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.