Report on “Radiation Disaster Recovery Studies” (1/8)

Report on “Radiation Disaster Recovery Studies”

Name: Masaya Tsujimoto○Regarding“Radiation Disaster Recovery Studies”
Radiation disasters cause not only acute impact but also medium and long-term impact. It usually
happens that both/either “exposure” by radiation and/or “contamination” by radionuclides become
problems. When human bodies are exposed by radiation, the radiation causes ionization andexcitation, and produces the secondary electron. This electron directory damages the DNA strand orproduces the radical by reacting the water molecule which is also harmful for DNA strand. The
human bodies have recovery mechanism of DNA strand’s damages, but if DNA damages are caused
more frequently by radiation with high dose rate, the damages cannot be recovered in time and it
appears as functional obstacles (acute effect). On the other hand, cancerogenesis and genetic effectoccur at a peak age of onset due to the accumulation of DNA mutation (late effect). On the other hand,
if the radionuclides are released to the terrestrial environment, and if foods contaminated byradionuclides are taken, that causes the internal exposure. In particular, some isotopes have anaffinity with organs and make them being continuously exposed. In Tokaimura nuclear accident
occurred in 1999, three workers were severely exposed to neutron from critical uranium solution,
which happened due to deviation of manufacturing manual, during the processing of nuclear fuel.
Those who lived around the business office were also exposed and forced to evacuate. On the otherhand, in Fukushima Daiichi Nuclear Power Station (FDNPS) accident occurred in 2011, FDNPS wascollapsed by tsunami triggered by the East Japan Great Earthquake, and caused the hydrogen
explosion. A lot of radionuclides were released into the terrestrial environment, and the residents
were forced to evacuate and to stay in the temporary housing for a long time. Sudden change of life
style and prolongation of situation led to high level of stress, and eventually caused somephysical/mental diseases, such as diabetes and depression. These disadvantages were caused not by
exposure or contamination directory, but by the situation of radiation disaster. In order to recoverfrom radiation disaster, we need the different countermeasures in each phase of the disaster. In theinitial stage, the protection of human life is taken on top priority. Existence or non-existence of those
who are exposed by radiation or contaminated by radionuclides must be confirmed. In addition, itReport on “Radiation Disaster Recovery Studies” (2/8)
must be considered whether or not the residents should evacuate. In next (or same time), the types ofradiation disaster must be determined (exposure and/or contamination). Since the radionuclides
with short half-life decay soon, the analysis of environmental samples must be conducted
immediately. At last, reassurance of people must be created through the risk communication anddialog with people based on the objective facts. The radiation disaster recovery is achieved in a

Radiation disasterstepwise approach.

Medical issues
Environmental issues

Recoveryfrom radiation disasterSocial issues

Acute problems >>>>>>>>>Long-term problems

Conceptual Scheme of Radiation Disaster Recovery○Title of Doctoral Thesis
Field Study on Transfer of Radioactive Cesium into Rice Plants Derived from Fukushima Daiichi
Nuclear Power Station Accident○Summary of Doctoral Thesis
1. Introduction
Among the radionuclides released from FDNPS accident, cesium-137 (137Cs) has a long half-life(30.17 years) considering the length of an average human life. Cesium-134 (134Cs) has acomparatively short half-life (2.06 years), and was released in similar quantities to 137Cs. Cesium(Cs) itself is a homologous element with potassium (K), and Cs and K have similar physical andchemical properties. Thus, 137Cs and 134Cs may contaminate the agricultural products by replacing K.
Since agriculture is the key industry of Fukushima Prefecture, and rice is a Japanese staple food, itis very important to ensure food safety. Rice contaminated by 137Cs was observed in the private
paddy farm in Fukushima City in 2013. The Japanese Society of Radiation Safety Management has
investigated the cause of the contamination1). In later years, we have started the studies about thefactors contributing to the transfer of radioactive cesium into rice plants. We found some correlations
between,A. The grain size distribution of the soil and the transfer factor (TF) of radioactive Cs from the soil

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to rice plants.
B. The concentration of exchangeable cations and radioactivity concentration of radioactive Cs in
the clay and silt fraction.
C. The concentration of exchangeable K in the clay and silt fraction and TF
D. The radioactivity concentration of potassium-40 (40K) and TF in each part of rice plants, such as
soil to rice roots, rice roots to leaves, and rice leaves to ears.
We believe that these are useful data as results of baseline study for developing the environmental
countermeasures of radiation disaster recovery.
2. Experimental methods
2.1. Fieldwork
We conducted the fieldworks in the private farm located in northwest 60
km from FDNPS from 2014 to 2018. There are four rice fields neighboringeach other (Fig. 1). For different aims, the fieldworks were conducted,
such as the sampling of paddy soil and rice plants and/or measuring ofrice plants’ growth (Table 1).
Table 1 Schedule of fieldworks
Year2014

2016

2018

Month/Date

9/25

4/26, 5/18, 6/15, 7/5, 8/2, 8/24

3/14

and 9/24
FieldSampling

A to D

C and D only

C and D only

Soil and riceSoil and rice plant

Soil onlyplantMeasuring

Fig. 1 Position of rice fields
Plant height and SPAD value

We collected the samples of the soil from the top 5 cm of soil and batch of rice plants of at a centerand four corners in each rice field in each year. In 2016, the plant height was measured manuallyand the SPAD (Soil and Plant Analyzer Development) values were measured by a SPAD-502Plus
chlorophyll meter at each sampling point.
2.2. Classification of grain size
To make the uniform soil samples, thegravel fraction (> 2mm) was removed by drysieving after drying at room temperature in
the laboratory for a few days. The soilsamples obtained in 2014 were separated
into more fractions for further analysis ofgrain size distribution according to “ theFig. 2 Classification of grain sizeReport on “Radiation Disaster Recovery Studies” (4/8)
method of classification of geomaterials for engineering purpose” by The Japanese GeotechnicalSociety (JGS0051) (Fig. 2).
2.3. Radioactivity measurement
The soil and rice plant samples were packed in U8 vessels and their radioactivity concentration of137Cs, 134Cs and 40K was measured by Ge semiconductor detector. The radioactivity concentration

was calculated by a following equation. The detection efficiency was estimated by measuring of thestandard source set. The correction constant of sum effect of 134Cs was estimated by measuring of thesoil samples obtained in the same fieldwork site.

2.4. Transfer factor (TF)
The transfer factor (TF) is the ratio of radioactivity concentration (a) divided by (b).
(a) Radioactivity concentration after the transfer (rice ears)(b) Radioactivity concentration before the transfer (paddy soil)
For the samples in 2016, three-phase TFs were calculated. TF1, TF2, and TF3 are the TF from soil toroots, from roots to leaves, and from leaves to ears, respectively.
2.5. Measurement of exchangeable cations
The concentration of four exchangeable cations, such as sodium (Na), magnesium (Mg), potassium(K) and calcium (Ca), in the clay and silt fraction, which were separated from the soil samples in2014, were measured by ICP-AES. The exchangeable cations were extracted from about 5 g of clayand silt by 1 M ammonium acetate. The extraction was conducted three times (1st: 40 ml forovernight, 2nd and 3rd: 30 ml for 1 hour). The molar concentration of each exchange cation wascalculated by a following equation.

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3. Results and discussion
3.1. Correlation between grain size distribution of paddy soil and TF
The fraction of clay and silt was rich in field C and D that that of A and B. The small fraction may be
carried from the upstream (A and B) to the down (C and D) (Fig. 3). The concentrations of 137Cs and134Cs in field C and D were higher than that of A and B by approx. 20%. On the other hand, theconcentration of 40K in field B was especially lower than the others by approx. 25%. In field B, the TF
of Cs-137 and Cs-134 was 1.5 ~ 2 times higher than the others (Fig. 4).

Fig. 4 Radioactivity concentration of paddy
Fig. 3 Grain size distribution of paddy soil in 2014

soil and rice ears and TF in 2014

When the percentage of clay and silt
increased, or medium sand relatively
decreased, the TF became lower (Fig. 5). It
is conceivable that the small fraction, suchas clay and silt, includes much minerals
having a layer structure known as “friededge site” where cesium ion is fixedstrongly. In the field where the clay andsilt fraction is enriched, the radioactive
cesium is fixed in the paddy soil, and it isFig. 5 Correlation b/w grain size distribution and TF in 2014.

difficult to transfer to rice ears.
3.2. Correlation between the concentration of exchangeable cations and radioactivity concentration
In the clay and silt fraction in 2014, there is a competitive correlation between the total
concentrations of exchangeable cations (Na, Mg, K, Ca) and radioactivity concentration of radioactiveCs (137Cs, 134Cs) (Fig. 6). However, the inverse number of the concentration of exchangeable K in theclay and silt fraction was proportional relation with TF (Fig. 7). The exchangeable K behavescompetitively with radioactive Cs on the transfer into rice plants, while the exchangeable cations doon the placing in the small fractions of paddy soil.

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Fig. 6 Correlation b/w exchangeable cations andFig. 7 Correlation b/w exchangeable K inradioactive Cs in the clay and silt fraction in 2014.

the clay and silt fraction and TF in 2014.

3.3. Correlation between three-phase TFs and radioactivity concentration of 40K
Transfer factor 1 of 137Cs and 134Cs from the soil to roots became high when 40K was poor in the soil,
while TF2 of 137Cs from roots to leaves and TF3 of 137Cs from leaves to ears became high when theradioactivity concentration of 40K in the roots and leaves were also high, respectively. It suggested
that existence of K is competitive on the absorption of Cs by roots, but corporative on thetransportation of Cs inside of rice plants.
(a) TF1 and 40K in the soil(b) TF2 and 40K in the roots(c) TF3 and 40K in the leaves

Fig. 8 Correlation b/w three-phase TFs and radioactivity concentration of 40K in each place (upper: Field C, lower:Field D)

3.4. Correlation between three-phase TFs and rice plants’ growth
The TF1 increased with the rice plants’ growth when the plant height increased or SPAD value
decreased, but TF2 and TF3 decreased (Fig. 9 and 10). In the early stage of the rice plants’ growth,
the roots is not likely to absorb the radioactive Cs while the once absorbed radioactive Cs istransferred to upper parts of rice plants, such as leaves.

Report on “Radiation Disaster Recovery Studies” (7/8)(a) TF1 and plant height(b) TF2 and plant height(c) TF3 and plant heightFig. 9 Correlation b/w three-phase TFs and plant height (upper: Field C, lower: Field D)(a) TF1 and SPAD value(b) TF2 and SPAD value(c) TF3 and SPAD value

Fig. 10 Correlation b/w three-phase TFs and SPAD value (upper: Field C, lower: Field D)

3.5. Yearly decay of radioactivity of radioactive Cs
The concentration of 137Cs and 134Cs decreased according to their half-life (Fig. 11). It can be
estimated that 3,500 Bq kg-1 of 137Cs and 134Cs (7,000 Bq kg-1 in total) remained in both field C and D
in March 2011.

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Fig. 11 Comparison of radioactivity concentration of 137Cs and 134Cs in the paddy soil with the estimated values(left: Field C, right: Field D)

4. Conclusion
Transfer factor of radioactive Cs from paddy soil to rice plant is subject to some parameters as below.
(1) the grain size distribution of soil (percentage of clay and silt fraction)(2) the concentration of exchangeable cations (in particular, K) in clay and silt fraction(3) the radioactivity concentration of 40K
Distribution of KCl for the enrichment of K ion in paddy has been recommended to prevent theabsorption of Cs by rice plant. But K inside of rice plant helps Cs to transfer into upper parts, such asleaves and ears, which is edible part for human. Since the radioactivity concentration of Cs decreases
by no reason other than own decay according to its half-life, the remove of small fraction of soil
and/or rice roots after harvesting is effective decontamination to reduce the concentration of Cs insoil.
Reference1)

Matsuda, N. and Nakashima, S.: Radioactive caesium in water and soil and its absorption byrice plant (in Japanese). Japanese J. Rad. Safety. Manag., 13, 89-91, 2014

○Other thesis published in academic research journals(1) Tsujimoto, M., Miyashita, S., Nguyen, T. H., and Nakashima, S.: A correlation between thetransfer factor of radioactive cesium from soil into rice plants and grain size distribution ofpaddy soil in Fukushima, Radiation Safety Management, 15, 1-8 (2016).
(2) Tsujimoto, M., Miyashita, S., Nguyen, T. H., and Nakashima, S.: Monthly change in radioactivity
concentration of 137Cs, 134Cs, and 40K of paddy soil and rice plants in Fukushima Prefecture,Radiation Safety Management (accepted in February, 2020).