A tsunami more than ten years ago caused a disaster at the Fukushima Daiichi nuclear power plant on the east coast of Japan. After the accident, large amounts of radioactivity contaminated the ocean, resulting in the imposition of a Sea exclusion zone and enormous reputation damage to the regional fish industry.
Since then, huge amounts of contaminated water have accumulated on the site. Water was needed to cool the damaged reactors, and groundwater contaminated from entering the site had to be pumped out and stored. Over 1,000 tanks were built on site for storage over a million tons of radioactive water.
But the storage space is running out and the tanks could leak, especially in the event of an earthquake or a typhoon. The Japanese authorities have given the site permission to discharge the stored radioactive water through a pipeline into the Pacific Ocean.
As an environmental scientist, I have been dealing with the effects of radioactive pollutants in the environment for more than 30 years. I think draining the sewage is the best option.
contaminated water
Before storage, the wastewater from Fukushima is treated to remove almost all radioactive elements. These include cobalt 60, Strontium 90 and Cesium 137. but tritium – a radioactive form of hydrogen – remains.
When one of the hydrogen atoms in water is replaced with tritium, radioactively tritiated water is formed. Tritiated water is chemically identical to regular water, making its separation from wastewater expensive, energy intensive and time consuming. A review of tritium separation technologies in 2020 found their inability to process the huge volumes of water required.
But in terms of radioactive elements, tritium is relatively harmless, and its existence as tritium water reduces its environmental impact. Chemically identical to ordinary water, tritiated water permeates organisms like water and therefore does not accumulate greatly in the bodies of living beings.
Tritiated water has a Bioaccumulation factor of about one. This means that exposed animals have roughly the same concentration of tritium in their bodies as the surrounding water.
In comparison, radioactive cesium-137, released in large quantities to Fukushima and from the UK’s Sellafield nuclear power plant in the 1960s and 1970s, has a bioaccumulation factor in marine environments of about 100. Animals typically have about 100 times more radiocaesium than the surrounding water because cesium increases the food chain.
Low radiation dose
When tritium decays, it emits a beta particle (a fast-moving electron that can damage DNA if ingested). But the tritium beta particle is not very energetic. A person would have to ingest a lot of it to receive a significant dose of radiation.
the World Health Organization‘s drinking water standard for tritium is 10,000 becquerels (Bq) per liter. This is many times the planned concentration of Fukushima’s wastewater.
The difficulty of separating tritium from wastewater and its limited environmental impact is why nuclear plants around the world have been dumping it into the sea for decades. The Fukushima Daiichi site plans to release approximately 1 petabecquerel (PBq – 1 followed by 15 zeros) tritium at a rate of 0.022 PBq per year.
That sounds like a huge number, but worldwide 50-70 PBq tritium is produced naturally in our atmosphere every year by cosmic rays. While the Cap de la Hague reprocessing plant in northern France releases about 4000 tons annually 10 PBq of tritium into the English Channel.
Significantly higher release rates from Cap de la Hague than planned at Fukushima no evidence shown significant environmental impacts and personal doses are low.
Secure Release
But the release of radioactive water has to be done properly.
Japanese Studies estimate that the effluent is diluted from hundreds of thousands of Bq per liter of tritium in the storage tanks to 1,500 Bq per liter in the effluent. dilution of the waste water before it is released, the radiation dose to humans is reduced.
The radiation dose to humans is measured in sieverts, or millionths of sieverts (microsieverts), with a dose of 1,000 microsieverts giving a 1 in 25,000 chance of dying early Cancer. the maximum estimated dose from Fukushima’s wastewater will be 3.9 microsieverts per year. This is much less than the 2,400 microsieverts that humans receive on average each year from natural radiation.
Japanese authorities must also ensure that the water released does not contain significant amounts of “organically bound tritium”. Here, a tritium atom replaces ordinary hydrogen in an organic molecule. The tritiated organic molecules can then be absorbed by sediments and ingested by marine organisms
In the mid-1990s, organic molecules containing tritium were released from the Nycomed-Amersham pharmaceutical factory in Cardiff Bay, Wales. The release resulted in bioaccumulation factors such as high as 10,000.
Treatment for other more dangerous radioactive elements also tends to do so leave small amounts of these elements in wastewater. The waste water stored in Fukushima is post-treated to ensure levels of these elements are low enough to allow safe discharge.
On a grand scale of the environmental problems we face, the discharge of wastewater from Fukushima is relatively small. But it is likely to do even more reputational damage to Fukushima’s struggling fishing industry. This is not helped by the political and media fuss likely to surround new releases of radioactive water into the Pacific Ocean.
JimSmithProfessor of Environmental Sciences, University of Portsmouth
This article is republished by The conversation under a Creative Commons license. read this original article.
