Tokyo Electric Power Co. (Tepco) found radioactive cesium in groundwater samples taken from the crippled Fukushima No. 1 nuclear complex, reversing an earlier announcement that contamination was negligible.
This news released Monday as Tepco was trying to secure the consent of local fishermen for dumping groundwater pumped out of wells at the site into the Pacific, saying it confirmed concentrations of radioactive substances are sufficiently low.
Tepco had said radioactive cesium in the groundwater was at a level that could not undergo detection by an instrument at the Fukushima No. 1 complex. But the same sample contained 0.22 becquerel of cesium-134 and 0.39 becquerel of cesium-137 per liter when checked at the nearby Fukushima No. 2 plant, where radiation levels are lower.
According to the utility, there was a problem in accounting for background radiation.
The revised amount of cesium-137 is still below the level that Tepco views as the upper limit for releasing groundwater, which is less than 1 becquerel.
Currently, about 400 tons of groundwater seeps into the crippled reactor buildings every day, where it becomes contaminated with radioactive substances. This means the total volume of toxic water increases by the same amount daily.
To slow the rate of accumulation, Tepco created a system to direct part of the groundwater into the ocean by pumping it out before it flows into the reactor buildings. The groundwater ends up stored in tanks before it discharges.
The utility has not been able to fully operate the system amid concern from fishermen that dumping groundwater will contaminate the marine environment.
The latest revelation could undermine the credibility of related data presented by Tepco, possibly making it hard for the utility to approval to discharge the groundwater and standing in the way of the overall plan to address the massive amounts of radioactive water at the plant.