TOKYO—A team from the U.N. nuclear agency arrived in Japan on Monday to assess preparations for the release into the ocean of treated radioactive water from the wrecked Fukushima nuclear plant. The six experts on the team from the International Atomic Energy Agency are to meet with Japanese officials and visit the Fukushima Daiichi plant to discuss technical details of the planned release, Japanese officials said. The government and the plant’s operator, Tokyo Electric Power Company Holdings, announced plans in April to start gradually releasing the treated radioactive water in the spring of 2023 to allow for the removal of hundreds of storage tanks to make room for facilities needed for the destroyed plant’s decommissioning. The plan has been fiercely opposed by fishermen, local residents, and Japan’s neighbors, including China and South Korea. Japan has requested assistance from the IAEA to ensure the discharge meets international safety standards and to …