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Home arrow Environment arrow Radiation Monitoring and Dose Estimation of the Fukushima Nuclear Accident

Methods

The external radiation dose for each staff member was measured using a personal dosimeter (ADM-112; Hitachi-Aloka Medical). For the internal radiation doses, body counters of radioactivity were determined using a whole-body counter consisting of an iron room, the 8 in. φ × 4 in. t NaI (Tl) with four photomultiplier tubes (Fuji Denki), and a digital multichannel analyzer (MCA7600; Seiko EG&G) (Fig. 17.1). Body content of radioactivity was determined for 10 min using the whole-body counter, 3–4 days after returning from the dispatch. A spectral stripping method was used for the pulse amplitude analysis of 131I, 134Cs, and 137Cs with the whole-body counter. The whole-body counter was calibrated using the human acryl phantom filled with KCl solution containing 40 kBq 40K. The conversion from the measured radioactivity in the body to the committed effective dose by internal exposure was evaluated with the MONDAL 3 (monitoring to dose calculation ver. 3) from the National Institute of Radiological Science (NIRS). MONDAL is a userfriendly tool for internal dose calculation based on the ICRP biokinetic models, by which the committed effective dose can be calculated from the whole-body counter results (Ishigure 2004). Volunteers stayed in the local hotel 100 km from TEPCO's plant. They consumed served food without radioactive contamination during the screening work. The intake route of radioactive particles was considered to be inhalation. All the radioactivities were assumed to be taken by inhalation of 1.0-μm aerodynamic diameter particles in this calculation. The effective doses were calculated using the last working day as the acute inhalation of the radioactive particles.

Fig. 17.1 KURRI whole-body counter. Left: exterior. The iron walls, ceiling, and floor of the iron room are 20 cm thick, with the inner surfaces lined with 3-mm Pb sheet. Right: Interior. The 8 in. φ × 4 in. t NaI (Tl) crystal scintillation counter and four photomultiplier tubes are 40 cm above the bed

Results

Initial Measurement of External Radiation and Internal Radiation Doses of Volunteers

The dispatch period, working place, and distance from TEPCO's plant, air dose rate, and external radiation doses of KURRI volunteers are listed in Table 17.1. The external radiation dose shows the 24-h doses for each volunteer using a personal dosimeter. As shown in Table 17.1, the data of screening teams 2–13, it is difficult to estimate the external exposure dose from the ambient dose rate because dose rate differs by place.

The internal radiation doses of 131I and 134Cs + 137Cs are listed in Table 17.2. The spectral stripping method for the NaI whole-body counter cannot distinguish between 134Cs and 137Cs. We estimated that the ratio of 134Cs–137Cs was 1:1, on the basis of the data from the measurements of the protecting masks using a germanium detector (Table 17.3). Four volunteers in Screening Team 1 showed 1,929, 1,816,

1,469, and 1,300 Bq of radiocesium (137Cs and 134Cs) and 118, 72, 52, and 48 Bq of

131I, respectively. The internal doses of radiocesium were estimated to be 28, 26, 21, and 19 μSv, and the internal doses of radioiodine were estimated to be 11, 7, 5, and 5 μSv, respectively.

Table 17.1 External radiation doses of KURRI volunteers

aThe maximum dose/day during the working days

bThe shelter in Iwaki City of team 9 is the different shelter from that of team 1

Table 17.2 Internal radiation doses estimated with human counter at KURRI

aThe maximum internal dose of the each group

bND not detected

Table 17.3 The cesium analysis of the protecting masks by a germanium semiconductor detector

Dispatched date

No of masks

Cs-134 (Bq/mask)

Cs-136 (Bq/mask)

Cs-137 (Bq/mask)

2011.3.20–3.22

11

2.2

0.2

2.5

2011.3.23–3.25

12

0.2

0

0.3

The doses from internal exposure were almost similar to the cumulative external doses for the dispatch period (March 20–22, 2011) when the radiation plumes following the explosions of Units 1 and 3 in TEPCO's Fukushima Daiichi nuclear plant had diffused over Fukushima City. The external radiation doses of the dispatched members after the second team decreased from one third to less than one tenth of the external doses of the first dispatched team. The internal radiation doses of 55 members after the second team was dispatched showed that 51 cases were undetectable and 4 cases showed doses of 2–15 μSv.

Follow-Up Data of Internal Doses for Two Volunteers

Two volunteers belonging the initial team (Screening Team 1) showed a rather high level of internal radiation dose because they visited Fukushima Prefecture three or more times. Volunteer A visited four times and volunteer B visited three times.

Fig. 17.2 Activity of 131I measured by KURRI whole-body counter. Diamond symbols show the activity of 131I of volunteer A. Volunteer A was dispatched joining in screening teams 1, 10, and 13 and KURAMA team 1. Black arrows show the dispatch period. Square symbols show the activity of 131I of volunteer B. Volunteer B had stayed in Sendai City on 3/11–3/13/2011. He dispatched joining in screening team 1 and KURAMA team 1. White arrows show the dispatched period

Volunteer A was dispatched joining in Screening Teams 1, 10, and 13 and KURAMA Team 1. Volunteer B had stayed in Sendai City on 3/11-3/13/2011. He joined Screening Team 1 and KURAMA Team 1. We followed up their internal radiation doses for 6 months, to 2012/08/17. The activities of 131I and 134Cs + 137Cs are shown in Figs. 17.2 and 17.3, respectively. Iodine-131 of volunteer A was detected until 2011/05/02, and iodine-131 of volunteer B was detected until 2011/03/31. Cesium-137 and -134 of volunteer A was detected until 2012/05/18, and cesium-137 and -134 of volunteer B was detected until 2011/08/08.

 
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