Fukushima Water Discharge: The Science Behind the Decision 


By Dr. Makoto Akashi

At the end of August 2023, the Fukushima Daiichi Nuclear Power Plant began a 30-year process to release more than one million tonnes of treated radioactive water into the sea, triggering global headlines and leading to concern from some quarters. Dr. Makoto Akashi explains the decision to discharge, why it is safe, and the importance of tackling scientific misunderstandings and misconceptions.

Almost twelve and a half years have passed since a huge earthquake struck the northeast coast of the main island of Japan, and the resulting tsunami caused serious damage to the Fukushima Daiichi Nuclear Power Plant (NPP) operated by the Tokyo Electric Power Company (TEPCO).  

In the disaster, the cooling system at operating reactor units was destroyed as well as at the spent fuel pools; the reactor cores overheated, and consequently the nuclear fuel melted. The containment vessels were also breached. Before the accident, the groundwater from the mountainside was pumped to the reactor through the sub-drains located around reactor buildings. The tsunami also damaged the sub-drains and pumps preventing groundwater from flowing into the buildings.  

Since the radioactive materials from the cores could not be contained in the reactor vessels, the groundwater flowing into the reactor buildings was mixed with radioactive debris resulting in contaminated water. Moreover, the groundwater entering the reactor buildings was also used to cool the fuel debris to keep the reactors in a stable condition. Rainwater also flowed into the building and was mixed with the fuel debris.

Treating the Radioactive Water

To remove the radioactive materials and reduce the radiation dose from the water, a system named the Advanced Liquid Processing System (ALPS) was developed. ALPS can remove most radionuclides from the contaminated water to below regulatory limits, including cesium (Cs) and strontium (Sr) which are the source of most of the radioactivity in the contaminated water.

TEPCO has been collecting and storing the ALPS-treated water on site, however the number of large tanks each containing 1,000 m3 of the ALPS-treated water had reached their capacity. Decommissioning of the plant is in progress and management of the increasing volume of the water is essential for the recovery from the disaster. It was for this reason that, in April 2021, the Government of Japan decided to discharge the ALPS-treated water at the TEPCO holdings’ Fukushima Daiichi Nuclear Power Station into the sea. The discharge of the water began in August 2023.

The problem is that a radioactive hydrogen isotope called tritium (3H) cannot be removed from the contaminated water by ALPS since 3H mostly exists as a form of water (tritiated water, see Fig. 1). Before being discharged, the ALPS-treated water is diluted with sea water more than 100 times until its radioactivity measures less than 1,500 Bq/L for 3H and 1% of regulatory standards for radionuclides other than 3H (Fig. 2). The World Health Organization’s (WHO) guidelines for drinking-water quality give a guidance level of 10,000Bq/L for 3H. Meanwhile, the water being discharged from Fukushima measures one-seventh of the WHO’s guidance levels, at 1,500 Bq/L for 3H.

Figure 1: Tritiated water. There are three forms tritiated water; one or two stable hydrogens are replaced with 3H.  Tritiated water has no color or odor like regular water H2O, © Makoto Akashi

The Effect of 3H in the Discharged Water on Health is Negligible

Hydrogen has two stable isotopes and a radioisotope (Fig. 3). Protium (1H) and deuterium (2H, D) are stable, and tritium (3H, T) is radioactive. Protium is the most common isotope of hydrogen, and its nucleus consists of a single proton with no neutrons. This isotope is usually called “hydrogen” and its formal name “protium’’ is rarely used.

Deuterium has a proton and a neutron, and between 0.0184% and 0.0082% of all hydrogen is this isotope. Tritium (3H, T) on the other hand is a radioactive isotope of hydrogen with a half-life of 12.43 years and decays to a stable helium isotope (3He) by emitting β-rays (electrons); thus, the amount of 3H decreases by half during this period. Tritium occurs naturally in the environment in very low concentrations, and the chemical behavior of tritium is the same as those of stable hydrogens. Therefore, it is common for tritium to exist in the human body.

Figure 2: Discharge of ALPS-treated water into sea. ALPS-treated water is diluted with sea water more than 100 times until less than 1,500 Bq/L for 3H and 1% of regulatory standards for the radionuclides other than 3H, © Makoto Akashi.

The β-rays emitted from 3H can be shielded with a thin sheet due to its low energy and can only travel 6 µm and 5 mm in water and in air, respectively. These β-rays are unable to penetrate the very outermost layer of human skin, known as the stratum corneum. Therefore, concerns about the health effects of 3H is not external exposure but internal exposure.

When tritiated water is incorporated into the body, it is uniformly distributed throughout the body. Tritiated water goes directly into soft tissues and organs, and is excreted like H2O. This is known as free water tritium, abbreviated to FWT. FWT has a biological half-life of 10 days, and therefore the amount of FWT in the body decreases by half for every 10 days in the body. On the other hand, almost 3 % of FWT incorporated into the body is converted to organically bound tritium (OBT) and, with a biological half-life of 40 days, remains in the body for a longer period than FWT.

If a five-year-old child drinks 2 liters of water with the same concentration of 3H as the discharged water every day, the radiation dose is 33.9×10-6 Sv per year.*

This dose is much smaller than 1mSv, the yearly dose limit of ionizing radiation recommended by the International Commission on Radiological Protection (ICRP) to members of the public.

Similarly, if this child eats 2kg of food containing 1,500 Bq/kg of tritium as OBT every day, the dose is 79.9 x10-6 Sv per year.**

Thus, the tritium level in the discharged water into the sea is much smaller than the guidance level of 3H for drinking water of WHO, and the effect of 3H in the discharged water on health is negligible.

Figure 3: Stable or radioactive hydrogens. Hydrogen has two stable isotopes and a radioisotope. Protium (1H) and deuterium (2H, D) are stable, and tritium (3H, T) is radioactive, © Makoto Akashi.

Lack of Scientific Knowledge Leads to Misunderstandings and Misconceptions

In Japan, we experienced the Fukushima nuclear accident and the outbreak of COVID-19. We have learned from these two events that lack of scientific knowledge leads to misunderstandings and misconceptions causing excess anxiety, discrimination, and prejudice.  

There is one more thing we learned from the Fukushima nuclear accident and COVID-19; scientifically correct information does not always resolve psychological and/or economic problems. CBRNe terrorism can also cause physical, chemical, and thermal injuries, and induce infectious diseases.  

Among these, radiation, radioactive materials, bacteria/viruses, or some chemical substances cannot be understood easily since they are not seen, heard, or felt. Therefore, psychological and/or economic problems frequently occur as a result of these events. Even after event, disaster, or consequences of the terrorist act have been stabilized and scientifically correct information has been provided, all problems cannot be resolved in society.

What we must consider is how to give people peace of mind from a public health perspective and to help people to feel safe. This is the most challenging and difficult task, and now it is time to discuss it.

Dr. Makoto Akashi is currently Professor Tokyo Healthcare University. He served as Executive Officer of National Institutes for Quantum Science and Technology (QST) from 2016 to 2018 and Executive Director of National Institute of Radiological Sciences (NIRS) from 2011 to 2015. Dr. Akashi started his medical career as a resident of internal medicine at the Jichi Medical School Hospital, Tochigi, Japan in 1981. He moved to the Division of Hematology/Oncology at University of California at Los Angeles (UCLA) in 1987 and joined NIRS in 1990. Dr. Akashi is President of the Japanese Association for Radiation Accident/Disaster Medicine. He was the representative of Japan to UNSCEAR from 2017 to 2019.

* (= 33.9 µSv, dose coefficient of FWT: 3.1×10-11 Sv/Bq)
** (= 79.9 µSv, dose coefficient of OBT: 7.3×10-11 Sv/Bq)

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