Basic Knowledge to Understand Radioactive Contamination

Abstract

This chapter will explain the basic characteristics of radioactive materials (radionuclides), which are necessary to understand the entire book, including the behavior of radiocesium in forests in Chap. 3 and countermeasures against radioactive contamination for exposure protection handled in Chap. 5 and later.

2.1 Radiation, Radioactivity, and Radioactive Materials (Radionuclides)

A material that has the ability to emit radiation (radioactivity) is called a radioactive material (radionuclide). There are different types of radiation, each with different characteristics.

A radioactive material (radionuclide) is a general term for a substance that has the ability to emit radiation (radioactivity). Since the nucleus of the atom is unstable, it changes into a different substance (nuclide ) over time by decay while emitting ionizing radiation. There are various types of radiation, and each type has different properties. The types of radiation emitted by each radionuclide is different.

There are three main types of radiation that are closely related to the nuclear accident: alpha (α) , beta (β) , and gamma (γ) . Alpha and beta rays are helium atoms and electrons emitted from the nuclei of radioactive elements, respectively, and are also called particle radiation. Gamma rays are not material, but electromagnetic waves, the same as X-rays used in X-ray examination. One of the characteristics of each type of radiation is its ability to penetrate objects (Fig. 2.1). Alpha rays have the lowest penetrating ability and can be stopped by a sheet of paper, beta rays can be stopped by a thin sheet of plastic or aluminum. However, gamma rays have a high penetrating ability and can pass through the human body, requiring lead or iron plates or thick concrete to stop them. Different types of radiation have different penetrating ability, and thus have different impact (exposure) on the human body. Since alpha and beta rays have low penetrating ability, they affect the tissues near the radioactive materials. On the other hand, gamma rays having a high penetrating ability affect the tissues inside the body while passing through the human body. Among radionuclides, cesium-134 and cesium-137 emit beta and gamma rays during decay.

Fig. 2.1

Types of radiation and their penetrating ability (Source: Adapted from Ministry of the Environment, Radioactive Waste Management Information Website “What is Radioactive Waste, Basic Knowledge of Radiation, Types and Characteristics of Radiation” [10])

Another important characteristic of radioactive materials is that their nuclei decay (change into other nuclides) over time, resulting in a decrease in the amount of radiation emitted. This phenomenon is called radioactive decay (or physical decay). The probability (or rate) that a radionuclide will decay in a certain amount of time is constant for each nuclide , and the decrease can be expressed as an exponential function. The time it takes for a radionuclide to decrease by half from its original amount due to radioactive decay is called its half-life (physical half-life), and the half-life of cesium-137, which was released in large quantities during the Fukushima nuclear accident, is about 30 years. For example, if there were 100,000 atoms of cesium-137, the number of atoms that would decay per day at that point would be about six, and it will take about 30 years for the number of decayed atoms to be halved to 50,000 (Fig. 2.2). After another 30 years, the number of cesium-137 decays to 25,000, half the number of cesium-137. It will take about 100 years for the radioactivity to decrease by a factor of 10, and 200 years for it to decrease by a factor of 100. The physical half-lives vary greatly depending on the nuclides (Table 1.1). Cesium-134, which, like cesium-137, was released in the nuclear accident, has a short half-life of about 2 years, so its radioactivity will decrease by a factor of 100 in 14 years. The physical half-life of iodine-131, which was released in the Fukushima nuclear accident in a quantity 10 times larger than cesium-137 and 134, is 8 days. Within 2 months after the accident, the radioactivity of iodine-131 was reduced to less than one-hundredth, and after 6 months, its effect was almost undetectable.

Fig. 2.2

Concept of physical half-life (In the case of a half-life of 30 years, 50% in 30 years and about 10% in 100 years)

In addition to the physical half-life, radioactive materials have various other half-lives, such as biological half-life, effective half-life, and ecological half-life:

• Biological half-life: This refers to the time it takes for radioactive materials taken into an organism through food , etc., to be discharged from the body (outside the tissues) through excretion and other metabolic processes, and to be reduced by half.

• Effective half-life: The half-life that takes into account both radioactive decay and biological half-life.

• Ecological half-life: The time it takes for radioactive materials within an ecosystem to be reduced by half due to changes in the environment such as material cycling and runoff via water.

In addition to the physical half-lives of different types of radionuclides, other half-lives vary greatly among radionuclides, organisms, and ecosystems. Understanding the various half-lives is important from the perspective of radiation protection, which will be discussed later, because it will help us understand how the amount of radioactive materials around us changes (Chap. 5). From now on, when we refer to “half-life”, we mean the physical half-life.

2.2 External Exposure and Internal Exposure

There are two main routes of radiation exposure.

Since the discovery of radiation at the end of the nineteenth century, mankind has explored and utilized the usefulness of radiation, while at the same time, research has been conducted to clarify the dangers of radiation, as the adverse effects of radiation on the human body became a problem. As a result, the concept of “radiological protection” was established to protect people from radiation. Depending on the degree of exposure, either immediate damage to the body’s tissues will affect the functioning of the body (deterministic effects), or there will be no immediate effects, but the probability of developing some form of cancer later on will increase (stochastic effects). The routes of exposure can be broadly divided into two categories: from outside and inside the body. Exposure to radiation emitted from radiation sources outside the body is called “external exposure”, while exposure to radiation emitted from radiation sources inside the body is called “internal exposure” (Fig. 2.3).

Fig. 2.3

Schematic diagram of external exposure and internal exposure (Source: Adapted from Ministry of the Environment, BOOKLET to Provide Basic Information Regarding Health Effects of Radiation, “Chap. 2 Radiation Exposure, 2.1 Exposure Routes, Internal and External Exposure” [1])

In more detail, radioactive materials emitted into the environment cause exposure to the human body through a variety of routes, but apart from the exposure during the passage of the plume immediately after the accident, the exposure routes from the environment after radioactive materials have fallen that require attention are:

• External exposure from radioactive materials contained in the surrounding environment (soil, etc.).

• Internal exposure due to ingestion of food and drinking water contaminated with radioactive materials.

Internal exposure from inhaling dust containing radioactive materials has also attracted attention, but it is considered to have less impact than the above two exposure routes.

2.3 Becquerel (Bq) and Sievert (Sv): Units for Radioactivity and Radiation Exposure Dose

Each of these units expresses the amount of radioactive material and the strength of the radiation effect on the human body.

We often hear the words “becquerel” and “sievert” used to describe radioactivity. What is the difference between them?

We explained in Sect. 2.1 that a radioactive material is a substance that has the ability to emit radiation (radioactivity). The becquerel (Bq) is used as a unit to express the intensity (amount) of radioactivity. When the nucleus of a radionuclide decays at a rate of one nucleus per second, the activity of the decay is defined as one becquerel. The becquerel is a physical quantity and can be measured with a high purity germanium (HPGe) semiconductor detector or a thallium-doped sodium iodide (NaI(Tl)) scintillation detector, by counting the gamma rays emitted from a sample placed in a shielded container (Fig. 2.4). Since the rate at which a radionuclide decays in a unit of time is constant, the becquerel can be regarded as the amount of radioactive material. Since the Fukushima nuclear accident, we have been measuring the number of becquerels of samples, which is the amount of radioactive material, to understand the dynamics of radiocesium in forests and other ecosystems.

Fig. 2.4

Instruments to measure the amount of radionuclides such as radiocesium. (a) High purity Germanium (HPGe) semiconductor detector, (b) sodium iodide (NaI(Tl)) scintillation detector (Photo taken by the author)

In contrast, the sievert (Sv) is used as a unit of intensity of damage (exposure) to the human body by radiation emitted from radioactive materials. The intensity of exposure varies depending on the type and amount of radioactive material, the distance from the radioactive material and the presence of shielding in between, as well as the age of the person exposed to the radiation and the part of the body. The sievert is a unit of measurement designed to evaluate the health effects of radiation from the perspective of radiation protection to protect humans from exposure. If the radiation dose evaluated in sievert is the same, the effect on human health is also considered to be the same, even if the type of radioactive material and the route of exposure are different.

When assessing the effects of radiation exposure on the human body, there are two ways to look at the exposure dose for each organ (equivalent dose) and the exposure dose for the entire human body (effective dose). Since the unit for both is the sievert, it is easy to confuse the two, but when considering exposure to radiocesium in the environment due to the Fukushima nuclear accident, the effective dose is often used because the human body is considered to be exposed uniformly to some extent regardless of the part or organ¹. The effective dose is calculated from the equivalent dose of each organ as weighted using tissue weighting factor. However, since it is not possible to measure the effective dose directly, other measurements such as those described below are used instead, or known coefficients are used. More detailed explanations can be found on the websites of the Reconstruction Agency of Japan (Basic Information Regarding Health Effects on Radiation) [11] and the Ministry of the Environment of Japan (BOOKLET to Provide Basic Information Regarding Health Effects of Radiation) [1].

To practically evaluate the effective dose due to external exposure, the air dose rate measured with a survey meter (ambient dose equivalent) and the personal dose equivalent measured with a personal dosimeter are used (Fig. 2.5). Both of these values are higher on the safe side than the effective dose. It should be noted that the external exposure is also affected by the time spent in the place where radiation is emitted (Sects. 5.4 and 6.1). The basic unit of the air dose rate is the sievert per hour (Sv/h), but in the environment in which people in Japan lived before the nuclear accident, it never exceeded 1 microsievert per hour (1 μSv/h, micro = 10⁻⁶), which is 1/1,000,000 of 1 sievert per hour (1 Sv/h). For this reason, the unit of microsievert (μSv/h) is used for the air dose rate in daily life, including in forests. As for internal exposure, the latest coefficients (effective dose coefficients, unit: Sv/Bq) for estimating exposure doses according to the type of radionuclides in food ingested and the amount of radiocesium (Bq) have been proposed by the International Commission on Radiological Protection (ICRP, Sect. 5.1) [12]. As a source of more detailed information, booklets and texts are prepared by ministries and public organizations of Japan. Please see the links at the end of this book.

Fig. 2.5

Various dosimeters and examples of their use. (a) a survey meter (air dose rate meter), (b) measurement of air dose rates in forests, using a pole with scale to measure at a fixed height, (c) personal dosimeters, and (d) personal dosimeters worn with the clip side facing outward from the body (Courtesy of Wataru Sakashita, the Forestry and Forest Products Research Institute (a, b), and photos taken by the author (c, d))

In this chapter, we explained what radioactive materials are, what radiation is, and how to measure it and their units. In the next chapter we will look at the behavior of radiocesium in the forest.

2.4 Column: Looking Back on that Time (2)

Relationship between research and society

Masamichi Takahashi

Technical Advisor, Japan International Forestry Promotion and Cooperation Center

Fellow, Forestry and Forest Products Research Institute

Following the Great East Japan Earthquake of March 11, 2011, the Tokyo Electric Power Company’s Fukushima Daiichi Nuclear Power Plant suffered a hydrogen explosion, and residents in the surrounding areas were evacuated during the tense days that followed. The actual situation in mountain villages was gradually coming out, and there were anxious concerns about the effects of radioactive materials deposited in forests. At the time, I was working at the Forestry and Forest Products Research Institute (FFPRI), where I was in charge of liaising with the Forestry Agency of Japan and other external organizations, and coordinating research within the institute. I was answering inquiries from the Forestry Agency, referring to papers on the Chernobyl nuclear accident. However, the institute had no equipment for measuring radioactivity. The only instrument we had was a Geiger counter, which we lent out to the Fukushima National Forest District Office. Apparently the forests were seriously contaminated. The Forestry Agency provided us with a supplementary budget for equipment such as germanium semiconductor detectors and for the renovation of the laboratory building, so with the cooperation of the administrative staff of the FFPRI, we hurried to design and construct the facility and install the equipment.

There were no experts on radioactive materials at the FFPRI. For this reason, we often exchanged information with organizations specializing in radiation and nuclear energy, but they did not know about nature, such as forests, mountains, and ecosystems. When I looked around the institute again, I found that we had many experts on forest structure, biomass estimation, material cycle, wood structure, soil, hydrology, mushrooms, and so on. Standing tree surveys and felling and sampling surveys in forests are routine tasks for us. A multi-disciplinary survey team was organized within the FFPRI. We were given special permission to cut down trees and we were dispatched to conduct the field survey in August 2011. In cooperation with Fukushima Prefecture, we also conducted a forest decontamination test. Due to concerns about the health effects of radiation, the survey was initially conducted by senior staff, including managers.

Most of the breaking data of forests that had already been published were only based on surveys in which only small parts of the leaves and branches of trees were cut off. The FFPRI aimed for a comprehensive survey that compares the distribution of radioactive materials in the entire forest ecosystem based on the difference between evergreen coniferous trees and deciduous broadleaf trees, as well as the contamination inside wood and this would be conducted at three locations in Fukushima Prefecture. Because of the urgency of the situation, the researchers acted behind the scenes, and the report was published as a press release by the Forestry Agency within the year. Our understanding of forest contamination has improved dramatically. Contamination inside wood could not be understood with the conventional academic common sense and was criticized by some experts. On the other hand, the comprehensive study of forest and wood was highly evaluated by nuclear experts.

After the press release, we received more requests to participate in public committees and review meetings. Interviews and lectures gave us more opportunities to communicate directly with newspaper reporters, citizens, and company executives. Scientific facts predicted the severity and prolonged duration of the effects, which further pushed the people affected by the accident to the edge. The reputation of the safety of nuclear power plants collapsed, and distrust of the government and science increased. Residents and local government offices, who were initially cooperative with the research, became frustrated because of the lack of progress in countermeasures. As a research administrator, I tried to be as sincere as possible, but it was always heartbreaking to talk with the people who suffered. I tried to encourage the use of the research results, but it took a long time due to conflicting opinions and adjustments among the related stakeholders. I was keenly aware that problem solving, which is the mission of researchers, cannot be realized without the understanding by society and politicians.

After a stormy few years, I think we are now able to write our papers calmly. The contents of this book show the world-class achievements in the field of environmental radioactivity. On the other hand, there are still many unresolved issues in Fukushima. I hope that the continuous efforts of researchers will lead to solutions step by step and brighten the future of Fukushima steadily.

Scenes of forest survey (Courtesy of Masamichi Takahashi)