The operator of the crippled Fukushima Daiichi nuclear power plant on Friday started a sixth release of treated radioactive water into the sea amid opposition from China and Russia. Tokyo Electric Power Company Holdings Inc. said it plans to discharge 7,800 tons of water through June 4, after confirming that the radioactivity levels meet both its own and the government’s standards. Since the operation began on Aug. 24 last year, about 39,000 tons of treated water have been released over five previous rounds. The utility plans to discharge a total of about 54,600 tons on seven occasions this fi…