TOKYO — About 100 tons of highly radioactive water leaked from one of the hundreds of storage tanks at the devastated Fukushima nuclear plant, its operator said Thursday, calling it the worst spill at the plant in six months.
The operator, the Tokyo Electric Power Co., said the leak was discovered Wednesday and stopped Thursday.
The company said it happened far enough from the plant’s waterfront that none of the radioactive water was likely to reach the Pacific, as has happened during previous spills.
Still, the episode was an uncomfortable reminder of the many mishaps that have plagued the containment and cleanup efforts at the plant, as well as the hundreds of tons of contaminated groundwater that still flows unchecked into the Pacific every day.
Tokyo Electric said it had traced the latest leak to a pair of valves that were left open by mistake.
The leaked water was among the most severely contaminated that Tokyo Electric has reported at the Fukushima Daiichi plant since March 2011, when damage caused by an earthquake and a tsunami led to meltdowns in three of the plant’s reactors.
Each liter of the water contained, on average, 230 million becquerels of particles giving off beta radiation, the company said. About half of the particles were likely to be strontium-90, which is readily taken up by the human body in the same way as calcium, and can cause bone cancer and leukemia.
That means the water was about 3.8 million times as contaminated with strontium-90 as the maximum allowed under Japan’s safety standards for drinking water. It also showed levels much more radioactive than a worrisome groundwater reading that Tokyo Electric announced earlier this month: 5 million becquerels of strontium-90 per liter.
Tokyo Electric has struggled to deal with the hundreds of tons of groundwater that seeps each day into the plant’s damaged reactor buildings.