The tritium-tainted water piling up at the crippled Fukushima No. 1 nuclear power plant has been found to contain other radioactive substances, defying the defunct plant’s special treatment system, Kyodo News has learned.

The government and Tokyo Electric Power Company Holdings Inc. are under pressure to dispose the treated water, which is accumulating in hundreds of tanks on the premises. One option is to dump it into the sea, as tritium, a normal byproduct of nuclear operations, is said to pose little risk to human health in diluted form.

If the plan goes through, the tritium-tainted water is expected to be diluted so it will likely lower the levels of the other radioactive materials before discharge.

But fishermen and residents are worried about the water discharge plan, and a government panel debating how to deal with it has mainly focused on the tritium rather than the other substances.

According to Tepco, a maximum of 62.2 becquerels per liter of iodine 129, far higher than the 9 becquerel legal limit, was found in the water filtered by the Advanced Liquid Processing System, which was reportedly capable of removing everything but tritium.

Iodine 129 has a half-life of 15.7 million years.

Tepco, which gathered data in fiscal 2017 through March, also detected a maximum 92.5 becquerels of ruthenium 106, shy of the 100 becquerel legal limit, as well as 59 becquerels of technetium 99 against the limit of 1,000 becquerels.

The Fukushima No. 1 complex was damaged by the 2011 earthquake and tsunami. Reactors 1 to 3 suffered fuel meltdowns as their cooling systems were crippled.

Water is injected perpetually to keep the fuel cold but it is extremely toxic. The water is filtered by the ALPS system but removing the tritium remains difficult.

As of August, around 920,000 tons of tritium-containing water are stored in some 680 tanks within the premises. But Tepco said it has not checked the concentration of radioactive materials in each tank.

Toyoshi Fuketa, who heads the Nuclear Regulation Authority, has been calling the ocean discharge plan the “only” solution.