Paper

Radiation Safety Management Vol. 15 (1–8)

A correlation between the transfer factor of radioactive cesium from soil into
rice plants and the grain size distribution of paddy soil in Fukushima
Masaya TSUJIMOTO1), 2), Sunao MIYASHITA1), 2), Hai T. NGUYEN1), 2),
and Satoru NAKASHIMA1), 2), 3)
1)

Phoenix Leader Education Program, Hiroshima University, 1-1-1 Kagamiyama, Higashi-Hiroshima 739-8524, Japan
2)
Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526 Japan
3)
Natural Science Center for Basic Research and Development, Hiroshima University,
1-4-2 Kagamiyama, Higashi-Hiroshima 739-8526 Japan
Received Jan. 7, 2016; accepted Feb. 25, 2016

The transfer factors (TFs) from paddy soil to rice plants of radioactive cesium (137Cs and 134Cs) derived
from Fukushima Daiichi Nuclear Power Plant (FDNPP) accidents in March, 2011 and grain size distributions were
estimated in four paddy fields, A through D, in Fukushima City. Soil and rice samples were obtained in September,2014. The soil was measured for 137Cs and 134Cs radioactivity by Ge semiconductor detector and separated by thedry sieving classification. The radioactivity of radioactive cesium in the soil was comparatively higher in fields C
and D (> 4000 Bq/kg), but the TF in field B was the highest (1.9E-03) among the four fields. Meanwhile, the soil infields A and B contained a relatively large amount of larger grains, such as medium sand. Grain size distribution
might be related to the TF. This study shows a new correlation between the grain size distribution and TF.
Key words: radioactive cesium, paddy soil and rice, grain size distribution, transfer factor, Fukushima[doi:10.12950/rsm.160107]

1.

Introduction

radioactive cesium from soil containing smaller grains (< 250

We are apprehensive about a migration of radioactiveμm) is difficult 3), and a computational chemistry revealed that

cesium (137Cs and 134Cs) from paddy soil into rice plants sinceradioactive cesium can hold a place in a frayed edge site instead

Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. Theof potassium and be fixed strongly.4) For a few years after the

East Japan Great Earthquake, which occurred on March 11th,

accident, the radioactivity of radioactive cesium in rice pro-

2011, caused the unimaginable tsunami and FDNPP accident. A

duced from Fukushima Prefecture was investigated and the

lot of radioisotopes were released due to some hydrogen explo-

excessive level of the radioactive cesium in some rice wassions in FDNPP and the ground and environment were contam-

detected.5) The migration of radioactive cesium from soil to riceinated.1, 2) Radioactive cesium, which is one of radioisotopes

plants has been investigated. The transfer factor of radioactive

obtained as a fission product in a nuclear reactor, has a compar-

cesium into brown rice was estimated.6) Furthermore, a correla-

atively long half-life (the half-lives of 137Cs and 134Cs aretion between the concentrations of the radioactive cesium andapproximately 30 and 2 years, respectively). It stays in the envi-

potassium in rice plants and transfer factors were investigated.7)

ronment for a long time. Meanwhile, it is known that cesium

But, there might be many factors to influence the transfer factor

itself is absorbed to biological bodies as well as potassiumof radioactive cesium. It is also very important to investigate

because both of the elements are grouped into alkali metal andhow a grain size distribution as a soil character affects the trans-behave similarly. It is known that the decontamination offer factor of the radioactive cesium. However, a correlation

Natural Science Center for Basic Research and Development,
Hiroshima University,
1-4-2 Kagamiyama, Higashi-Hiroshima 739-8526 Japan

TEL: 082-424-6291 FAX: 082-424-0700
E-mail: snaka@hiroshima-u.ac.jp

1What determines the transfer factor of cesium?

between transfer factor of radioactive cesium and grain size distribution has been unrevealed. It is conceivable that radioactive
cesium migrates into rice plants from larger grains in paddy soil,(a)given smaller grains have the specific site, such as the frayededge site, and fix radioactive cesium so strongly.
Contaminated rice was observed in the rice from the paddy
field in Fukushima City, which is northwest, approximately 60
km far from FDNPP. The situation was investigated bySubcommittee concerning chemical treatment, Japanese Societyof Radiation Safety Management in 2013.8) In the present study,
we actually have visited paddy fields in Fukushima and obtained
soil and rice samples. We investigated how the grain size distribution in the soil influences the transfer factor.

2.

Materials and methods

2-1. Sampling and preparations
Paddy soil and rice plant samples were obtained in four(b)paddy fields in Oguraji, Fukushima City in September, 2014,
which are located northwest approximately 60 km far from
FDNPP and in Abukuma river basin (Fig. 1). The paddy fields
are irrigated mainly from a pond, but a little water is supplied
from Abukuma river. We sampled five soil samples from thesurface (0–5 cm) at the center and four corners and single rice
plant sample at the center in each field (A through D). All samples were, first, dried at room temperature for one day, and thenthe soil samples were dried completely at 105 °C for 24 hours by
a convection oven (NDO-401W, EYELA Tokyo Rikakikai Co.,
LTD.). The mass of the soil samples was measured by an electronic balance (GH series, Kensei Co., LTD.) before and after(c)complete drying in order to calculate a moisture ratio. The
moisture ratio was calculated in a following equation.
The moisture ratio (%) =The mass of water contained in the soil sample* (g)¥100
The mass of the soil sample before complete drying (g)*The mass of water contained in the soil sample= (The mass of the soil sample before complete drying) − (Themass of the soil sample after complete drying)
In the meantime, unhulled rice was removed from rice plant
samples and contained in U-8 vessels. Its mass was measured by
the electronic balance.
2-2. Dry sieving
2-2-1. Classification of soil
Soil can be classified by its grain size. We categorized the2Fig. 1. (a) The location of Fukushima Prefecture in Japan. (b)Positional relationships among sampling paddy fields, Abukuma
River, and FDNPP. (c) A view of the paddy fields and a pond asa source of water.

Tsujimoto et al.

100

soil samples in accordance with the method of classification ofgeomaterials for engineering purposes by The Japanese
Geotechnical Society (JGS0051). The soil samples with a grain
2 mm < X) were grouped into clay, silt, sand, and gravel, respectively. The sand samples were additionally grouped into fine,medium, and coarse sand by a grain size Y (75 μm < Y < 250μm, 250 μm < Y < 850 μm, and 850 μm < Y < 2 mm).

95
Sieving ratio (%)

size X (X < 5 μm, 5 μm < X < 75 μm, 75 μm < X < 2 mm, and

X1
X2
Y1
Y2

90

85

2-2-2. Pretest for dry sieving
To optimize experimental conditions, a dry sieving test byusing sea sands and a clay mineral was done. The sea sands
whose grain size is 300–600 μm, 425–850 μm, and 850–1400
μm (191–15955, 196–08175, and 190–11405, Wako Pure
Chemical Industries, Ltd.), and Montmorillonite (281522,
Sigma-Aldrich Co. LLC.) were mixed as test samples of soil

80
0

30

90
60
Sieving time (min)120

Fig. 2. Results of the dry sieving test. X and Y stand for the
results of the sieving of 1 g/cm2 and 0.1 g/cm2-test samples,
respectively. Two tests were performed in each condition. Whiteand black circles and triangles stand for test number X1, X2, Y1,
and Y2, respectively, shown in Table 1.

containing sand and clay and passed through a 10 cm diameter,
75 μm mesh size testing sieve (JIS Z 8801, NONAKA RIKAKI
Co., LTD.). The mass of the mixed sea sands andenough time (> 3 hours) (Fig. 2). After sieving, each grain (clayMontmorillonite is shown in Table 1. The mass of the test sam-and silt, fine sand, medium sand, coarse sand, and gravel) were22ples per sieving area was adjusted to 1 g/cm and 0.1 g/cm . The

contained in U-8 vessels, and the mass of each grain was meas-

dry sieving was performed in a shaker (FMC-100, EYELA

ured by the electronic balance. The result was plotted on a grain

Tokyo Rikakikai Co., LTD.) for 10, 20, 30, 60, and 120 min, andsize accumulation curve.

a sieving ratio was calculated in a following equation after sieving in each time.
The sieving ratio (%) =The mass of Montmorillonite passed through the sieve in the time (g)¥100
The mass of Montmorillonite mixed in the test sample (g)2-3. Measurement of radioactivity
Radioactivity of 137Cs, 134Cs, and 40K in the soil andunhulled rice samples was measured by Ge semiconductor
detector (GEM 30-70, ORTEC). The radioactivity of the soil
samples was measured in advance before dry sieving and that ofsand after dry sieving was done as well. The radioactivity in the

2-2-3. Dry sieving classificationsoil in each field was estimated as an average of that of the five

The soil samples were separated by using the 20 cm diame-soil samples. The radioactivity in the rice was done as a result ofter sieves (JIS Z 8801, SANPO Co., LTD.) as previously men-that of the single unhulled rice sample. To calculate the radioac-

tioned in chapter 2-2-1. This dry sieving was performed with thetivity of 137Cs and 134Cs, the 662 and 794 keV g-rays were tar-appropriate quantities of the soil samples (< 0.1 g/cm2) for

geted to be detected, respectively. The measurement was3What determines the transfer factor of cesium?

performed for enough time until the standard error of NET count10

of 794 keV g-rays becomes smaller than 5 %. The count efficiency, which comes from the differences of heights of the sam-

8
Moisture ratio (%)

ple sources and energies of the detected g -ray, is estimated bymeasuring a radioactivity standard gamma volume source set(MX033U8PP, Japan Radioisotope Association). The countefficiency of the 794 keV g-ray was calculated by logarithmic
and logarithmic interpolation between the count efficiency of
the 662 keV (derived from 137Cs) and 835 keV (derived from

6

4

2

54

Mn) g -ray. In addition, in order to estimate a coefficient, whichcalibrates the underestimation of 134Cs radioactivity because of0A

Sum effect (a Sum peak formed by 605 and 794 keV derived
from

134

Cs), some soil samples were placed 20 cm and 1 cm

from the detector to measure their radioactivity. This coefficient

B

C

D

Fig. 3. Moisture ratio of soil in paddy fields A through D. The
error bars show the standard deviation.

to calibrate Sum effect was calculated in a following equation.
The coefficient to calibrate Sum effect =The ratio of 134/137Cs radioactivity at 20 cm from the detector
The ratio of 134/137Cs radioactivity at 1 cm from the detector

3-2. Profile of soil and irrigation water
The soil in the fields A and B included slightly more water

The decay correction gave the radioactivity of 137Cs and 134Cs on

than that of C and D (Fig. 3). This may be because the fields A

the date of sampling as the result.

and B are located close to a pond, which supplies the irrigationwater, and the fields C and D are done in the ends of the running

2-4. Calculation of transfer factors

water. This locational fact might be also able to explain the dif-

Transfer factors in which the radioactive cesium migrated

ference among the grain size distributions (Fig. 1c). Accordingfrom the paddy soil to rice plants were calculated in a following

to the grain size accumulation curves, a composition of soil isequation.

similar between A and B, and between C and D (Fig. 4(a)). The
soil in the fields A and B had slightly less clay and silt than that

The transfer factor =The radioactivity in the unhulled rice (Bq/kg)
The radioactivity in the soil before sieving (Bq/kg)

of C and D. The running water might carry the clay and silt fromthe fields A and B to C and D. However, medium sand aboundedin the fields A and B than C and D (Fig. 4(b)). Whereas the soil
was composed chiefly of sand this time, in our experiment, some2-5. Handle of errors
For the case that the two and more data were obtained as aclay and silt might be still attached on surface of sand.

result, a standard error was calculated from a standard deviation

Additionally, according to the previous study, radioactivity ofas an external error. The data of radioactivity of 137Cs, 134Cs, and

137

40

Abukuma river makes little differences and is sufficiently low.8)

K in the soil had standard errors, and internal errors were also

Cs and 134Cs in the both of irrigation water of the pond andcalculated by conforming to the rule of error propagation.
Larger one of them was adopted (Table 2(a) and (b)).

3-3. Radioactivity in the soil and rice and transfer factor
The radioactivity of 137Cs and 134Cs in the soil before sieving

3.

Results and discussion

in the fields C and D was higher than that of A and B, and theradioactivity in the sand after sieving denoted the same tendency

3-1. Dry sieving test
Montmorillonite could be separated from the test samples by(Fig. 5(a) and (b)). However, the radioactivity of 137Cs and 134Cs

over 95% for up to 2 hours when the dry sieving tests were per-in the unhulled rice in the field B was specifically the highest and2formed with the test samples of 0.1 g/cm (Fig. 2). On the otherthe transfer factor in the field B was also as well (Fig. 5(c)).

hand, only less than 90% of Montmorillonite could be separated

Although the transfer factor of 137Cs in the field A was placed

from the test samples of 1 g/cm2.

second behind that of B, the radioactivity of 134Cs in the rice inthe field A, C, and D could not be estimated due to the size of4

Tsujimoto et al.

error. At any rate, the radioactivity of 137Cs and 134Cs between

transfer factor of 137Cs was high in the field where the soil was

the soil and unhulled rice was not simply correlated with eachcomposed mainly of medium sand and, in contrast, less silt andother. It was inspired that the higher radioactivity of radioactive

clay (Fig. 6). The transfer factor was relatively higher in thecesium in soil does not always cause more migration of radioac-

fields A and B where the soil contained more large grains, such

tive cesium into rice plants. Meanwhile, the lack of 40K was

as medium sand (and less small grains, such as clay and silt). As

found in the soil in the field B (Fig. 5(d)). The same tendency

the specific site to adsorb the cesium, say frayed edge site,

was confirmed in sand (Fig. 5(e)), though the radioactivity ofabounds in small grains in soil, it is presumable that the migra-

137

Cs andtion of the radioactive cesium into rice plants from soil contain-

Cs in the total soil was 0.8–0.9 times higher in the sand, but

ing the small grains is unfavorable. Just for the record, the

Cs,

134

40

Cs, and K was different: the radioactivity of

137

134

40

that of K in the total soil was 1.0–1.2 times higher in the sand.

moisture ratio of the soil in the fields A and B was also, coinci-

The transfer factor of 40K was the highest in the field B (Fig.

dentally, relatively higher as well as the transfer factor, though5(f )) because rice plants absorb the determinate amount ofwater is likely to be a carrier of radioactive cesium.

potassium independent of the concentration in the paddy soil.

4.
3-4. Correlation between the transfer factor of 137Cs andgrain size distribution

Conclusion
It has been thought that a concentration of potassium in

paddy soil can mainly contribute to a transfer factor of radioac-

The amount of medium sand in the soil influenced thetive cesium into rice plants. And, in our knowledge, a correla-migration of the radioactive cesium into the rice plants. Thetion between a grain size distribution of paddy soil and a transfer

5What determines the transfer factor of cesium?

100

Accumulated ratio (%)

(a)

75
A

50

B
C
D

25

0
1.E+01

1.E+02

1.E+03Grain size (µm)

1.E+0440(b)Rate of grains (%)

30
A
B

20

C
D
10

0
Clay and Silt(b) 4000

2000
1000

(c) 201.E−02

1.E−03

40

Cs

K

3000
2000
1000
0

0
A

B

C

B

C

500
250

A

B

C

D

B

C

D

(f) 1000

750
500
250
0

0

1.E−04
A

Radioactivity (Bq/kg)

Radioactivity (Bq/kg)

750

5

D

(e) 1000(d) 1000

10

0
A

D

15

1.E+00

750
500
250
0

A

B

C

D

1.E−01
A

B

C

D

Fig. 5. The results of radioactivity and transfer factor (TF) of 137Cs and 134Cs (a) in the soil before sieving, (b) in the sand after
sieving, and (c) in the rice. (d), (e), and (f) show those of 40K, respectively. The error bars show the standard error.

6

TF

3000

134

Cs

TF

Radioactivity (Bq/kg)

137

Radioactivity (Bq/kg)

Gravel(a) Grain size accumulation curves. (b) Rate of each grain contained in the soil. The error bars show the standard error.

(a) 4000

Radioactivity (Bq/kg)

Medium sand Coarse sandRadioactivity (Bq/kg)

Fig. 4.

Fine sandTsujimoto et al.

Rate of grains (%)
0

10

20

30

Clay and Silt

Medium sand40

TF

1.E-02B

B

1.E-03
A

A
C

D

D

C

1.E-04
Fig. 6.

A correlation between the transfer factor of 137Cs and rate of grains. The error bars show the standard error.

factor has been unrevealed. However, this paper could havedecreasing the proportion of medium sand in paddy fields, bygiven a consideration for this correlation. In this study, we actu-

design, might be able to block the migration of radioactive

ally used the samples of the soil and rice plant obtained incesium from soil into rice in spite of the loss of potassium andFukushima. The dry sieving classification has been performed

the contamination level of radioactive cesium, though it shouldadequately based on the result of the pretest, and the radioactiv-

be considered whether that grain size distribution is appropriate

ity of the radioactive cesium has been measured and calculated

for growing rice plants. However, decline of the radioactivity

properly with the revisions as far as we can see. We found theover time will be shown, and the continued observations aredifferences of the moisture ratio and grain size distribution withnecessary. Therefore, future research should include follow-up

a few % among the soil samples in fields A through D, evenworks how the transfer factor, grain size distribution and theirthough the locations of these fields neighbor to each other. And

correlation will change.

moreover, there were the difference in the radioactivity ofcesium among fields and the lack of potassium in field B. The

5.

Acknowledgement

radioactivity of cesium was not corresponded with the differ-

We appreciate the cooperation of Mr. Kato, which provided

ence in the transfer factors. They were affected by the concen-

the soil and rice plants as the samples from his paddy fields. Ge

tration of potassium and difference in the grain size

semiconductor detector of Phoenix Leader Education Program,

distributions. The migration of radioactive cesium from paddy

Hiroshima University was used for the radioactivity measure-7)

soil to rice plants easily occurs in a poor-potassium field. In ourment.

experimental fields, the radioactivity of 40K in the field B was
comparatively low, and the transfer factor of the radioactive
cesium was high. It exactly supports the previous study.
However, the transfer factor of radioactive cesium in the field A
was also relatively higher than that of C and D, even thoughthese fields likely had an enough concentration of potassium(the radioactivity of 40K was more than 600 Bq/kg in thesefields). The rate of sand, especially medium sand in soil, wasalso one of key factors to determine the transfer factor ofradioactive cesium from soil to rice plants. These findings provided fresh insight into the migration of radioactive cesium into
rice plants. Additionally, this study indicated a possibility toprevent radioactive cesium from moving to rice plants by controlling the grain size distribution in paddy soil. For example,

6.

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