No detectable amount of tritium was found in the first fish samples taken in waters near the crippled Fukushima nuclear plant, where the discharge of treated radioactive water into the sea began this week, the Japanese government said Saturday.

The water discharge began Thursday despite persistent concerns among local fishermen and some neighboring countries about the environmental impact, while the U.N. nuclear watchdog has said the procedure aligned with global safety standards.

Photo taken on Aug. 26, 2023, in Tagajo, Miyagi Prefecture, shows a researcher checking the concentration level of tritium in a fish taken from waters off Fukushima Prefecture. (Kyodo)

The fish samples, a gurnard and olive flounder, were collected Friday within 5 kilometers of the discharge outlet of the Fukushima Daiichi complex, the Fisheries Agency said on its website.

The water had been used to cool melted nuclear fuel at the plant but has undergone a treatment process that removes most radionuclides, except tritium.

The remaining tritium is then diluted to one-40th of the concentration permitted under Japanese safety standards before being released into the Pacific Ocean via an underwater tunnel 1 km from the seaside plant, wrecked by a massive March 2011 earthquake and tsunami.

The agency plans to continue to take daily fish samples for analysis and provide updates on the results for about a month.

The Environment Ministry also collected seawater samples within around 50 km of the plant and is expected to announce the results as early as Sunday.

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