Tokyo Electric Power Co. said it suspects human error may be behind the leak of about 100 tons of highly radioactive water discovered Feb. 19 from a storage tank at the Fukushima No. 1 nuclear power plant.
Tokyo Electric Power Co. said it suspects human error may be behind the leak of about 100 tons of highly radioactive water discovered Feb. 19 from a storage tank at the Fukushima No. 1 nuclear power plant.
The utility said workers may have mistakenly left open valves to a pipe that leads to the holding tank in the H6 area of the facility.
Because of that, the contaminated water overflowed from the tank and spilled into a rain gutter.
According to TEPCO, at around 2 p.m. on Feb. 19, an alarm warning that water levels in the tank were exceedingly high sounded.
But workers did not investigate, believing a malfunctioning water level gauge mistakenly set off the alarm. Only at around 11:25 p.m. that night did workers discover that water was leaking outside the barrier surrounding the tanks via the rain gutter drainage system.
Tests by TEPCO indicated 240 million becquerels of beta-ray emitting radioactive substances per liter of water, including radioactive strontium, were contained in the water that leaked. Their test also showed the actual radiation levels in the water were at 50 millisieverts of beta rays per hour.
TEPCO is currently looking into the operation records involving the valves. The utility said Feb. 20 that it believed none of the water reached the ocean, which is about 700 meters from where the leak occurred.
In a similar leak discovered in August 2013, about 300 tons of radioactive water escaped from a different tank in the plant. Since then, the plant’s operator has strengthened measures to prevent radioactive water from leaking from the tanks.
However, increasing volumes of water at the plant are making it difficult for TEPCO to contain the situation. More and more groundwater is seeping into reactor buildings and other facilities, where it is then being contaminated by radioactive substances.
TEPCO has also employed special equipment to remove cesium and other radioactive substances from the water. Despite their efforts, about 400 tons of highly radioactive water is still being moved into storage tanks every day.
To reduce the risk resulting from the highly radioactive water, TEPCO plans to increase the number of ALPS (advanced liquid processing system) devices, which can remove most of the radioactive substances from the water.
The government also plans to develop new, higher-performance equipment to decontaminate the water, with the aim of processing all of it by the end of the next fiscal year, which ends in March 2015.
As of Feb. 18, the volume of radioactive water stored in the 1,000 holding tanks at the plant totaled about 520,000 tons. Of that, 340,000 tons is highly radioactive, according to TEPCO.
The utility said it is employing various measures to prevent further leakage.
For example, TEPCO said it has increased the number of workers whose job is to patrol and observe the tanks and pipes by sixfold to 60 and increased the number of patrols from two to four a day.
More water level gauges have also been added to each tank, the company said.
In addition, TEPCO raised the height of the barriers surrounding the tanks to prevent radioactive water from escaping from the area.
Water leakage has often occurred in tanks where the steel panels are fastened with bolts. Therefore, the government and TEPCO plan to replace those tanks with ones whose seams are welded shut, making them more reliable and leakproof.
The holding tank discovered leaking on Feb. 19 was of the bolt-type design.
(This article was written by Akira Hatano and Shunsuke Kimura.)