China has rejected Japan's proposal that it join an international verification framework for assessing the results of radiation level monitoring in treated water being released into the sea from the crippled Fukushima nuclear power plant, sources familiar with the bilateral relationship said Tuesday.

China is staunchly opposed to the ocean discharge and has banned all seafood imports from Japan since it began in late August, calling the water "nuclear-contaminated." Beijing has also ignored Tokyo's repeated proposals for a science-based dialogue involving experts on the matter, Japanese officials said.

A large screen in Beijing shows on Aug. 24, 2023, footage of tanks holding treated radioactive water at the crippled Fukushima Daiichi nuclear power plant in Japan's northeast. (Kyodo)

Under the framework, participating countries will compare, analyze and evaluate the results of the monitoring carried out respectively by the Japanese government and the International Atomic Energy Agency on seawater off Fukushima Prefecture, northeastern Japan.

In order to guarantee the objectivity of international evaluations, Japan does not participate in the framework.

The research institutions participating in the framework are selected by an entity related to the IAEA. It currently involves institutions from the United States, France, Switzerland and South Korea.

Since early this year, Tokyo has repeatedly asked Beijing through diplomatic channels to participate in the monitoring efforts, but has had its requests rejected by China under the argument that the framework "does not guarantee" independent analysis of the released water, the sources said.

A Japanese government source pointed out that China likely believes joining the framework would be "tantamount to endorsing the ocean discharge."

The water has undergone a treatment process that removes most radionuclides except tritium. Before being discharged into the ocean, the remaining radioactive substance is diluted to one-40th of the concentration permitted under Japanese safety standards.

Tritium is known to be less harmful to human health than other radioactive materials such as cesium and strontium as it emits very weak radiation and does not accumulate in the body, experts say.

Worldwide, nuclear power plants routinely release treated water containing low concentrations of tritium and other radionuclides into the environment as part of normal operations, according to the IAEA.