High levels of a toxic substance called strontium-90 have been found in groundwater at the devastated Fukushima nuclear power plant in Japan, the utility that operates the facility said on Wednesday.
Strontium-90 is a by-product of the fission of uranium and plutonium in nuclear reactors as well as nuclear weapons, according to the website of the U.S. Environmental Protection Agency.
The discovery of rising levels of such radioactive material is likely to complicate efforts by the utility, Tokyo Electric Power Co, to get approval to release what it describes as water contaminated with low levels of radiation into the Pacific Ocean.
"This contaminated water should not be released to the ocean," said Michiaki Furukawa, a nuclear chemist and professor emeritus at Nagoya University. "They have to keep it somewhere so that it can't escape outside the plant."
Tepco is being overwhelmed with contaminated liquids as it flushes water over the three reactors at the seaside plant that had meltdowns after an earthquake and tsunami two years ago knocked out power and cooling systems.
High levels of tritium, a less harmful substance, had also been found, Toshihiko Fukuda, a general manager at Tepco, told a news conference.
Tepco did not believe any of the strontium-90 found in groundwater tests had leaked into the ocean, Fukuda said. The company has constantly revised announcements about radiation levels and other problems at the plant since the disaster.
Explosions that rocked the plant at the height of the crisis discharged large amounts of radioactive material into the atmosphere and surrounding land and ocean.
Testing of groundwater outside the turbine building of reactor No. 2 had shown the level of strontium-90 had increased by more than 100 times between December 2012 and May this year, Fukuda said.
He said it was likely that radioactive material entered the environment after water poured over the melted fuel in unit No. 2 and leaked out via the turbine building, which is located between the reactor and the ocean.
Testing of groundwater showed the reading for strontium-90 increased from 8.6 becquerels to 1,000 becquerels per litre between Dec. 8, 2012 and May 24, Fukuda said. The elevated reading of strontium is more than 30 times the legal limit of 30 becquerels per litre.
"Tepco needs to carry out more regular testing in specific areas and disclose everything they find," added Furukawa, the nuclear chemist.
Testing also showed 500,000 becquerels per litre of tritium on May 24, compared with the legal limit of 60,000 and 29,000 on Dec. 8, 2012. A becquerel is a measure of radioactive decay.
Tepco has struggled with the clean-up of Fukushima, the site of the world's worst nuclear disaster in 25 years. It said in April it was running out of capacity to store the water contaminated in its still-makeshift cooling system.
Adding to its difficulties, about 400 tonnes of groundwater flow daily into the reactor buildings only to be mixed with highly contaminated water that comes from cooling the melted fuel.
It has been trying to convince sceptical local fisherman that it is safe to dump 100 tonnes of the groundwater a day into the ocean to take some of the strain off its storage facilities.
Earlier this month the company reversed a claim that the groundwater flowing into the damaged basements of reactor buildings was not contaminated.
Recent mishaps, including two power outages, have heightened concerns about Fukushima's stability and called into question Tepco's ability to decommission the plant, which may take more than 30 years.