More than 12 years after the Tōhoku earthquake and tsunami that cost Japan 20,000 lives, the cleanup continues. The International Atomic Energy Commission (IAEA) has formally approved Japan's plan to release treated radioactive water from the damaged Fukushima Daiichi nuclear power station into the sea.
When the Tōhoku disaster struck Japan in 2011, it was the worst earthquake ever to have struck the country and the fourth worst on record. With little to no warning, the resulting tsunami caused an almost unimaginable loss of life and property and the videos of the event are a humbling reminder of nature's power.
One of the victims of the disaster was the Fukushima Daiichi nuclear power plant, which was an obsolete boiling water design of six reactors that was commissioned in 1971. On March 21, 2011, it was battered by 14-m (45-ft) ocean waves that were far beyond its design capacity to withstand. The facility was severely damaged and the backup diesel systems that should have come online to keep the reactors cooled were swamped. Worst of all, the damage to the surrounding area was so great that it was impossible for emergency crews to reach the plant before the reactors went into meltdown and some were severely damaged by a hydrogen gas explosion.
In the years since, there has been a herculean effort to clean up the facility and to properly decommission it. However, there remains the problem of 1.3 million m³ (340 million gal) of contaminated water stored on the site in stainless steel tanks.
The water has already been largely decontaminated. Strontium, cesium, and other heavy radioactive elements have been removed by means of chemicals that precipitate or capture the contaminants. What's left was desalinated and passed through the Advanced Liquid Processing System (ALPS), which uses a series of membranes and filters to remove 62 kinds of radioactive particles.
The end result, from a chemical point of view, plain water. The problem is that this isn't the sort of water that comes out of a kitchen tap. It contains a form of heavy or tritiated water where some of the hydrogen atoms are an unstable radioactive hydrogen isotope called tritium whose nucleus is made up of a proton and two neutrons.
Tritium is naturally occurring, being produced by cosmic rays striking the Earth's atmosphere. It's also created in nuclear reactors as the cooling water is exposed to the radioactive environment. With a half life of 12.32 years, it's quite radioactive, but this only takes the form of beta particles that can't penetrate any depth of living tissue.
It's not dangerous unless it's in very high concentrations, which is annoying because highly concentrated tritiated water can be separated from ordinary water, but this isn't practical in the low concentrations seen at Fukushima.
It may seem irresponsible, but the safest way to dispose of such irradiated water is to release it into the sea. Seawater already contains natural tritium in vastly greater quantities than nuclear power plants could ever hope to produce and it's a standard procedure to discharge treated reactor water into bodies of water to be quickly diluted as part of nuclear operations.
The trick is to make sure that the water is already highly diluted and that the discharge is made in such a gradual way that the tritium can't significantly increase at the point of release and cause local damage. This is the reason for the intense two-year review of the release plan by the IAEA, the Japanese government and Japan’s Tokyo Electric Power Company (TEPCO) that operates Fukushima. The review was to make sure the plan met national and international safety standards, including ensuring that the tritium does not concentrate in the food chain.
"The IAEA have taken time and due care and attention in preparing this report, commensurate with the somewhat unique situation," said Prof Robin Grimes, Steele Chair of Energy Materials, Imperial College London. "They have made it clear they will continue to monitor the release. Independent verification is always to be welcomed. However, the concentration of tritium, the remaining radionuclide in the water to be discharged, is very low and well below levels of any environmental concern. The state of the tritium is important – in this case it is a component of water molecules (tritiated water) but not bound to more complex compounds. There is no established mechanism for tritiated water bioaccumulation so discharge will further dilute these low levels of tritium enormously. It will be interesting to see if any increase in tritium in the discharge area is even detectable over natural tritium generated by cosmic ray processes. Certainly the concentration of tritium will be well below levels of naturally occurring radionuclides although comparing the environmental impact of different radionuclides is quite a challenge."
Source: IAEA