The Mainichi Shimbun answers some common questions readers may have about the characteristics of tritium, and why it is hard to dispose of water containing the radioactive element.
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Question: I heard the term "water containing tritium" used when talking about the treatment of contaminated water at the Fukushima No. 1 Nuclear Power Plant operated by the Tokyo Electric Power Co. (TEPCO).
Answer: It refers to treated water including tritium. The element cannot be removed using the current purification method used at the crippled nuclear power plant. The government and TEPCO are considering ways to dispose of the liquid, which is continuing to fill waste water tanks at the plant.
Q: What kind of substance is tritium?
A: Tritium is a radioactive isotope of hydrogen containing one proton and two neutrons while the ordinary hydrogen nucleus contains just one proton. It has a half-life of about 12.3 years, which is the time required to reduce half of its radioactivity.
Q: Is tritium found only in the treated water from the damaged nuclear plant?
A: Tritium can also develop when oxygen and nitrogen in the atmosphere react to cosmic neutrons. Around 70 quadrillion becquerels appear naturally per year, and around a total of 223 trillion becquerels are contained in Japan's annual rainfall, according to data compiled by the Ministry of Economy, Trade and Industry (METI). Coolant in normal operating nuclear reactors also carries tritium. At the Fukushima No. 1 Nuclear Power Plant, tritium is generated in groundwater pouring into the buildings that house reactors, and in water used to cool melted fuel debris.
Q: Why is it difficult to dispose of tritium?
A: Other radioactive substances can be removed using specific disposal equipment for filtration and absorption to levels below the allowed ceiling. However, separation is very hard for water containing tritium because its characteristics, including the boiling temperature, are similar to those of normal water.
Q: What about the impact it will have on human health, as it is radioactive?
A: Tritium emits beta radiation that has weak energy, and will mostly pass through the body if drank. Its effects on the human body are said to be minimal compared to radioactive cesium. Nuclear power plants around the world are disposing water containing tritium according to regulations, in oceans and other places, once it has been diluted to a radiation level that falls below standard limits. According to METI, Japan released into oceans around 380 trillion becquerels of tritium per year on average for five years before the Fukushima nuclear disaster.
(Answers by Riki Iwama, Science & Environment News Department)