Friday 8 November 2013 from 2.30 pm to 4 pm
‘Environmental radiation in Fukushima’
Mr Kimiaki Saito, Fellow, Fukushima Environmental Safety Centre, Japan Atomic Energy Agency
The Mitchell Library, Glasgow
This Japan Matters public lecture was organised by Japan Desk Scotland, in cooperation with Glasgow Life, and with grant from the Great Britain Sasakawa Foundation. The lecture was free and open to the public.
This is my third visit to Glasgow. It was more than ten years ago that I first visited Glasgow to attend a meeting for International Commission on Radiation Units and Measurement (ICRU) Report Committee. We published ICRU Report 53, ‘Gamma-Ray Spectrometry in the Environment’, which was about in situ measurements using portable Ge detectors, and evaluating radionuclide ground deposition density and air dose rate. The basic data for evaluation of environmental measurements in this Report is actually being used in measurements in Fukushima now.
My second visit was last year. We had a big conference of International Radiological Protection Association in Glasgow, and I presented some results from Fukushima measurements.
I worked for a long time on environmental radiation measurement, and in the 1990s I visited Chernobyl every summer for about one month. We did measurement there and made a contamination map around Chernobyl, using helicopter survey. JAEA provided the map to the national authority of Ukraine. After that, I stopped working on environmental studies. I was organising research on life sciences in JAEA, and I was away from environmental studies for more than ten years.
However, just one year before my retirement, Fukushima accident happened. JAEA set up a new Department to support recovery from Fukushima accident and I moved to the new Department. Since then I have been working on a large-scale environmental monitoring and mapping, and today I would like to present some results from the projects I have been working on.
a) Radiation exposure in the environment
Radioactive materials or radioactive nuclide emits radiation, like a torch emits light. Radioactivity originally means ability to emit radiations, but the word is also used as radioactive materials. The unit of radioactivity is Becquerel, and the intensity of radiation is expressed by Sievert or Gray.
There are two different kinds of radiations. One is non-penetrating radiations like alpha rays and beta rays. Alpha rays can’t penetrate paper, and beta rays can penetrate paper, but can’t penetrate thin aluminium. Other group is penetrating radiations including gamma rays and neutron beam. It needs a large mass of materials to shield gamma rays and neutron beams. Water is very effective to stop neutron beams. If you intake radionuclide emitting alpha rays and beta rays, they play an important role in internal exposure, while gamma rays and neutrons contribute both to external and internal exposures.
Exposure pathways in the environment:
Radionuclides released from nuclear facilities stay in the air for a while, making external exposure from the air. If you inhale radioactive gasses, then you get internal exposures. After a while, radionuclides deposit on the ground or other objects, and then they become sources for external exposure. If you eat contaminated food, you get internal exposure. We must be noted that there exist natural radionuclides, and we get some amount of radiation from natural radionuclides.
Radiation sensitivity depends on the organ or tissues, so we use effective dose (E), which is defined by this formula.
E = ∑wT・DT
DT : Dose that an organ or tissue T receives
wT: Weigthing factor for considering radiation sensitivity
Effective dose E is convenient indicator able to measure radiation risk irrelevant to irradiation conditions, and is widely used in dose evaluation in the environment. Effective dose rate is difficult to measure directly, so it is obtained from measurable dose rates, such as air dose rate and radioactivity, and conversion coefficients.
b) About Fukushima prefecture
Fukushima City, the capital of Fukushima Prefecture, is located 240 km north of Tokyo. The population of Fukushima Prefecture is about 2 million, and the number of evacuees is estimated about 148,600 as of 9 August 2013.
Fukushima Prefecture is mountainous, and most people live in plains. These plains are divided into three regions: Hama Dori (Coastal street), where Fukushima Daiichi Nuclear Power Plant is located; Naka Dori (Middle Street), where many people are living; and Aizu.
Fukushima Prefecture produced a lot of rice, the 5th largest supplier of rice in Japan, and a variety of fruits, such as peach, apple, cherry, strawberry and persimmon. The prefecture generated electricity using nuclear, water, thermal, geothermal, and wind, and manufactures information and communication apparatus. These industries were damaged by the big earthquake and the nuclear accident.
2. Environmental radiation in Fukushima
a) National mapping projects
On 11 March 2011, an earthquake of magnitude 9 attacked Eastern Japan, and a big tsunami, with a height of more than 30m, followed. Then there was an accident at Fukushima Daiichi nuclear power plant. A large amount of radiation was released into the atmosphere. Radiation level rose suddenly. I was living in Ibaraki Prefecture, 100 km south of the plant, but still we observed a sudden increase of dose rate.
According to the estimated radioactive releases into the atmosphere from the Fukushima accident made by Nuclear Industry Safety Agency on 6 June 2011, the released amount of cesium was about one tenth of that from Chernobyl accident.
The released radionuclides were carried into different directions. Several radionuclides with longer half-life will remain in the environment. In order to estimate the impact of the Fukushima accident and take appropriate countermeasures, it has been necessary to obtain precise information on the contamination conditions.
The Ministry of Education, Culture, Sports, Science and Technology (MEXT) commissioned the Japan Atomic Energy Agency (JAEA) to construct detailed contamination maps based on reliable environmental monitoring. Actually, just after the accident a lot of environmental monitoring data was obtained by different organisations, but methods and accuracy were quite different and it was difficult to integrate them. A systematic observation by JAEA was required.
JAEA has completed three series of mapping projects in collaboration with many organizations.
1. The first campaign [June – November 2011]
We paid special attention to use reliable common methods.
2. The second campaign [December 2011 – June 2012]
It was already known that quite wide areas were contaminated, and we tried to cover wide areas contaminated down to 0.2 µSv/h, from Iwate Prefecture to Tokyo.
3. The third campaign [July 2012 – March 2013]
We started the construction of prediction models based on analyses of accumulated monitoring data. We have repeated the same kind of measurements several times, so we have found some temporal tendencies.
The tasks for mapping projects were:
1. Mapping of radionuclide deposition and dose rates in air
2. Studies on radionuclide migration, cesium migration, in natural environment
3. Construction of a database
4. Prediction of contamination condition in future
b) Radionuclide deposition maps
In the first campaign, the regions within 80km from the Fukushima site were divided into small areas of 2×2 km grid, and the region between 80 to 100 km and the rest of Fukushima Prefecture were divided into 3×3 km grid. The total number of areas was about 2,200. In each area, we selected one appropriate location for soil sampling. More than 400 volunteers took part in the soil sampling.
The data obtained in the soil sampling was written by hand in the first campaign, , and later data was input into a personal computer. This process produced a lot of mistakes. In the second campaign we devised an automatic data collection system. We directly input data into a tablet PC. The recorded data were transferred through a cellular phone network, and sampling location (GPS) was identified on a tablet PC. It was effective.
We collected top 5cm soil. We collected about 11,000 samples in the first campaign, and they were analysed at 21 laboratories.
In the second campaign, we started to use in-situ measurements, using portable Ge detector, to determine average deposition density of radionuclides.
Dose rates in air are expected to decrease rather rapidly within several years, according to physical decay of Cs-134 (half year 2.065 years). Weathering effects will accelerate the dose rate decrease.
From the radiation maps, it is clear that the areas in north-west direction of the Fukushima site are heavily contaminated, and that the middle part of Fukushima Prefecture, Naka Dori, is relatively highly contaminated.
Summary on regional distribution of radionuclides on ground:
1. Cesium is much more important than other nuclides from a viewpoint of exposure doses in future.
2. Plutonium and Strontium originating from the accident were detected, but the activity was not large.
3. Cs-134 deposition density has certainly decreased since June 2011.
4. Activity ratio of I-131, Te-129m or Ag-110m to Cs-137 has regional dependency:
a) high at south areas for I-131, Te-129m
b) high at north-west areas for Ag-110m.
c) Air dose rate maps: fixed locations, car-borne
Air dose rates were measured in terms of ambient dose equivalent which always overestimates effective doses.
Distribution of areas having different dose rate ranges within the 80 km zone
• Areas more than 0.2 µSv/h are decreasing, less than 0.2 µSv/h increasing.
• Nearly 70% of the total area has dose rates below 0.5 µSv/h.
Comparison of dose rates in air at 1m between June 2011 and Aug. 2012
• Dose rates in air decreased by more than 30% (Physical decay: 25%)
• There exist locations showing large dose-rate reduction.
Measurement of dose rates and GPS data per every 3-10 seconds were made available with KURAMA systems in a moving car, which allowed immediate data transfer through a cellular phone network and conversion of dose rates inside a car to those outside of the car.
KURAMA-II system is compact and easy to operate. 100 systems are distributed to about 200 local governments. Each local government makes a survey as it would like.
Comparison of air dose rates measured by car-borne survey
• Dose rates decreased by about 40% from June 2011 to March 2012.(Physical decay 20%)
• Dose rates decreased by about 55% from June 2011 to Sep. 2012. (Physical decay 25%)
• Dose rate reduction tendency was analyzed in connection with different land uses: Fast in urban and water areas, while slow in ever-green forest area
Summary on regional distribution of dose rates in air:
1. Dose rates in air above roads have decreased much faster than those at undisturbed flat fields: Which is close to living conditions?
2. Dose-rate reduction tendency has land-use dependency.
3. Decontamination is considered to reduce dose rates in air by a factor of 2-5.
4. Car-borne and air-borne surveys are effective to obtain precise dose-rate distributions.
3. Return of evacuees and decontamination
a) Arrangement of evacuee areas
After the safety of the nuclear power station was ensured by completion of Step 2 on 26 December 2011, restricted zone and planed evacuation zone have been rearranged into 3 areas responding to the annual cumulative doses
Areas where it is expected that residents will face difficulties in returning for a long time (> 50 mSv/y): About 25,280 evacuees
Areas in which residents are not permitted to live (20 – 50 mSv/y): About 24,600 evacuees
Areas to which evacuation orders are ready to be lifted (< 20 mSv/y): About 34,000 evacuees
In total there are about 148,600 evacuees.
b) Decontamination activities
Decontamination pilot projects have been carried out by JAEA. Based on the knowledge obtained in the pilot projects, full-scale decontamination has been performed.
Decontamination techniques for buildings:
high-pressure water washing; removal of dirt, sludge, fallen leaves, etc. in roof gutters, and street gutters; removal of peeling agent
Decontamination techniques for farmland:
plow; turf stripping; topsoil removal; spraying fixation agent
Mr Saito ended his talk by showing pictures of festivals held in Fukushima Prefecture after the accident.