Fukushiman ydinonnettomuus: Tritiumpitoisuus käsitellyssä vedessä alle Japanin käyttörajan

International Atomic Energy Agency (IAEA) has confirmed that the tritium level in the fourth batch of diluted treated vesi, which Tokyo Electric Power Company (TEPCO) started discharging on 28 February 2024, is far below the Japan’s operational limit.

Experts stationed at the site of the Fukushima ydin- teho station (FDNPS) took samples after the treated vesi was diluted with merivesi in the discharge facilities on 28 February. The analysis confirmed that the tritium concentration is far below the operational limit of 1,500 becquerels per litre.

Japan is discharging the treated vesi from the FDNPS in batches. The previous three batches – a total of 23,400 cubic metres of vesi – were also confirmed by the IAEA to have contained tritium concentrations far below operational limits.

Since the accident in 2011, vesi is needed to continually cool the melted fuel and fuel debris at the Fukushima Daiichi NPS. In addition to the vesi pumped in for this purpose, groundwater also seeps into the site from the surrounding environment, and rainwater falls into the damaged reactor and turbine buildings. When vesi comes in contact with melted fuel, fuel debris and other radioactive substances, it becomes contaminated.

The contaminated vesi is käsitelty through a filtration process known as Advanced Liquid Processing System (ALPS) which uses a series of chemical reactions to remove 62 radionuclides from contaminated water before being stored. However, tritium cannot be from the contaminated water through ALPS. Tritium can be recovered when it is highly concentrated in small amounts of water, for example at ydin- fusion facilities. However, the stored water at the Fukushima Daiichi NPS has a low concentration of tritium in a large volume of water and so the existing technologies are not applicable.

Tritium is a naturally occurring radioactive form of hydrogen (half-life 12.32 years) that is produced in the atmosphere when cosmic rays collide with air molecules and has the lowest radiological impact of all naturally occurring radionuclides in seawater. Tritium is also a by-product of operating ydin- power plants to produce electricity. It emits weak beta-particles, i.e., electrons, with an average energy of 5.7 keV (kiloelectron-volts), which can penetrate about 6.0 mm of air but cannot penetrate the body through human skin. It may present a radiation hazard if inhaled or ingested but is only harmful to humans in very large doses.

Currently, contaminated water produced at the Fukushima Daiichi NPS is treated and stored on site in specially prepared tanks. TEPCO, the plant operator, has installed roughly 1000 of these tanks at the Fukushima Daiichi NPS site to hold around 1.3 million cubic meters of the treated water (as of 2 June 2022). Since 2011, the volume of water in storage has increased steadily, and the current tank tila available to store this water is nearing full capacity.

Vaikka parannuksia on tehty saastuneen veden tuotantonopeuden vähentämiseksi merkittävästi, TEPCO on päättänyt, että tarvitaan pitkäaikaista loppusijoitusratkaisua, joka auttaa varmistamaan paikan jatkuvan käytöstä poistamisen. Huhtikuussa 2021 Japanin hallitus julkaisi peruspolitiikkansa, jossa hahmotellaan ohjeet ALPS-käsitellyn veden hävittämiseksi valvotuilla päästöillä mereen, joka aloitetaan noin 2 vuoden kuluttua, edellyttäen, että kotimainen viranomainen hyväksyy.

On 11 March 2011, Japan was shaken by the Great East Japan (Tohoku) Maanjäristys. It was followed by a tsunami which resulted in waves reaching heights of more than 10 meters. The maanjäristys and tsunami led to a major accident at the Fukushima Daiichi ydin- Power Station, which was ultimately categorized as a Level 7 on the International ydin- and Radiological Event Scale, the same level as the 1986 Chernobyl onnettomuus Fukushiman kansanterveysvaikutukset ovat kuitenkin paljon lievempiä.

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