Tokyo Electric Power Co. said radioactive water may still be leaking into the sea from the crippled Fukushima No. 1 nuclear power plant after enhanced levels of radioactive tritium were detected in the port area in front of the facility.
The plant operator said June 24 it is investigating the matter with some urgency as the finding suggests that radioactive water, generated on the plant premises, may be leaking into the sea from the ground.
TEPCO said tritium levels of 1,100 becquerels per liter of seawater were recorded June 21 north of the water intakes for the No. 1, No. 2, No. 3 and No. 4 reactors in the port area.
The reading compares with 500 becquerels per liter measured on June 10. It was the highest concentration since the nuclear crisis began to unfurl in March 2011.
Officials said tritium levels of 910 becquerels per liter of seawater were detected at a different location near the water intakes for the No. 1 and No. 2 reactors and close to a well, up from the previous reading of 600 becquerels per liter on June 14.
The previous record for seawater in the port, 920 becquerels per liter, was taken in October 2011. Tritium levels in the port generally hovered 100 and 200 becquerels per liter during the past 12 months.
Tritium, a naturally existing radioisotope of hydrogen, is generated in coolant water for nuclear reactors. Unlike radioactive cesium, tritium, which exists in the form of water, is difficult to remove by way of absorption. Tritium, once ingested, flushes out of the human body relatively quickly.
Miniscule amounts of tritium are released into the environment during the normal course of operating a nuclear reactor.
The government has set an upper limit of 60,000 becquerels per liter for seaborne tritium concentrations outside a nuclear facility.
TEPCO had said June 19 that 500,000 becquerels of tritium was detected in late May per liter of water from a well on the sea side of the No. 1 and No. 2 reactors.
The utility said the radioactive water likely entered the ground immediately after the nuclear crisis began, and later mixed with groundwater and flowed into the well.
TEPCO also said it would inject a sealing agent into the ground between the well and the sea to prevent the radioactive water from spreading to the ocean and conduct drilling near the well to monitor radioactive concentrations more closely.
"We need more investigations to identify the cause," a TEPCO official said. "We will keep a close watch on the situation."
Tetsu Nozaki, who heads the Fukushima Prefectural Federation of Fisheries Cooperative Associations, expressed concern that fishing operations in local waters could be affected.
"I will be waiting for TEPCO to explain the cause and present its mitigation measures," Nozaki said. "Although I cannot yet gauge the magnitude of the latest development, I am afraid that a growing number of our members would feel anxious if similar incidences were to recur."
(Jin Nishikawa contributed to this article.)
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