Chapter 42
Radiation Disasters and Their Health Effects
Field of expertise: Radiation Science

Masatoshi Suzuki & Koichi Chida
SummaryRadiation disasters, which occur less frequently than natural disasters, pose the risk of exposure
across national and prefectural borders when large-scale disasters occur. It is highly expected that
promoting radiation education contributes to disaster prevention and mitigation through fosteringboth human resources who can respond to radiation disasters and a society that prevents
reputational damage based on correct understanding. It is required as the future tasks to elucidate
and accumulate the scientific knowledge about radiation effects especially in low dose areas forwhich knowledge is currently lacking, to further the development of radiation education.
Keywords: radiation disaster, nuclear power plant accident, deterministic effect (tissue reaction),
stochastic effect, low dose/low dose-rate exposure, medical exposure, radiation education

Introduction
The effects of radiation exposure on the human body are understood based on the lessons
learned from past radiation exposure cases. It is necessary to accurately evaluate the exposure
dose and dose rate when a radiation disaster occurs and to investigate the relationship of
biological and health effects with radiation exposure. It is difficult to conclude the radiation effectsof low-dose areas by epidemiological studies alone, and verification by the experiments usingstate-of the-art life science technology is also required.

1: Problems Revealed by the Great East Japan Earthquake
What happened?
Nuclear power generation utilizes a fission reaction between uranium-235 contained innuclear fuel and neutrons, and the neutrons emitted from this reaction react with other uranium235 to induce fission in a chain reaction. If the number of neutrons becomes excessive, nuclear
fission will go out of control, resulting in a large-scale radiation disaster such as the Chernobyl
nuclear accident. Therefore, control rods that absorb neutrons are used to control the fission toreact within an appropriate range. When power generation in the nuclear power plant is suddenly
stopped due to a natural disaster or other reason, electric power is required to function both thecontrol rods and the cooling system in order not to leak radioactive materials outside the reactor.
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The 2011 off the Great East Japan Earthquake caused a shaking with a seismic intensity of
6-upper in the area where the Fukushima Daiichi Nuclear Power Plant (FNPP) was located. FNPP
lost its external power supply due to the collapse of the steel tower from the shaking caused by theearthquake, but Tokyo Electric Power Company (TEPCO) reported that the power supply from theemergency power generation triggered an automatic shutdown system for all the reactors thatwere operating at the time of the earthquake. However, FNPP finally lost all the power supply due
to the tsunami that arrived about 50 minutes after the earthquake, and the reactor cooling functionin the two emergency shutdown systems stopped working. As a result, the core was damaged
from the temperature rising and a large amount of hydrogen was generated. In addition, theincreased pressure caused the structure to distort, resulting in radioactive materials and hydrogenleaking into the reactor building. Because of the ventilation system reducing the pressure inside
the building and the hydrogen explosion, radioactive materials were released into the environment
from the reactor building resulting in a radiation disaster. In this way, the FNPP accident became amulti-hazard disaster in which a natural disaster indirectly caused a radiation disaster.
The reality of the damage
The International Atomic Energy Agency (IAEA) and the Organization for Economic Cooperation and Development/the Nuclear Energy Agency (OECD/NEA) classify abnormal eventsand accidents at nuclear facilities into seven levels according to their severity by the International
Nuclear Event Scale (INES). The FNPP accident was considered to be the level 7 serious accident
which is the same level as the Chernobyl accident. However, the total amount of radioactive
material released by the FNPP accident is 77x1016 becquerels (Bq), which is about 6.8 times lessthan that of the Chernobyl accident (520 x1016 Bq). In the Chernobyl accident, the nuclear fission
went out of control and the nuclear fuel fragments were released into the environment. By contrast,in the FNPP accident, radioactive materials leaked from the reactor were released into theenvironment and suppressed the release of fuel fragments. It is thought that this was the cause oflower release of radiation in the FNPP accident.

2: Paradigms Destroyed by the Earthquake
Conventional wisdom and necessary responses
Human health effects of radiation exposure are estimated with reference to theepidemiological surveys of past radiation exposure cases. The Life Span Study (LSS) of atomic
bomb (A-bomb) survivors of Hiroshima and Nagasaki is the representative epidemiological survey,
which was conducted on a large-scale and continuously. The result of LSS has been referenced byvarious international organizations as an international gold standard.
Radiation effects are classified into two types, one is the deterministic effect (tissuereaction) and another is stochastic effect. The deterministic effect appears under exposure to acertain dosage (threshold dose). Symptoms become worse as the exposure dose increases. On
the other hand, the stochastic effect has no threshold dose and its incidence increases as theexposure dose increases. The symptom will not appear when radiation-induced damage can berepaired in the cells constituting tissue and organs, while the symptoms will appear as adeterministic effect when radiation induced cell death due to the residual irreparable damage. The
worsening of symptoms with dosage in the deterministic effect is thought to be due to theincreasing radiation-induced cell death. In the case of the stochastic effect, if cells with radiationinduced gene mutation survive, such cells potentially have a risk to be the origin of radiation2

induced carcinogenesis by accumulating other changes. The mortality analysis of LSS revealed
that cancer mortality increased from the range exceeding 100 mSv compared to spontaneouscancer mortality, and the mortality subsequently increased depending on the exposure dose. It isinternationally agreed that it is difficult to estimate the risk of radiation carcinogenesis using
epidemiological data in the dose range of less than 100 mSv because the incidence of radiation
carcinogenesis less than 100 mSv is similar to that of spontaneous carcinogenesis. Therefore,
from the viewpoint of radiation protection, the risk of radiation carcinogenesis of less than 100 mSv
is estimated by a linear non-threshold model (LNT model) extrapolated from the data in the highdose range.
Childhood thyroid cancer increased in the Chernobyl accident. Since the distribution offoods such as milk contaminated with radioactive iodine was not restricted, the radioactive iodinetaken into the body accumulated in the thyroid gland and caused internal exposure. The internal
exposure dose was estimated to be 300 mSv or more, and exposures exceeding 1 Sv (= 1,000mSv) were also reported.
The risk of health effects can be suppressed by reducing the exposure dose when aradiation disaster occurs. “Low dose” is often defined as 200 mSv or less. Since the threshold dose
for the deterministic effects was set as 500 mSv, it is important to focus on reducing the risk of thestochastic effects such as leukemia and solid tumors in case of low dose exposure. Therefore, themonitoring is necessary to limit the distribution of food contaminated with radioactive substances
resulting in reducing internal exposure.

3: A New Approach
Evacuation orders after the FNPP accident and restrictions on the shipment of foods thatexceed the standard values are effective to reduce radiation exposure, and these are lessonslearned from the Chernobyl accident. According to the Fukushima Prefectural Health Survey,
99.8% of the survey subjects, excluding those who have experience in radiation work, reported
that the external exposure dose was 5 mSv or less (average 0.8 mSv, maximum 25 mSv), which isa lower dose than the threshold dose for the deterministic effects. This is consistent with the factthat no deterministic effects from the accident have been reported so far, indicating the importanceof reducing the exposure dose when a radiation disaster occurs.
The inspection of meat contamination was always conducted after the livestock wereslaughtered; therefore, the meat contaminated in excess of the food standard value must bedisposed of. In addition, all livestock in the evacuation area (ex-evacuation zone) designated after
the FNPP accident were euthanized. If it can be determined before the livestock are slaughteredthat the contamination of edible parts exceeds the standard value, excretion of the radioactive
substances from the body can be promoted by feeding non-contaminated, clean feed, and as aresult, the meat can be shipped after the contamination decreased to below the standard value.
We have established a technique for estimating the concentration of radiocesium in the muscles bycounting from the surface of living cattle using the NaI survey meter. In addition, we obtained
experimental findings that can estimate the feeding period of non-contaminated clean feed
required for radiocesium to excrete below the standard value from the ratio of radiocesium
concentration in blood and in muscle as well as the biological half-life (Suzuki et al., 2019). The
practical application of this technology is expected to contribute to animal welfare because it canavoid unnecessary slaughter when radiation disasters occur, in addition to ensuring food safety.
Analysis using the plasma of cattle living in the ex-evacuation zone revealed that oxidativestress slightly increased. We also succeeded to evaluate the changes in antioxidant capacity,which is expected to be related to increasing oxidative stress, by the i-STrap method using3

electron spin resonance (Sun et al., 2018). Since it can be evaluated with a small amount of blood,
it is expected to be put to practical use in triage, health effect evaluation, and estimation ofexposure dose when a radiation disaster occurs.

4: Achievements and the Future
A new approach to disaster science
Through the experience of the public lecture held in Fukushima City after the FNPP
accident and the nuclear accident consultation in the office of Miyagi Prefectural Government, it
seemed that the general public had many misunderstandings related to radiation exposure at thetime of the FNPP accident. It was strongly realized that the dissemination of correct andfundamental knowledge is an urgent issue to prevent reputational damage (Chida, 2019).
Promoting medical exposure-related research is indispensable for low-dose exposure research as
an important research platform. In addition, the ongoing research of wild Japanese macaquesliving in the ex-evacuation zone is expected to provide insights to further understanding of the lowdose radiation effects on the human body.

Conclusion - from the author
Unnecessary exposure during a radiation disaster should be avoided, but after a radiation
disaster, there may be factors that bring about social changes such as low-concentration
radioactive substances remaining after decontamination and treatment of tritiated water. Since
there is no such thing as zero-risk in many events, including radiation, building acceptable risk
standards based on accurate information can become the driving force for reconstruction fromradiation disasters.

References
Chida, K. (2019). Seven years after the Great East Japan Earthquake: Build Back Better andRadiological Science. Japanese Journal of Radiological Technology, 75(8), 787-798.
https://doi.org/10.6009/jjrt.2019_JSRT_75.8.787 (In Japanese)Sun, L., Inaba, Y., Sato, K., et al. (2018). Dose-dependent decrease in anti-oxidant capacity ofwhole blood after irradiation: A novel potential marker for biodosimetry. Scientific Reports, 8(1),7425. https://doi.org/10.1038/s41598-018-25650-y
Suzuki, M., Suzuki, H., Ishiguro, H., et al. (2019). Correlation of Radiocesium Activity between
Muscle and Peripheral Blood of Live Cattle Depending on Presence or Absence ofRadiocontamination in Feed. Radiation Research, 192(6), 589-601.
https://doi.org/10.1667/RR15418.1

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