First priority is given to safety and security Measures related to “contaminated water” in Fukushima How much radiation would disposal of ALPS-treated water release into the environment?
(English ver.) 2020-06-10
- Salient points of this article:
- Effects of atomic radiation are estimated on the assumption that all the ALPS-treated water currently stored is to be discharged over the course of just one year, either into the sea, or into the atmosphere in the form of vapor. Both of these methods have been utilized before.
- Discharge by either of the two methods is estimated to release radiation as low as or even less than one-thousandth of that arising from existing, naturally occurring radiation sources.
- Discharge into the sea is estimated to result in less than half of the radiation release than for vapor release into the atmosphere.
Our previous articles presented basic and updated information explaining “contaminated water” with high concentrations of radioactive substances generated at TEPCO’s Fukushima Daiichi Nuclear Power Station (Fukushima Daiichi NPS). In this article, we discuss how we estimate the level of radiation resulting from the discharge of “ALPS-treated water”, which is a critical issue in studying various methods of disposal.
“Contaminated water” and “ALPS-treated water”
Contaminated water with high concentrations of radioactive substances has been generated as a result of the accident at TEPCO’s Fukushima Daiichi NPS. As explained in our previous article, it is still being generated at a certain rate and various countermeasures are being taken based on our three basic operating principles, namely 1. No leakage, 2. No inflow of water, 3. Removal of contaminants.
Contaminated water, after being purified to reduce risks associated with radioactive substances, is being stored in tanks on the site of Fukushima Daiichi NPS. Multiple pieces of equipment are used for purification. The most efficient is “Multi-nuclide Removal Equipment (known as the Advanced Liquid Processing System = ALPS)” that removes 62 kinds of radioactive substances. Water treated by ALPS is referred to as “ALPS-treated water”, which contains approximately one-millionth of radioactive substances contained in contaminated water before this treatment.
ALPS-treated water still contains tritium that cannot be removed by ALPS. Discussions are being held by experts on how to deal with this issue at the “Subcommittee on Handling of ALPS-treated Water (ALPS Subcommittee)”.
Estimating radiation effects resulting from discharge of “ALPS-treated water”
The “Tritiated Water Task Force”, having examined the technical feasibility of the various disposal options, compiled a report in 2016. As a result of classification and screening, the task force presented in its report the following five options:
- Five disposal options presented by the “Tritiated Water Task Force”
- (1) Geosphere injection
(2) Discharge into the sea
(3) Vapor release
(4) Hydrogen release
(5) Underground burial
The ALPS Subcommittee assessed the effects of radiation resulting from each of the above five options.
When it comes to radiation exposure or radioactive substances, it is not enough to discuss whether any radiation “exists or not”, or how much radiation is estimated in terms of becquerels. It is much more important to consider and discuss “exposures of the human body to radionuclides” expressed in terms of sieverts. This methodology is dubbed “public exposure assessment” or “radioactive effects assessment”.
The Tritiated Water Task Force has examined various options on the condition that the disposal of radioactive substances, conforming to the regulatory standards, must not cause significant, scientifically verifiable damage to the environments in which we live. Therefore, radiation effects of each of the five options provided are estimated to be well below 1 sievert per year.
However, there is no way to simply compare those options in terms of radiation effects. Only two of the five options, namely discharge into the sea and vapor release into the atmosphere, have been utilized before in reality. The other options can only be evaluated theoretically, making it difficult to allow a simple comparison of the five methods.
As for discharge into the sea and vapor release into the atmosphere, the “United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR)” published a methodology for comparison in the following report:
“Sources, Effects and Risks of Ionizing Radiation”, Annex A
“Methodology for Estimating Public Exposures due to Radioactive Discharges” (PDF format: 1.96 MB)
Estimated radioactive effects of discharge into the sea and vapor release into the atmosphere using UNSCEAR’s methodology were presented to the ALPS Subcommittee.
Estimating radioactive effects of discharge into the sea and vapor release using UNSCEAR methodology
UNSCEAR was established to broadly assess the sources of radiation and effects on human health and the environment. The Committee, comprised of 27 member states, conducts investigations and assessments based on independent, scientific inquiry.
UNSCEAR’s methodology was formulated for the purpose of “estimating public exposures due to radioactive discharges”. Its main characteristics are summarized below:
(1) It can be used to estimate individual doses resulting from discharges into the atmosphere and the sea, in terms of doses received by a typical person living in the area around the discharge point.
(2) It can be used to estimate collective doses. A collective dose is the dose received by all members of a specified population over a defined period of time, even an unrealistically long period, for example “100 years of continuous discharge”. The results are then assessed on a yearly basis.
(3) It can take into consideration a number of routes of public exposure for vapor release into the atmosphere and discharge into the sea as described below:
Vapor release into the atmosphere;
Exposures external to the body through atmosphere
Exposures external to the body through sediments in the soil.
Exposures inside the body through inhalation.
Exposures inside the body through ingestion of ground organisms.
Discharge into the sea;
Exposures external to the body from beaches and ocean water.
Exposures inside the body through ingestion of marine organisms.
The estimation presented to the ALPS Subcommittee is based on the following conditions:
- It takes into account eating habits of people living in the Asia-Pacific Region. (For vapor release, annual consumption of meat and crops. For discharge into the sea, annual consumption of fish and crustaceans.)
- The density of tritium contained in the water is “one million becquerels per liter”. Any necessary dilution is to be conducted before discharge, to meet regulatory standards.
- Other radionuclides (radioactive substances) have been removed with the density of remaining substances being below the statutory limits. Data from direct analysis of the water stored in the tanks is used. The water is stored in conformity with statutory standards.
The above table shows the results of assessment, using UNSCEAR methodology, of radiation effects arising from vapor release and discharge into the sea based on the disposal amount of tritium over 1 year (known as “disposal speed”). Three different disposal speeds are used, namely approx. 860 trillion becquerels per year, approx. 86 trillion becquerels per year, and approx. 8.6 trillion becquerels per year.
The figure “approx. 860 trillion becquerels” represents the total amount of tritium contained in the water stored in tanks on the site of Fukushima Daiichi NPS as of October 31, 2019.
Calculations based on this methodology indicate that if all the ALPS-treated water currently stored on site was discharged over one year, radiation effects would be approximately 0.000052 – 0.00062 millisieverts per year in the case of discharge to the sea, and approximately 0.0013 millisieverts per year in the case of vapor release to the atmosphere.
The national average exposure to radiation in daily life, which arises from naturally occurring sources such as radioactive substances contained in the air, is 2.1 millisieverts per year in Japan. Therefore, even if 860 trillion becquerels were discharged over the course of one year, radiation effects would be very low, approximately one-thousandth of the existing, natural exposure to radiation. According to the UNSCEAR methodology, it was also concluded that radiation effects resulting from discharge into the sea would be less than half of those resulting from vapor release into the atmosphere.
Comparison of radiation effects: 860 trillion becquerels discharged over one year vs. natural exposure to radiation
At the ALPS Subcommittee, discussions are being held by experts on the scientific facts as described above. The subcommittee is also considering social issues such as reputational damage and countermeasures against it. Based on the forthcoming recommendations by experts, the government of Japan will explore the best possible way to dispose of ALPS-treated water with a view to pursuing both “reconstruction of the area” and “decommissioning of the NPS”.
Division in charge
About the article
Nuclear Accident Response Office, Electricity and Gas Industry Department
About Special Contents
Research and Public Relations Office, Policy Planning and Coordination Division, Commissionerʼs Secretariat
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