Radioactive Materials Released by the Fukushima Nuclear Accident

Abstract

In this chapter, the release of radioactive materials and the characteristics of the forest ecosystem will be described to get an overall picture of the radioactive contamination of forests caused by the Fukushima Daiichi Nuclear Power Plant accident.

1.1 How Were the Radioactive Materials Dispersed from the Power Plant?

The fate of radioactive materials released into the atmosphere as a result of the Fukushima nuclear accident was affected by meteorological conditions such as precipitation and wind direction, and the amount deposited on the ground surface varied by more than 1000 times depending on the location.

The release of radioactive materials from the Fukushima Daiichi Nuclear Power Plant occurred as a result of the massive tsunami triggered by the March 11, 2011 earthquake, which caused the plant to lose power and the ability to cool its reactors. The release of radioactive materials occurred from several reactors, and the major releases and depositions are believed to have occurred between March 12 and 21. The main radioactive materials (nuclides) released were xenon-133 (¹³³Xe), iodine-131 (¹³¹I), cesium-134 (¹³⁴Cs) and cesium-137 (¹³⁷Cs). Among these, iodine-131, cesium-134, and cesium-137 are the most dominant nuclides, and although there is still some room for discussion about the exact value, the emitted amount of each nuclide is estimated to be 160, 18, and 15 PBq (P = peta = 10¹⁵ = thousand trillion, and Bq = becquerel; becquerel is explained in Sect. 2.3) according to the data compiled by the Ministry of the Environment of Japan (Table 1.1). The half-lives (the time it takes for a radionuclide to decay to half of its original amount, see Sect. 2.1) of these radionuclides are very different: 8 days (¹³¹I), 2 years (¹³⁴Cs), and 30 years (¹³⁷Cs), respectively. A short half-life means that the radionuclide decreases quickly through decay. Therefore, the amount of iodine-131 was high immediately after the accident, but rapidly decreased, and radiocesium contamination became a major problem several months after the accident. According to Table 1.1, which is an estimate for the first few months after the accident, the ratio of cesium-134 to cesium-137 was 1.2:1, but according to many subsequent studies it was considered that the emitted ratio was approximately 1:1 at the time of the accident. In addition, 7 years after the accident, the amount of cesium-134 decreases to less than one tenth of the initial amount. After that, contamination with cesium-137, which has a longer half-life, becomes a long-lasting problem. Since cesium-134 and cesium-137 have the boiling point of 671 °C and become gases when nuclear fuel is melted, and then become particles when the temperature drops below the melting point of 28 °C, most of the radiocesium in the air was considered to be in the form of small particles and was diffused by the wind. In the case of the Fukushima accident, radioactive plutonium (²³⁸Pu, ²³⁹Pu, ²⁴⁰Pu) and strontium (⁹⁰Sr) were released in very small amounts, so they were not a major problem. Although xenon-133 was released in much larger quantities than cesium-134 and 137, it has a very short half-life of 5 days and is an inert gas, so it is considered to have little impact on the human body or the environment.

Table 1.1 Half-lives of dominant radionuclides and amount released to the environment by the Fukushima Daiichi Nuclear Power Plant accident and Chernobyl Nuclear Power Plant accident

The Fukushima nuclear accident is often compared to the Chernobyl Nuclear Power Plant accident that occurred in 1986. According to the scale of an international organization, both accidents were serious accidents of level 7, but if we compare the amount of radionuclides released, we can see that the amount of radioactive iodine and cesium released by the Fukushima nuclear accident was less than a fraction of the amount released from the Chernobyl nuclear accident, and the amount of strontium and plutonium was less than 1/100 to 1/1000 of the amount released by the Chernobyl nuclear accident (Table 1.1). Furthermore, the area contaminated by the Fukushima accident was much smaller than that of the Chernobyl nuclear accident, which contaminated a large part of Europe (Figs. 1.1 and 1.2).

Fig. 1.1

Map of air dose rate in and around Fukushima Prefecture prepared by airborne monitoring. Decay corrected as of May 31, 2012 (Source: Data from Nuclear Regulation Authority of Japan, “(i) Results of Airborne Monitoring Survey in Hokkaido and (ii) Revision to the Results of Airborne Monitoring Survey over the Eastern Part of Japan with Detailed Consideration of the Influence of Natural Radionuclides” [3])

Fig. 1.2

Map of radiocesium (cesium-137) distribution in Europe after the Chernobyl Nuclear Power Plant accident. Including not only cesium-137 emitted from the Chernobyl nuclear accident but also derived from the past nuclear tests (global fallout: see Sect. 4.4) (Source: Reprinted from Saenko et al. [4], original map: De Cort M, Dubois G, Fridman ShD, et al. (1998) Atlas of Caesium Deposition on Europe After the Chernobyl Accident. EUR Report no.16733. Luxembourg: Office for Official Publications of the European Communities. https://op.europa.eu/en/publication-detail/-/publication/110b15f7-4df8-49a0-856f-be8f681ae9fd)

The released radioactive materials drifted in the air as a plume according to the wind at that time. Although the amount of radioactive materials released changed from moment to moment, and the flow was greatly affected by weather and wind direction at the time of release, most (about 80% in the case of cesium-137) flowed out to sea [2]. However, land areas, mainly in eastern Japan, were also widely contaminated. The contamination was particularly pronounced in the area extending in a northwestern direction from the Fukushima Daiichi Nuclear Power Plant, which was considered to have been mainly contaminated by the wind and rain in the afternoon of March 15 (Fig. 1.1).

Contamination of land is mainly caused by deposition , a phenomenon in which radioactive materials fall from the atmosphere to the land surface and adhere to objects on the surface. The higher the amount deposited, the more heavily the land is contaminated with radioactive materials. Deposition can be divided into two main processes. Wet deposition occurs when precipitation such as rain falls, and dry deposition occurs when fine particles of radioactive aerosols drifting in the air hit and adhere to the plant leaves and the ground surface (Fig. 1.3). In addition, there is fog water deposition caused by fog. In areas where the amount of deposition was high, it is thought that most of the deposition was caused by wet deposition [5, 6].

Fig. 1.3

Various deposition processes of radioactive materials

To understand a wide area of contamination, regional surveys using aircraft were conducted, which is commonly called airborne monitoring. In airborne monitoring, gamma (γ) radiation from the ground is measured and corrected using ground-based observations to create a map. The airborne monitoring revealed the extremely uneven spatial distribution of the contamination: even within a distance of 10–80 km from the plant, the air dose rate was around 0.1 μSv/h in the less contaminated areas, while in the highly contaminated areas northwest of the plant, there were points where the air dose rate exceeded 50 μSv/h. In addition, it has become clear that the amount of radiocesium deposited immediately after the accident ranged from less than 10 kBq/m² (k = kilo = 10³) to 10 MBq/m² (M = mega = 10⁶) at high-dose points, a difference of more than 1000 times. The airborne monitoring was not only used to provide the public with a visualization of the spatial extent of contamination, but was also widely used by researchers as a rough indicator of the contamination level at the study sites.

1.2 Characteristics of Forests in Fukushima

Fukushima is one of the areas in Japan where forests are widely distributed.

Approximately 67% of Japan’s land area is covered by forests (Fig. 1.4). The forest coverage in Fukushima Prefecture is 71%, which is higher than the average for Japan, and the forest area is 970,000 hectares [7]. Artificial forests (forests that have been planted and managed) cover 380,000 hectares, and natural forests (forests that have sprouted and grown by nature, or forests that have been unmanaged by humans for a long time) covers 580,000 hectares [7]. By type of ownership, national forests cover 410,000 hectares and privately owned forests cover 570,000 hectares. There are a wide variety of tree species, but the main ones are evergreen coniferous trees such as Japanese cedar (Cryptomeria japonica) , cypress (Chamaecyparis obtusa), and red pine (Pinus densiflora), and deciduous broadleaf trees such as konara oak (Quercus serrata) (Fig. 1.5). Evergreen trees have leaves throughout the year, while deciduous trees drop all their leaves in autumn and spend the winter without leaves on their branches, and then sprout new leaves in spring.

Fig. 1.4

Distribution of forests in and around Fukushima Prefecture. Forests are shown in green areas, and prefectural borders are indicated by black lines (Source: Data from Ministry of Land, Infrastructure, Transport and Tourism, based on Land-use Maps from “Digital National Land Information” [8])

Fig. 1.5

Forests in Fukushima Prefecture (Left: Japanese cedar forest; Right: konara oak forest in winter, which has no leaves because konara oak is a deciduous tree) (Source: Reprinted from IAEA, TECDOC-1927 [9], courtesy of Shinta Ohashi, the Forestry and Forest Products Research Institute)

In particular, the forests in the Abukuma Highlands (Abukuma Mountains, see Fact Sheet), which stretch to the east of Fukushima Prefecture, have been actively cultivating trees for mushroom logs, mainly konara oak, but they have been greatly affected by the contamination caused by the accident (Sect. 6.5).

1.3 Forest Ecosystems Are Unique and Different from Cropland

Time scales of forests range from a few decades to 100 years.

Forests are similar to cropland in the sense that plants grow on top of the soil . In reality, however, forests are different from cropland in many ways, causing significantly different behavior of radiocesium (Fig. 1.6). First of all, compared to cropland, where most plants (crops) are annuals, trees in forests have a long life span (perennial), ranging from several decades to more than 100 years. Over time, trees can grow to heights of 10 to 30 meters above the ground, spreading their branches and leaves to form a multi-layered structure that covers the ground surface. If you look at the surface of the ground, you will find layers of mineral soil, which are made up of minerals that have been weathered and mixed with decomposed humic organic matter, and a soil surface organic layer (also known as a litter layer), which is made up of organic matter such as fallen leaves and branches (Fig. 1.7). In cropland, there is no natural organic layer, although grass clippings may be used to mulch the cropland. In cropland soils, the plow layer (the soil near the surface that is used to grow crops) is created artificially by adding compost every year or by tilling and furrowing the soil with a tiller. On the other hand, in forest soils, the soil surface is sometimes disturbed by soil erosion and shallow landslides, but there is no artificial soil disturbance like in cropland. Therefore, the concentrations of various substances in the soil are uniform within the plow layer of cropland, but in forest soils, the concentration of substances and the soil quality vary with depth. In general, the concentration of nutrients tends to decrease with depth in forest soils.

Fig. 1.6

Schematic diagram of structural differences between forest and cropland

Fig. 1.7

Cross-sectional view of a forest soil . An organic layer consists of leaves or twigs at the soil surface decomposed slightly or strongly, and the darker materials below are mineral soil. Roots can be seen at the top of the mineral soil layer (Courtesy of Shinji Kaneko, the Forestry and Forest Products Research Institute)

In a forest, major nutrients such as nitrogen, phosphorus, and potassium, as well as trace elements (minor nutrients) necessary for plant growth, are circulating within the system mentioned above. For example, when it rains in a forest, some of the rain adheres to the trees, while the rest goes directly into the soil. Some of the rain on the trees will evaporate, but the rest will fall to the ground immediately or take a little time to travel through the trees and enter the soil. Some of the water that enters the soil penetrates deeply, while some water is absorbed by the trees. Trees are perennial, but as they grow, they renew their foliage by cutting off old leaves and branches and dropping them to the ground. The organic matter that falls to the ground, such as leaves and branches, becomes a source of nutrients for microorganisms and soil animals, and is decomposed by their biological activities. Some of the organic matter accumulates in the soil as humus that does not decompose quickly. Humus also decomposes little by little each year, and some of the nutrients released from it are absorbed by the trees again. The function of forests to circulate and utilize nutrients through litterfall is called self-fertilization, and is a major characteristic of forests.

The degree of human involvement is also very different. In the case of cropland, humans are heavily involved throughout the year in planting, fertilizing, harvesting, and tilling. Forests are also cut down, planted, and thinned, but on a time scale of decades, and human intervention is not as frequent as in cropland. Normally, there is no fertilization or tilling, and the cycling of materials of the ecosystem is left to nature. In addition, while cropland is mostly a place to produce food, the main product of forests is wood. It takes a long time to grow and harvest wood, usually 40 to 50 years, but in some cases harvesting can take over 100 years. In addition to the main product of wood, which is used for building materials, furniture, chips, and logs for mushroom production, various by-products are collected depending on the region, such as mushrooms, wild vegetables, lacquer, Japanese paper, dyes, and honey.

1.4 Column: Looking Back on that Time (1)

I’m proud that I’ve been a researcher in the field of environmental radioactivity

Keiko Tagami

Group Leader, Quantum Science and Technology Research Organization

The day after the 11th annual “Environmental Radioactivity” workshop in Tsukuba City, Japan, the Tohoku-Pacific Ocean Earthquake struck while I was working at my laboratory in Chiba City, which was also affected by the strong quake. Soon after the earthquake, after I finished checking that everyone was safe, I heard the TV program (no time to watch it at that time) reporting about the huge tsunami that was going to hit the Pacific coast of Tohoku near the epicenter. However, because I heard that the nuclear power plants in operation on the coastal areas had been shut down properly, I had no doubt that the safety devices in these plants were working properly; and I never imagined that it would become such a big nuclear accident.

On the next day, Units No. 1–3 of the Fukushima Daiichi Nuclear Power Plant were not cooled down enough and the situation became critical and started to release large amounts of radionuclides into the environment. By March 15th, relatively high air dose rates were recorded even in the Kanto area (Tokyo metropolitan and adjacent prefectures including Chiba). Because I and my colleagues are radioecologists, even in such conditions, we started to collect environmental samples. In particular, we wanted to know, “What sort of radionuclides were deposited on the ground and in what amounts?”. This information would give us the severity of the nuclear accident. Together with this sampling activity, because we were a limited number of environmental radioecologists at that time, we had to take care of many things. Since there were concerns about internal radiation exposure through food and drink, we conducted research on the removal of ¹³¹I detected in tap water, reduction of radioactive materials by food processing, absorption of radionuclides on plant surfaces, translocation of radiocesium in plants, etc. At the same time, we exchanged information with overseas countries and provided information to government agencies to reduce exposure doses.

Unfortunately, it was not an easy situation for us to open our research results to the public. At that time in the media, every day, I found unfamiliar faces speaking as environmental radioecologists (are they really experts?) and listened to their comments exaggerating how harmful the radiation exposure was, I felt sad every time. Yet, I measured environmental samples to understand the present situation, and thought about how to obtain data that would be useful in the future; for that purpose I’ve just kept working with nature. When I measured radioactivity in plants regularly, I noticed that plants simply responded as we had learned before the accident. Thus I thought, “This means that Japan is still safe, and we can reduce radiation exposure”. Later, however, I recognized that this is true but only in places where the contamination level is limited (not harmful to human beings).

Particularly in the area northwest of the Fukushima Daiichi Nuclear Power Plant as a highly radioactive plume passed through the area. No matter how beautiful nature is now, there is still high radiation there that can threaten people’s lives. When I think about it, I feel sorry because although I am an expert in environmental radioactivity, I can’t do anything about it. At least, I would like to know and inform the people how much radiation we are exposed to (or have been exposed to) from our living environments. To do so, we need to provide environmental parameters that can be used in mathematical model assessments, not just lists of raw data. I am glad that I’ve been a researcher in the field of environmental radioactivity who can do this.

It has been 10 years since the accident. We still have to tackle radiation. I would like to provide the people with useful information so that they can live with some peace of mind.