Excessive levels of highly toxic strontium have been detected in seawater and groundwater at Japan's Fukushima Daiichi nuclear complex, the plant operator said Monday, a development that suggests an increased risk of radioactive contamination further entering the food chain.
Also underscoring the difficulties of trying to stabilize the stricken facility, operator Tokyo Electric Power Co. said six more workers have received more than the permitted annual emergency levels of radiation exposure.
The Strontium-89 and Strontium-90 isotopes are believed to have been released from the damaged reactors when the fuel cores overheated and melted after the March 11 earthquake and tsunami, Tokyo Electric, also known Tepco, said at a briefing. In all, the amount of contaminated water now flooding the basements and the connected trenches of the plant's reactor buildings is estimated at more than 100,000 tons.
Environmental experts said the discovery of the strontium heightens the risk of contaminated seafood in the area, now complicated by the arrival of seasonally heavy rains.
"With the arrival of the rainy season, more and more radioactive fallout is being washed into groundwater and the sea, raising the levels of strontium contamination," said Ikuro Anzai, professor emeritus at Ritsumeikan University.
Strontium acts like calcium and accumulates in bones. Unlike other radioactive materials, such as cesium and iodine, strontium doesn't emit powerful gamma rays, and therefore, its harmful effects are limited unless it is ingested or inhaled. But once inside the body, it can cause bone cancer or leukemia.
"Japanese people often eat small fish, such as sardines, whole, including the bones and head. There is therefore a risk of consumers taking in strontium from contaminated small fish," Prof. Anzai said.
The government already has undertaken a program of testing of seafood in the nearby area and didn't announce any additional measures because of the latest disclosure. Prof. Anzai said there should be close monitoring for potential contamination because there is a risk of the radiation spreading through the food chain.
The six workers were found to have been exposed to more than the annual limit of 250 millisieverts now set for an emergency situation, during continuing check-ups on an estimated 3,700 staff who have worked at the plant during the crisis. Tepco said the six are likely to have been exposed to radiation from 265 to 498 millisieverts, according to preliminary results.
Two male workers were previously confirmed to have exceeded the annual limit, according to Tepco, while two women workers were found to have exceeded the limit of 5 millisieverts set for females in any three-month period. The discoveries come from the testing of all workers at the site. Of the staff, 2,400 have so far gone through check-ups for internal radiation.
Tepco continues to struggle with ways to reduce the amount of radiation at the site. It announced separately on Monday that it now expects a newly developed system for treating highly radioactive water to start full operation by the end of this week, to filter the rising amount of radioactive water produced by the continuous cooling of the reactor cores.
The decontamination system was originally scheduled to start operating Wednesday and delays add to concerns that the plant will run out of storage space for the contaminated water.
The system is made up mainly of zeolite-containing cartridges, developed by Kurion Inc. of the U.S., to absorb radioactive cesium and iodine, and of equipment built by French energy company Areva SA to remove radioactive materials. It can treat up to 1,200 metric tons of water daily.
After the water is decontaminated, it will be either used as coolant for the reactors or released into the ocean, according to Tepco.
Tepco said a separate device, set up near the coastline, started operating Monday to remove radioactive materials from seawater. Water is being pumped through the zeolite-filled device to collect cesium and other radioactive materials. But Tepco said it was not clear if it would remove strontium as well.