Knowledge on Distribution of Radioactive Materials and Air

Dose Rates

Fukushima Environmental Safety Center

Sector of Fukushima Research and Development Japan Atomic Energy Agency

Q1-4-5 Is it possible to predict how air dose rates in the environment will decrease in future?

Flowchart of questions about distribution of radioactive materials and air dose rates

Q1-5-1 How has the distribution of radioactive material been investigated?

Q1-4-1 How has the distribution of air dose rates changed over time?

Q1-4-2 What are the characteristics of air dose rate reductions? Do land uses or human activities affect air dose rates?

Q1-3-2 Air dose rates at flat locations without soil disturbance have decreased faster than expected just by radioactive decay. Where did the radioactive cesium move to?

Q1-6-1 Where can one find the data and results?

Q1-6-2 Is it possible to make community-based radiation maps?

Q1-5-2 How has the distribution of air dose rates been investigated?

Q1-1 What kinds of radioactive materials were released into the atmosphere and how much?

Q1-2 Where were radioactive materials deposited and how much?

Q1-3-1 How has the distribution of radioactive material changed over time?

Q1-4-4 How have air dose rates decreased in forests?

Q1-4-3 How are air dose rates in the residential environment related to data obtained by monitoring posts and car-borne surveys?

Measurement methods

Release of results

Radioactive materials

Air dose rates

Large amounts of many kinds of radioactive material were released by the accident of the Fukushima Dai ichi Nuclear Power Stat ion. The amount and types of radionuclides released were est imated by various organizations.

Q1-1 What kinds of radioactive materials were released into the atmosphere and how much?

Large amounts of radioactive nuclides were released over several events between March 12 and 21, 2011.

Radioactive nuclides were mainly released from Units 2 and 3.

According to most estimates, the total amount of 137Cs released was greater than 1016 Bq.

Amounts of radionuclides (Bq) released by the accident of the Fukushima Daiichi Nuclear Power Station

Results estimated by the Nuclear and Industrial Safety Agency (June 6, 2013)

133Xe 134Cs 137CS

89Sr

238Pu

129mTe

90Sr

239Pu 240Pu 241Pu

131I

Nuclide Half life Unit 1 Unit 2 Unit 3 Total

It was confirmed that radioactive iodine and tellurium deposited as well as cesium just after the accident. But now, radiation exposures are mainly caused by radioactive cesium.

Q1-2 Where were radioactive materials deposited and in what quantities?

Distribution of air dose rates at 1 m height measured by air-borne monitoring (November 19, 2013)

Maps of deposited radionuclides estimated by analyzing soil samples (June, 2011)

Radioactive nuclides that were confirmed to be deposited in many places by the accident were cesium (134Cs and 137Cs), iodine (131I), tellurium(129mTe), silver (110mAg), plutonium (238Pu, 239+240Pd and 241Pu), and strontium (89Sr and 90Sr).

137Cs Plutonium

Radioactive cesium deposited across a wide region of east Japan.

The amount of radioactive strontium and plutonium deposited in soil was within the range of levels that existed before the accident.

Radiation exposures were estimated based on measurement data. Long-term radiation exposures will mainly be caused by radioactive cesium, while the contribution of plutonium and strontium to radiation doses is expected to be low.

131I

(Cited from the Report on Investigation of Radiation Distribution：http://radioactivity.nsr.go.jp/ja/contents/6000/5235/24/5253_20120615_1_rev20130701.pdf, http://radioactivity.nsr.go.jp/ja/list/504/list-1.html)

(Cited from website：http://ramap.jaea.go.jp/map/)

ea

The amount of cesium (Bq/m2) deposited in flat locations where litt le soil disturbance occurs has decreased almost according to the half- l i fe of the nuclide. The amount of horizontal migration of cesium is small in uncultivated places without flooding.

Q1-3-1 How has the distribution of radioactive material changed over time?

Distribution of 134Cs deposited in soil at two different periods Time-dependence of averaged amount of cesium deposition

It was confirmed that the amount of 134Cs and 137Cs deposited in soil has decreased almost based on the half-life of the nuclides. Analysis of cesium deposition and field studies on environmental dynamics have revealed that radioactive cesium migrates

deeper into the ground over time, while amount of migration in horizontal directions is low.

The amount of cesium deposition is normalized by that of 137Cs in June, 2011

(These figures were made by JAEA based on results obtained in work for the Nuclear Regulatory Agency in 2014.)

1st measurement June 2011

5th measurement November 2014

Amou

nt o

f ces

ium

dep

osite

d in

soi

l (r

elat

ive

valu

e)

Physical decay based of the half-life

Measured value

Time after the accident (Years)

1F 1F

Amount of 134Cs deposition

Radioactive cesium gradually moves deeper into soil over time. When cesium moves downwards, air dose rates above the ground lower due to the shielding effect of the soil.

Q1-3-2 Air dose rates at flat locations without soil disturbance have decreased faster than expected by the half-life. Where did the radioactive cesium move to?

Time-dependence of depth where 90% of cesium is located (L90%)

Most cesium stays within the top 5 cm of the ground

Time-dependence of air dose rates

Reduction of air dose rates at flat locations can be explained by the shielding effect of the soil.

2011年6月21日の空間線量率を 1.0 としている。

Geometric mean value

Aver

aged

air

dose

rate

(rel

ativ

e va

lue)

Air dose rate is normalized to that on June 21, 2011. Time after March 11, 2011 (Days)

L 90%

dep

th (c

m)

Time after March 11, 2011 (Days)

Dec 2011 (n=84) Aug 2012 (n=82) Dec 2012 (n=82) Jul 2013 (n=82) Nov 2013 (n=82) Aug 2014 (n=24) Nov 2014 (n=78)

Physical decay based on the half-life Results on the ground measured by survey meter Decay considering shielding effect of soil

(These figures were made by JAEA based on results obtained in work for the Nuclear Regulatory Agency in 2014.)

Air dose rates and the amount of radiocesium in soil have decreased over time. Except for forests, air dose rates are decreasing much faster than those estimated by the half- l i fe of the nuclides.

Q1-4-1 How has the distribution of air dose rates changed over time?

Air dose rates on roads measured by car-borne surveys have decreased rapidly compared with those in flatlands without disturbance.

This is because horizontal migration of cesium in flatland without disturbance is slow, while cesium deposited on roads and their surroundings moves more easily.

Changes in the air dose rate distribution measured by car-borne surveys Time-dependence of averaged air dose rates

2011年6月からの経過日数 空間線量率（相

対値

）

March 2012 November 2014 Time after June, 2011 (Days)

Air dose rate is normalized by that in June, 2011.

Air d

ose

rate

(rel

ativ

e va

lue)

(These figures were made by JAEA based on results obtained in work for the Nuclear Regulatory Agency in 2014.)

1F 1F

Air dose rate Physical decay of radioactive cesium Air dose rates at flat locations without disturbance measured by survey meter Air dose rates measured by KURAMA-II

● ▲

----

It has been clarif ied that the tendency of reductions in air dose rates depends on land use and human activit ies.

Q1-4-2 What are the characteristics of air dose rate reductions? Do land uses or human activities affect air dose rates?

The reason that air dose rate reductions outside of the evacuation zone are faster than inside the evacuation zone is that human activities including decontamination work accelerates the reduction of air dose rates.

Decay of air dose rates on roads around buildings is fast, while that near forests is slow.

Characteristics of reductions in air dose rates has be found by analyzing the results of radiation measurements from car-borne surveys.

Effect of land use Difference between inside and outside

of evacuation zone

空間線量率（相対

値）

2011年6月からの経過日数

空間線量率（相対

値）

2011年6月からの経過日数

Air d

ose

rate

(rel

ativ

e va

lue)

Air d

ose

rate

(rel

ativ

e va

lue)

Time after June, 2011 (Days) Time after June, 2011 (Days)

Decay of air dose rates outside of the evacuation zone is faster than that of inside.

Differences in the reductions of air dose rates among land uses are due to 1) cesium adsorbed on artificial materials is more easily decontaminated, and 2) there is little cesium migration from forests.

(These figures were made by JAEA based on results obtained in work for the Nuclear Regulatory Agency in 2014.)

Physical decay

Forest

Buildings

●

▲

Physical decay

Inside of evacuation order zone

Outside of evacuation order zone

●

▲

Person-borne surveys have been carried out to investigate the characteristics of air dose rates in residential environments. It was found that air dose rates in residential environments are on average higher than those on roads, but lower than those in flat areas with soil.

Q1-4-3 How are air dose rates in the residential environment related to data obtained by monitoring posts and car-borne surveys?

In order to clarify the characteristics of air dose rates in residential environments, person-borne surveys were carried out in districts of various conditions within 80 km from the Fukushima Daiichi Nuclear Power Station.

It was clarified that air dose rates in residential environments are on average higher by 10%-20% than those on roads, but lower than those in flatlands without disturbance.

It was confirmed that air dose rates in residential environments fluctuate even in a small area of 100 m2.

Person-borne survey

Person-borne surveys were carried out over a wide area

Comparison between person-borne and car-borne surveys

(Cited from the Report on Investigation of Radiation Distribution：http://fukushima.jaea.go.jp/initiatives/cat03/pdf07/Part1-4.pdf)

Air dose rate measured by car-borne survey (µSv/h) (Average of 100 m mesh)

Air

dose

rate

mea

sure

d by

par

son-

born

e su

rvey

(µ

Sv/

h) (A

vera

ge o

f 20

mes

h)

Regions where air dose rates are lower than 1 µSv/h (n=217,967)

1F

The tendency of reductions in air dose rates has been investigated at many spots in forests. It was observed that air dose rates in forests have decreased basically following the half- l i fe of radionuclides.

Q1-4-4 How have air dose rates decreased in forests?

Air dose rates were measured in 2014 at the same spots in forests where the Forestry Agency took measurements just after the accident.

The air dose rates fluctuate depending on the kinds of tree. But on the whole, air dose rates in forests have decreased at the same rate as expected by radioactive decay alone.

Investigation on dynamics of cesium has revealed that most of cesium remains in forests and the amount of cesium migration to outside forests is small.

Change of air dose rates in forests

2011年6月からの経過日数 空間線量率

（相

対値

）

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

0 300 600 900 1200空

間線

量率

の変

化率

経過日数（平成23年12月1日からの）

初期値を1とした場合の低減率

【本事業成果（林野庁再測定地点）】

アカマツ

アカマツ・広葉樹

スギ

広葉樹

物理減衰

初期値（H23.12.1時点）

(Cited from the Report on Investigation of Radiation Distribution： http://fukushima.jaea.go.jp/initiatives/cat03/pdf07/Part2-1.pdf)

Air dose rates were measured at 1 m height using survey meters.

（2011年12月1日からの） Air dose rates have been measured at many

spots in forests in Fukushima Prefecture.

Decreasing ratio where the air dose rate on December 1, 2011 is unity.

Time after December 1, 2011 (Days)

Rat

io o

f cha

nges

in a

ir do

se ra

te

Initial value

Physical decay curve based on half-life

Japanese red pine Japanese red pine, broad leaf tree Japanese cedar Broad leaf tree

Spots where the Forestry Agency once measured

Q1-4-5 Is it possible to predict how air dose rates in residential environments will decrease in future?

Future air dose rates were predicted based on an empirical equation derived following the Chernobyl Nuclear Power Plant accident. Tendencies of air dose rate reductions were predicted using an empirical equation that is approximated by combining two exponential functions (fast and slow decay components).

Air dose rates on a 100 m mesh were predicted using best fit parameters in each 100 m mesh cell, determined on the basis of previously measured air dose rates.

Maps for future air dose rates up to 2041 were made considering the changes in land uses and human activities.

Large amounts of data on air dose rates that have been obtained following the accident were analyzed statistically. The tendencies of air dose rate changes were clarif ied. On the basis of the analyzed results, future air dose rates were predicted.

Flowchart for prediction of future air dose rates

5 years after March 15, 2011

Map of predicted future air dose rates in the 80 km area surrounding the Fukushima Daiichi Nuclear Power Plant

30 years after March 15, 2011

(Maps shown here were made by JAEA based on results obtained in work for the Nuclear Regulatory Agency in 2014.)

Utilization of obtained knowledge

Accumulation of data on air dose rate

Distribution of air dose rate as standard

Measurement at 1 m height

person-borne survey

Car-borne survey

Air-borne monitoring

Monitoring by Fukushima Prefecture

Monitoring by TEPCO

Future distribution of air dose rate

Future distribution of deposition

Half life in environment of fast decay component Rate of fast decay component (Set up at every mesh)

Compensation coefficient for air dose rate in residential environment

Air-borne monitoring

Results for analysis of Cs depth distribution

Method for long-term prediction of air dose rate

Application of two component compartment model and statistical analysis Half life in

environment of slow decay component

Other factors (Half life by physical decay, etc.)

Measured data in forests

Amount of soil erosion

Cooperation with development of model

Land use Evacuation zone

Preparation of environmental parameters

Q1-5-1 How has the distribution of radioactive material been investigated?

In order to investigate species, concentrations, and depth distributions of radioactive nuclides deposited in soil, appropriate methods for investigation have been applied according to the purpose and situation.

◆ Investigation of depth distribution: Depth distribution and migration behavior of radioactive cesium in soil are investigated.

・Soil is taken at every 0.5~several cm depth, paying attention not to mix the soil at different depths.

Sampling using a scraper pate

Relationship between concentration of 137Cs and depth

(Method 1) Analysis in laboratory after taking samples

(Method 2) Field measurement using portable germanium semiconductor detectors

◆ Measurement of amounts of deposition: Investigation of species and deposition quantities of radioactive nuclides in soil surface

・(Merit) Not only the amount of deposited nuclides but also their chemical states and soil properties can be obtained.

・(Merit) There is no need to take and carry samples. Averaged values of deposited amounts of radioactive materials can be obtained at each measuring spot.

Radioactive nuclides are analyzed using germanium semiconductor detectors in a laboratory.

Concentration of radiation (Bq/kg)

Depth

(cm

)

(The figure was made by JAEA based on results obtained in work for the Nuclear Regulatory Agency in 2014.)

Q1-5-2 How has the distribution of air dose rates been investigated?

For measurement of air dose rates, appropriate methods for investigation have been applied according to spatial resolution of the method and the scale of the area: from wide regions to specific spots.

◆ Car-borne survey・Person-borne survey：A radiation detector is loaded on a car or on a person. Analysis of continuous changes in air dose rates. (Spacial resolution: from several tens of meters to several hundred meters)

(Cited from website of Kyoto University Research Reactor Institute)

(Cited from website of the Nuclear Safety Technology Center)

◆ Survey meter: Air dose rate in a spot depending on the height and position where we want to know can be measured.

◆ Gamma plotter: Radiation measuring instruments on a pole. Detectors are set at 5 cm and 100 cm height Air dose rates at two different height can be measured simultaneously. Changes in air dose rates at a specific spot such as surroundings of a house can be observed.

（Detector loaded on a bus）

Detector

・Appropriate survey measures are used depending on the radiation level.

◆ Airborne monitoring：Radiation detectors are loaded on a helicopter Two-dimensional radiation map across a wide area (Spatial resolution: several hundred meters)

KURAMA-II system

(A man carrying KURAMA-II on his back)

Height 150m Data

Nuclear Power Station Ground

Ground surface contaminated with radioactive materials Data analysis using software

for data processing of airborne survey

Detector

Data analysis・Data sending board

Lowest value 0.32

Highest value 0.51

The results obtained by mapping investigations are summarized in reports, and they are open on the JAEA website.

In addition to the data from mapping investigations, the environmental data obtained by relevant ministries and local governments are combined, and a database was constructed. The database is released in website on “Monitoring Data of Radioactive Materials”.

Q1-6-1 Where can one find the data and results?

● “Results for Investigation on the Distribution of Released Radioactive Materials (first investigation)” (FY 2011).

● “Investigation on Secondary Distribution of Radioactive Materials Released by the Accident of Fukushima Daiichi Nuclear Power Station” (FY 2011).

● “Establishment of Method to Clarify Long-term Effect of Radioactive Materials Released by the Accident of Fukushima Daiichi Nuclear Power Station” (FY 2012).

● “Establishment of Method to Clarify Long-term Effect of Radioactive Materials Released by the Accident of Fukushima Daiichi Nuclear Power Station” (FY 2013).

● “Report on Airborne Monitoring” http://fukushima.jaea.go.jp/initiatives/cat03/entry03.html

Reports on Mapping Investigations Website on “Monitoring Data of Radioactive Materials”

Data download and their visualization are possible.

http://emdb.jaea.go.jp/emdb/

In the website on “Monitoring Data of Radioactive Materials”, JAEA provides supporting tools for data visualization by users, such as creating a detailed radiation map and making a graph for plotting time-dependence of radiation. (h t tp: / /emdb. jaea.go. jp/emdb/mappingtool .html)

Q1-6-2 Is it possible to make community-based radiation maps?

– Among the environmental monitoring results released on the website, about 60 kinds of data such as those of air dose rates and car-borne survey can be visualized.

– Community-based maps and graphs can be created by combining with free software such as Google Earth.

15

We can see the radiation time-dependence at a certain spot.

Main characteristic of supporting tool for visualization

Map Graph

We can zoom in and zoom out on the focused region.