Monitoring Activity and Data of the University of Tokyo
Targets and Methods
The two primary targets of the surveillance were (1) the ambient radiation dose (μSv h−1) and (2) the specific radioactivity (Bq kg−1) of rainwater and of soil around the surface of the ground, which was used to show the level of contamination.
To measure the ambient dose, well-calibrated NaI(Tl) scintillation survey meters of the energy compensation type were used. The elevations at which measurements were performed were adequately selected as 1 m (for the standard height of adults in the Kashiwa campus and the Hongo campus), or 15 m for the airborne radioactivity survey in the Komaba campus.
To measure the specific radioactivity of soil, NaI(Tl) scintillation and/or Ge semiconductor spectroscopy systems were used. Soil materials were sampled from the surface of the ground, from a layer between 0 and 1 cm depth (for surface layer target method) or from a layer between 0 and 5 cm depth (for standard target method). The sampled soil was weighed and measured as is (without being dried). Specific radioactivity of samples of rainwater was also measured. The ambient dose data had been reported every day on the website. In addition, the data had been published in the portal site magazine of the university. Soil contamination data were reported occasionally and periodically.
Data and Information
Since the morning of March 15, 2011, continuous survey of the ambient radiation dose rate has been officially performed at the selected representative locations on the campus sites. The first peak dose of 0.72 μSv h−1 was observed around 2:30 pm on March 15, 2011, in the Kashiwa campus, and the second peak dose of 0.80 μSv h−1 was observed around 11:00 am on March 21, 2011, when the first rain fell after the NPP accident. The elevation at which these measurements were performed was 1 m above the ground.
The large reduction of ambient radiation dose rate was observed after 20 days following March 21, 2011, and afterward the ambient radiation dose gradually reduced until the continuous surveillance ended at the end of March 2012. At that time, the representative ambient dose rates of the campuses were measured to be about 0.23 μSv h−1 for the Kashiwa campus and about 0.11 μSv h−1 for the Hongo campus (at 1 m above the ground). These measured values included the natural background dose rate.
Area distribution of ambient radiation dose was measured at 1,516 points with a distance of 6 m between them in the site of the Kashiwa campus from October 10 to November 10, 2011. At the elevation of 1 m above the ground, the average dose rate was 0.3 μSv h−1, and more than 80 % of measurements yielded values less than
0.35 μSv h−1. Some measurement spots where the radiation dose indicated more
than 0.6 μSv h−1 at the elevation of 1 m and more than 1.0 μSv h−1 at the elevation of 5 cm have been decontaminated, following the corresponding plan determined by
the Kashiwa local government. These spots were found in drainage pathways, such as rain gutters and underneath downspouts. After the contaminated surface soil was removed, the observed radiation dose at the elevation of 5 cm above the ground dropped by about 70–80 % from the level of the predecontamination values.
From March 13, radiation experts in the Komaba campus had started measuring the ambient radiation dose rate. An automatic ambient radiation monitoring system was installed about 15 m above the ground. During the measurement period, the maximal level of the ambient radiation dose rate in the Komaba campus was about
0.52 μSv h−1, observed at 7:00 pm on March 15, 2011. By the end of April 2011, the radiation dose rate at the elevation of 15 m was reduced almost to the same level as the natural background radiation (0.05–0.06 μSv h−1).
As of mid-April 2011, 0.2–0.35 kBq kg−1 of 131I and 1.0–1.5 kBq kg−1 of 134+137Cs
were measured in soil samples in the Kashiwa campus using the standard target method. On March 29, 2011, 7.8 kBq kg−1 of 131I and 2.7 kBq kg−1 of 134+137Cs were measured in soil samples in the Hongo campus using the surface layer target method. On March 22, 2011, 3.2 kBq l−1 of 131I and 0.18 kBq l−1 of 134+137Cs were measured in a rainwater sample on the Hongo campus site.
Box 5.1 indicates a member list of the special corresponding team appointed to survey the environmental radiation status in the University of Tokyo. We wish to express our sincere gratitude to the members for their enormous contribution in the related activity.
An example outline of the environmental radiation status in and around Tokyo immediately after the nuclear disaster of the TEPCO Fukushima Dai-ichi NPP has been presented. The background status and technical information of the related measuring activities and the estimated data were introduced, focusing on activities of three local governments and the University of Tokyo.
Kashiwa City and Nagareyama City are now continuing monitoring of environmental radiation and operating their decontamination program following the local government policies. Their newest data can be checked and traced through their official website.
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