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

Results

Measurement of the Radioactivity in Surface Soils

Results of the measurements ranging from April 2011 to July 2012 have already been published [6, 8, 9]. Almost all the radioactive nuclides from the Fukushima Daiichi NPP decayed to approximately background levels by the middle of 2011, except for 134Cs and 137Cs. Figure 10.4 shows a time trend of 137Cs activity in ratios for the April 17, 2011 sample. Figure 10.4 shows a radical decrease between September 2011 and October 2011 as a result of water exposure of the sampling place by a typhoon [No. 15 (2011), named Roke]. It is surmised that the water exposure carried away thickly contaminated surface soils from the place, bringing lightly contaminated muds. After a few months, an increase is seen between April 2012 and May 2012 because of the change of sampling soil type. Until April 2012, I sampled sand-type soil that is similar to the ground surface soil before the water exposure. Since May 2012, I sampled mud-type soil that is presumed to have come with water exposure. With this change, 137Cs activity increased. Furthermore, the 137Cs activity decreased drastically again between January 2013 and February 2013. In this period, decontamination of the sampling point was carried out by Fukushima City, and as a result the 137Cs activity decreased.

Fig. 10.4 Time trend change of 137Cs activity of surface soil samples. Horizontal axis, date; vertical axis, specific activity; circles, measured activities

Fig. 10.5 Ratios of activity. Vertical axis, ratio in activity for 137Cs; upper horizontal axis, date for 134Cs/137Cs ratio; lower horizontal axis, date for 131I/137Cs and 129mTe/137Cs ratio. Circles show 131I/137Cs activity ratios, squares show 129mTe/137Cs activity ratios, and triangles show 134Cs/137Cs activity ratios. 131I and 129mTe decreased following their physical half-lives

The data show that activity of 137Cs was in a decreasing trend with time and the gradient of the trend was faster than the physical half-life of 137Cs decay.

For more detailed analysis, activity ratios of other radioactive nuclides for 137Cs activity are shown in Fig. 10.5. From this result, the ratios of radioactive nuclides are following their physical half-lives, which means the radioactive nuclides moved together with 137Cs in the general environment after the fallout.

Dose Rate Distribution Survey

Some of the earlier results of this measurement have been published [6, 8, 9], showing dose rate distributions in the urban part of the Fukushima Naka-Dori area had changed. The change in the dose rate in paved areas (with time) is described in this chapter.

A characteristic dose rate distribution change (Fig. 10.6) is shown by dose rate distributions of the Fukushima station neighborhood close to the soil sampling point. Figure 10.6a shows the dose rate distribution on April 17, 2011, and Fig. 10.6b shows the dose rate distribution on October 13, 2012.

Fig. 10.6 Dose rate distributions in Fukushima City. Lines indicate the survey route and their color the measured dose rate on April 17, 2011 (a) and October 13, 2012 (b)

The dose rate on October 13, 2012 was lower than that on April 17, 2011 (Fig. 10.6), and the dose rate distribution became much clearer on October 13, 2012. The higher dose rate areas (Fig. 10.6b) were where the streets were paved with cushion, the average dose rate areas were where the streets were paved with blocks, and the lower dose rate areas were where the streets were paved with normal asphalt (Fig. 10.7). The dose rates of these three points decreased with time (Fig. 10.7). The dose rate decreasing rate appears different among the three points in Fig. 10.7.

For more detailed analysis, I compared measured dose rates with calculated dose rates. The calculated dose rate trends were determined as follows.

The air dose rate at Fukushima city is composed of the environmental background dose rate and the dose rate of radioactive materials from Fukushima Daiichi NPP, as shown in Eq. (10.1), where Dcalc is the calculated dose rate, DBG is the environmental background, and DRM is the dose rate from the radioactive materials from Fukushima Daiichi NPP.

Dcalc = DBG + DRM . (10.1)

Background dose rate at Fukushima City was set at 0.044 (μGy/h), calculated from total background dose rate, 0.075 (μGy/h) [=8.6 (μR/h)] [10], and cosmic muon dose rate, 0.031 (μGy/h). The muon dose rate is calculated from 0.031 (μSv/h) at sea level [11] and a height correction equation [11]. The conversion from Sv to

Fig. 10.7 Time dependence of the dose rate for different paving materials. Horizontal axis is date and vertical axis is dose rate. Circles show measured dose rates at the cushion-paved area, squares show measured dose rates at the block-paved area, and triangles show measured dose rates at the normal asphalt-paved area; solid lines show calculated dose rates

Gy was executed with a muon radiation weighting factor, 1.0. Time trend of dose rate from the radioactive materials was calculated from elapsed time from a normalization date, activities of the detected radioactive materials measured in previous studies [6, 8, 9], and the air kerma-rate constant as shown in Eq. (10.2), where A is normalization constant, i is a number of detected radioactive materials measured in the previous work, Ci is air kerma-rate constant [12], SAi is activity in the surface soil [6], λi is a decay constant, and t is elapsed time from the normalization date.

From this formula, the dose rate changes that do not include movement of the radioactive materials can be estimated. A relative decay effect for dose rate change can be estimated from a sum of products of Ci and SAi. Absolute changes can be set up by the normalization with a measured dose rate. The normalization constant A was determined from the measured dose rate at each point on a normalization date. In this analysis, the normalization date is April 29, 2011, because the consecutive dose rate measurement for the same places shown in Fig. 10.3 started at April 29, 2011. The calculated results are shown as solid lines in Fig. 10.7.

Some differences between calculated and measured dose rates are found in Fig. 10.7. The measured dose rates become smaller than calculated dose rates in all three points with time, and the difference in blocks area looks larger than the

Fig. 10.8 Ratios of the deviations between measured and calculated dose rates for different paving materials. Horizontal axis is elapsed dates since the normalization date; vertical axis is a ratio of the deviations between measured and calculated dose rates; circles show the ratio at the cushionpaved area; squares show the ratio at the block-paved area; triangles show the normal asphaltpaved area

difference in cushion area. The dose rate differences between calculated and measured were changed to ratios for a calculated net dose rate as shown in Eq. (10.3) to compare decreasing trends.

Rdev = (Dmeas - Dcalc ) / DRM . (10.3)

Rdev is the ratio for a calculated net dose rate and Dmeas is a measured dose rate (Fig. 10.8). The decrease in gradient of the cushion-paved area was smallest, and the gradient of the normal asphalt-paved area was largest of the three areas (Fig. 10.8). The ratios for elapsed time were fitted with Eq. (10.4), where k is fixation ratio and r is a decreasing constant to estimate the effect of fixed radioactive materials.

Rdev = k ´{exp (-r ´ t ) -1}. (10.4)

k describes a ratio of the fixed radioactive materials and r describes a gradient of the decrease. Fitted lines are shown as solid lines in Fig. 10.8, which show different gradients and saturation values. k and r for each point are shown in Table 10.1. The fixation ratio of the cushion area dose rate is highest and that of the normal asphalt area is lowest in the three points.

From these analyses, it became clear that the dose rates on paved areas decrease faster than the physical decay of radioactive nuclides that were speculated to come

Table 10.1 Fixation rate and decreasing constant for different paving materials

Paving material

Cushion

Blocks

Normal asphalt

k r

0.13 ± 0.05

0.0033 ±0.0027

0.55 ± 0.06

0.0033 ± 0.0008

0.81 ± 0.08

0.0035 ± 0.0008

from the Fukushima Daiichi NPP, and the decreasing trends are correlated with the paving materials.

This phenomenon is believed to occur by capture activity differences of the paving materials for radioactive materials.

 
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