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TOKYO — About 100 tons of highly radioactive water leaked from one of the hundreds of storage tanks at the Fukushima nuclear plant, its operator said Thursday, calling it the worst spill at the plant in six months.

The operator, Tokyo Electric Power Co., or Tepco, said the leak, discovered Wednesday and stopped Thursday, happened far enough from the plant’s waterfront that none of the radioactive water was likely to reach the Pacific Ocean, as has happened during previous spills. The incident was nonetheless a reminder of the many mishaps that have plagued the containment and cleanup efforts at the plant and of the hundreds of tons of contaminated groundwater that flow unchecked into the Pacific every day.

Tepco said it had traced the latest leak to a two valves that were left open by mistake.

The leaked water was among the most severely contaminated that Tepco 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. A becquerel is a unit used to measure radioactivity. 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.

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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 groundwater reading that Tepco announced this month. That reading — 5 million becquerels of strontium-90 per liter — which was detected at a location closer to the ocean than the latest spill, prompted criticism of Tepco because the company waited five months to report it publicly.

Critics have assailed the company since the accident, saying it has been slow to acknowledge problems at the plant and has disclosed too little information about the conditions inside. Even so, the government has left the company largely in charge of the cleanup work there.

Tepco has struggled to deal with the hundreds of tons of groundwater that seeps each day into the plant’s damaged reactor buildings, where it is contaminated by the melted nuclear reactor cores. To keep the radioactive water from running into the Pacific, it is pumped out of the reactor buildings and stored in rows of huge tanks on the plant grounds.

So far, about 340,000 tons of water have accumulated in the tanks.

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