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5.1 Internationally Agreed-upon Approach to Radiation Protection by the International Commission on Radiological Protection (ICRP)

The ICRP has put together a list of ideas for reasonably reducing exposure to radiation, depending on the situation.

The International Commission on Radiological Protection (ICRP) is a private, academic organization of experts from around the world who volunteer to formulate consensus on the basics of radiological protection and publish their recommendations in reports. Since 1928, the ICRP and its predecessors have been discussing the concept of preventing radiation hazards to human health and dose limits, and have repeatedly issued recommendations. The most recent recommendation on a system of radiological protection (ICRP Publication 103) [12] was issued in 2007, and ICRP Publications 109 [103] and 111 [104] were published in 2009 to advise on the application of Publication 103. However, these were all technical books for experts and were difficult for the general public to understand. Therefore, a commentary book on ICRP 111 was published in Japan in response to the radioactive contamination caused by the Fukushima nuclear accident [105]. In the following, we will mainly follow this understandable guide to explain the latest thinking on radiation protection.

The main points of the thinking behind ICRP 103 can be summarized in the following three principles.

  • The principle of justification : Consider radiation exposure as a risk and allow activities only when the benefits of exposure outweigh the risks (disadvantages).

  • The principle of optimization of protection: Making efforts to keep exposure reasonably low while maximizing the benefits to be gained from activities involving exposure, taking into account economic and social circumstances.

  • The principle of application of dose restrictions : Stepwise reduction of exposure by setting dose targets (reference levels) according to the exposure situation from emergency to normal through recovery period.

This way of thinking is actually reflected in the measures taken by the government after the Fukushima nuclear accident. It is also useful for individuals to think about how to protect from and deal with radioactive contamination of forests by adopting the above three principles.

5.2 Approaches to Radiation Protection in Forests

It is important to consider forest contamination in a balanced manner.

What can be expected if the ICRP ’s concept is applied to forests where radioactive contamination has occurred? As described in Chap. 2, the main exposure routes for humans can be roughly divided into external exposure from radioactive materials in the environment such as soil and internal exposure from ingestion of radioactively contaminated food . Forests tend to have a higher air dose rate than residential areas because radiocesium stays there for a longer period of time (Sect. 6.1). In addition, wild mushrooms, wild plants, and other blessings obtained from forests tend to have higher radiocesium concentrations than crops grown in the agricultural fields (Sect. 6.4). Therefore, people living in mountain villages potentially could be exposed to higher levels of radiocesium when they stay in the forest for a long time or eat wild mushrooms than when they do not. Therefore, it is necessary to make reasonable efforts to reduce the additional exposure (optimization) so as not to exceed the criteria of annual doses (dose restrictions). To reduce external and internal exposure, it is possible to check the air dose rate in the forests one has been in and avoid staying in high radiation places for a long time, and to reduce the amount of radioactive materials in food through cooking (Sect. 6.4).

On the other hand, for people who have been using forests before the accident, the fact that the forests are unable to use as before may have a negative impact on them physically and mentally. It is necessary for each person to make a decision (justification) after considering the additional exposure from using the forests as a risk (cost ) and weighing the benefits of using the forests on the other hand. The research on the actual situation of radioactive contamination of forests, that has been conducted since immediately after the Fukushima nuclear accident, will provide the objective evidence for promoting such optimization and justification measures.

It is not the purpose of this book to allow or recommend additional exposure for individuals. However, we feel that until now, domestic opinions after the Fukushima nuclear accident have emphasized the risks of radiation exposure. As the ICRP states, risks should be weighed against benefits, and we should act on the concept of radiological protection as our own.

5.3 Countermeasures in Contaminated Areas: The International Atomic Energy Agency’s (IAEA ) Approach

Each measure against radioactive contamination has its own advantages and disadvantages.

The International Atomic Energy Agency (IAEA) is an autonomous international organization within the United Nations system that aims to promote the peaceful use of nuclear energy and to prevent the military use of nuclear energy. In 2006, the 20th anniversary of the Chernobyl Nuclear Power Plant accident, the IAEA published a report on its experience with the environmental impact and remediation of the accident [39]. It also describes measures for forests. There are two main types of measures: one is a technology based countermeasure and the other is a management based countermeasure (Fig. 5.1). The former includes felling and removal of trees, removal of surface organic layer and mineral soil (decontamination), soil mixing, and potassium fertilization. The latter includes access control (zoning) according to the degree of contamination and regulating the consumption of contaminated food. In practice, there are also intermediate measures that use both.

Fig. 5.1
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Conceptual diagram of types of countermeasures and their benefit (advantage) and cost (disadvantage)

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The two types of measures are known to have their own merits (advantages) and demerits (disadvantages). For example, soil removal, which is one of the measures to apply the technology, is effective to some extent in reducing radiation levels, but it is known to generate huge costs and huge amounts of waste, and it is not always clear how cost effective it is. There is also the issue of radiation exposure of the decontamination workers. The latter, on the other hand, has the advantage of being less costly, but requires a longer period of time than decontamination before the contamination drops to a certain level, since radioactive materials can only be reduced by radioactive decay. Whatever measures are applied, as recommended in the IAEA report, there is a limit to the number of measures that can be applied to forests, and it is important that the effectiveness, costs, and benefits are thoroughly examined before implementation [106]. In addition, communication and understanding with local residents are essential for implementation, and decisions cannot be made based on scientific rationality alone.

5.4 Concept of Setting Criteria in Japan

In Japan, various criteria have been set in consideration of the estimated annual exposure dose.

To control the exposure dose due to radioactive contamination, Japan adopted the ICRP approach and established criteria of annual exposure doses. The governmental regulations for exposure protection after the Fukushima nuclear accident are divided into two categories: limit values set by laws and regulations, and limit values provisionally set by notices issued by the departments in charge of the ministries and agencies. The former includes limits of annual exposure dose for the general public and workers involved in decontamination work, and the concentration of radiocesium in food, applied as “standard limits”. In addition, for the use of forest products other than foodstuffs, the Forestry Agency of Japan has provided guidance to related industries in the form of notices using the term “index values” for radiocesium concentrations, which constitutes slightly more flexible but still substantial regulation as required by the Government of Japan. In this section, criterion/criteria (standard limits) is explained. The latter index values will be discussed in Sect. 6.2, which describes examples concerning the regulations for use of wood.

In the areas where radioactive contamination has occurred, the long-term goal is to keep individual exposure doses below 1 mSv per year, which is the reference level of exposure dose in normal times. However, it is not practical to set a uniformly strict criterion for all forms of exposure protection, since the situation varies depending on the degree of contamination, the relative importance of different sources of exposure and the time that has passed since the contamination occurred. Therefore, as mentioned earlier, the goal is to realize realistic and effective exposure protection by setting reference levels according to the contamination and social conditions and lowering the reference level in stages (optimization) [107]. The numerical value of 1 mSv is a target for effective implementation of radiation protection measures, and does not indicate that there is necessarily a health hazard if the exposure dose is higher than this value [108].

5.4.1 Criteria of Air Dose Rates

In environmental exposure protection, it is necessary to consider both external and internal exposures. As criteria for zoning and regulation of activities to reduce external exposure, air dose rates calculated by considering estimated annual exposure dose accompanying indoor and outdoor activities were used. In this section, we will discuss the criteria using air dose rates that are also related to activities in forests, including (1) the limit used to establish evacuation order zones (3.8 μSv/h, 20 mSv per year), (2) the target value for decontamination in the living area (0.23 μSv/h, 1 mSv per year), and (3) the limit of air dose rate which requires dose control in decontamination and other work (2.5 μSv/h, 5 mSv per year) (Fig. 5.2) [109].

  1. 1.

    Limit used to establish the evacuation order zones (3.8 μSv/h, 20 mSv per year)

In the immediate aftermath of the accident, emergency measures were necessary, and the ICRP set such a situation as an “emergency exposure situations” and set a reference level of 20–100 mSv per year as a reasonable exposure dose to promote recovery from the emergency situation while avoiding excessive exposure. In Japan, 20 mSv per year, the lowest of the reference levels, was set as the limit for residence. The air dose rate at which the external exposure dose from daily life is 20 mSv or less per year is 3.8 μSv/h. This calculation assumes that people are outdoors for 8 h and indoors for the remaining 16 h of the day, and that the indoor air dose rate is 40% of that outdoors.

  1. 2.

    Target value for decontamination in the living area (0.23 μSv/h, 1 mSv per year)

Next, the ICRP sets a reference level of 1–20 mSv per year for the situation (existing exposure situations) where contamination exists but the goal is to further reduce the effects after the stage where emergency measures should be taken. Therefore, to achieve the long-term target of 1 mSv per year, a target value of 0.23 μSv/h was set as the air dose rate in the living area through decontamination. As is the case with the criterion for evacuation order zones, the air dose rate is 0.19 μSv/h based on the assumption that people will be outdoors for 8 h a day and indoors for 16 h a day. Furthermore, adding the air dose rate from natural radiation that existed before the accident (average of 0.04 μSv/h) yields 0.23 μSv/h.

  1. 3.

    Limit of air dose rate where individual dose control is required in decontamination and other operations (2.5 μSv/h, 5 mSv per year)

Exposure to radiation when performing work such as decontamination in contaminated areas is called occupational exposure and is distinguished from public exposure. While occupational exposure permits a higher exposure dose than public exposure, strict individual dose control is required according to a governmental regulation (Ionizing Radiation Ordinance for Decontamination)Footnote 1 [110]. The limit for the air dose rate to determine whether or not the work requires such dose control (work under a specific dose) is 2.5 μSv/h. This value is derived by dividing the annual exposure dose of 5 mSv by the work hours (40 h per week × 50 weeks). The limit of the regulation is also applied to forests, and is used as a guide for decontamination work in forests and normal forestry activities (Sect. 6.1).

Fig. 5.2
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Calculation formulas on which the three representative air dose rate criteria are based (Source: Adapted from the Forestry Agency, Considerations for Prevention of Radiation Hazards in Operations in Forests and Other Areas (Q&A), “Reference Air Dose Rates” [109])

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5.4.2 The Reason for the Criterion of 100 Bq/kg for Food

As of April 1, 2012, the standard limit for general food has been set at 100 Bq/kg for the total of cesium-134 and cesium-137. The reason for this is that the annual internal exposure dose to humans from continuously eating food with a concentration of the standard limit under certain conditions was estimated to be 1 mSv (Fig. 5.3). Some assumptions are made in the calculation. The Codex Alimentarius (an international intergovernmental organization for the purpose of protecting the health of consumers and ensuring fair trade in food) and the EU have also set the criterion for food to keep internal exposure below 1 mSv per year. However, they set the criterion of radiocesium concentration in food at 1000 Bq/kg, which is higher than in Japan. This is due to different assumptions such as the ratio of foods containing radiocesium (50% in Japan, 10% in Codex and EU) [111].

Fig. 5.3
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Formula for calculating the standard limit of radiocesium concentration in general food (Source: Adapted from Ministry of the Environment, BOOKLET to Provide Basic Information Regarding Health Effects of Radiation, “Chap. 8: Radioactive Materials in Food, 8.1 Approach for Calculation of Standard Limits (1/2)” [1])

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Radioactivity inspection of food products was conducted based on the standard limit of radiocesium concentration. If the inspections show that the relevant food products widely exceed the standard limit within a municipality, shipping restrictions will be imposed. Wild mushrooms (mushrooms that occur naturally in the forests and fields) and wild plants (edible tree buds and shoots, bamboo shoots, and ferns, etc.), which are blessings of forests, have higher levels of radiocesium than other agricultural products, and shipping restrictions have been imposed over a wide area (Sect. 6.4).

5.4.3 8000 Bq/kg: Criterion for Waste

8000 Bq/kg is a standard limit for the safe disposal of waste. In the case of waste disposal, the amount of exposure was expected to vary depending on the type of waste and the work process. Therefore, the exposure doses from radiocesium in waste were estimated for residents living around the landfill site and for workers at the landfill site according to their work activities [112]. In the estimation of exposure doses for workers, it was assumed that half of the annual working hours were spent working with waste. As a result, it was calculated that the exposure dose from the landfill work of dewatered sludge, etc. was high, and it was confirmed that the concentration of waste should be kept below 8000 Bq/kg to keep the annual exposure dose from this work below 1 mSv. If the concentration is below 8000 Bq/kg, the waste can be disposed of in the same way as ordinary waste, but if it exceeds 8000 Bq/kg, the waste will be disposed of as designated waste under the management of the government.

In the case of forest contamination, the concentration in the bark of trees is high, and to ensure that the bark from the lumbering process does not exceed the standard limit for waste, areas in Fukushima Prefecture where wood can be used were set based on air dose rates. In addition, lower index values were set for firewood and charcoal because burning wood concentrates radiocesium in the combustion ash. These are discussed in Sect. 6.2.