have shown that a small radioactive plume had passed over. A radioactive peak seemed to have formed

Fig. 8.2 Change in air dose rate along Tohoku Expressway

Fig. 8.3 Dose rate inside a car along the Tohoku Expressway. Closed circles and open squares indicate data from the Geiger Mueller tube (GM-tube) measurements and official material, respectively

around Nihonmatsu where a high air dose rate on the expressway was reported in May 2011 by the Fukushima Prefecture government [4]. The dose rate dipped inside the Fukushima tunnel, about 900 m in length, through Mt. Atago in Fukushima City. Although dose rates inside the long tunnel rapidly decreased because of the negligible direct radioactive deposition, these values gradually increased with time as the result of transit of contaminated traffic and air.

The calibrated dose rates obtained inside a car along the Tohoku Expressway on May 26, 2011, are shown in Fig. 8.3. A peak dose rate of 0.94 μSv/h was recorded at Nihonmatsu City, which is 55 km from the Fukushima Daiichi NPP and is the nearest point to the plant on the Tohoku Expressway. A similar measurement was carried out using a NaI scintillation counter on May 22, 2011; this was part of the official data published by the Fukushima Prefecture government [4]. The results of the long-distance mobile radiation survey (shown in Fig. 8.3) indicate that both measurements provide comparable results, except for three points in Fukushima and Koriyama City, and that they validate the use of calibrated GM-tube counting.

Fig. 8.4 Measure of contamination based on distance from Fukushima Daiichi Nuclear Power Plant (NPP)

Table 8.2 Comparison of dose rate between in Tohoku Shinkansen and on the Tohoku Expressway

Location

Calibrated dose rate (μSv/h)

Nasushiobara

0.50

0.20

Shirakawa

0.47

0.15

Koriyama

0.55

0.25

Date

May 2011

August 2011

Route

Tohoku Expressway

Tohoku Shinkansen

A comparison of dose rate trends north and south of Nihonmatsu City (Fig. 8.4) provided the status of the radioactive contamination spread in the environment. Both trends show a similar decrease up to 75 km. Different trends, however, are shown from 75 to 125 km. The values obtained in the northern location decreased to a certain constant value, defined here as the background value. In contrast, the values obtained in the southern location are two to five times higher than the background value and exhibit some small peaks, which could be considered hot spots. These trends indicate that larger amounts of radioactive material were widespread in the southern location.

8.3.2.2 Tohoku Shinkansen

The results of measurements carried out in August 2011 aboard the Tohoku Shinkansen (bullet train), which runs almost parallel to the Tohoku Expressway, are listed in Table 8.2. The dose rates obtained in the Shinkansen decreased to 40 % of those obtained along the expressway in May 2011. According to “Extension Site of the Distribution Map for Radiation Dose” [5], the radiation dose at three locations decreased from a range between 0.5 and 1.0 μSv/h to a range between 0.2 and

Fig. 8.5 Temporal variation of radiation dose rate in Fukushima City

0.5 μSv/h in the same period. The same decreasing tendency suggests the validity of using a calibrated GM-tube for radiation dose measurement in a rapidly moving vehicle at a speed of around 200 km/h.

8.3.2.3 Fukushima City

In Fukushima City as well as on the Tohoku Expressway, air dose rates measured several times on the same street between the Prefectural office (center of Fukushima City) and Iizaka Onsen (northern part of the city) are shown in Fig. 8.5. The dose rate in the center of the city was higher, and the highest values were obtained at the north end of the Shinobuyama tunnel, about 700 m in length through Mt. Shinobuyama (altitude 275 m). The distance between the prefectural office and Iizaka Onsen is about 9 km, and they are located 60 and 64 km from the Fukushima Daiichi NPP, respectively. In spite of similar distances from Fukushima Daiichi, a large variation in the air dose rate at these locations was observed. Further, a dip in the air dose rates inside the Shinobuyama tunnel was observed: the rates were 6–8 μSv/h at both ends and decreased to 1.5–2 μSv/h at the center of the tunnel.

8.3.2.4 Across Fukushima Prefecture

We made a round trip to Iwaki City on March 20. The temporal change in the air dose rate is shown in Fig. 8.6. The rates along the Banetsu Expressway and in Iwaki

Fig. 8.6 Change in air dose rate on 1 day of screening activity

City were considerably lower than those along Tohoku Expressway and in Fukushima City as already described. This trend across Fukushima Prefecture agreed with the results from airborne monitoring in April 2011 [6], which would suggest the distribution of contamination (mainly radioactive cesium) had roughly stabilized since March 20 at the latest.

Screening activity was conducted in Samegawa Village and Shirakawa City on March 21 and 22, and their dose rates were approximately 1 and 2 μSv/h, respectively. Low air dose rates were obtained on both the Banetsu Expressway and in Samegawa Village in the Abukuma highlands. The Abukuma highlands served as a barrier to the diffusion of the radioactive plume released from the Fukushima Daiichi NPP [7]. Consequently, the plume streamed along the basin at the foot of the highland where Fukushima City and Tohoku Expressway are located and high radioactive contamination was detected.

Surface Contamination

The instrument readings (uncalibrated value) obtained near the ground surface in and around Ueno Park, 2 weeks after the nuclear accident, are shown in Fig. 8.7. Some of the values obtained are beyond the calibration value, and hence, the original instrument reading is adopted. The measurement values and the width of their distribution increased in the following order: park ground, road asphalt, and street gully. The removal of several millimeters of the surface of park ground provided a decrease in the measured dose rate by a factor of 2–4, which showed that radioactive material contaminated only the park ground surface and barely penetrated deeper. The nature of the ground surface affected the measurement values, although all the

Fig. 8.7 Amount of relative radioactivity (instrument reading indicated in μSv/h)

obtained near the ground surface

Fig. 8.8 Average activity on masks used during screening activity

monitoring locations were very close to each other. Our observation discovered “hot spots” where radioactive material would accumulate with fine soil particles from its original deposit location owing to wind and rain.

A measurement of ground surface was conducted at two points in August 2011 in Namie town, which is located about 30 km northwest of the Fukushima Daiichi NPP. The result values were 54 and 175 μSv/h (uncalibrated), which are 10–100 times higher than the results obtained at Ueno Park on March 30, 2011. In other words, serious contamination was observed.

Radiation Exposure

The radioactivity of the masks used on each day is shown in Fig. 8.8. Among detected nuclides, only 136Cs was found in masks used on March 20 with 131I showing a slightly different trend, and the radioactivity decreased faster than their own physical half-lives, which was caused by the difference in location of screening activity and time spent outdoors. During this screening activity, the effective dose (external exposure) was 0.1 mSv, and the committed effective dose (internal exposure) was 0.0327 mSv.

 
Found a mistake? Please highlight the word and press Shift + Enter  
< Prev   CONTENTS   Next >
 
Subjects
Accounting
Business & Finance
Communication
Computer Science
Economics
Education
Engineering
Environment
Geography
Health
History
Language & Literature
Law
Management
Marketing
Mathematics
Political science
Philosophy
Psychology
Religion
Sociology
Travel