By MARTIN FACKLER

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 the leak the worst spill at the plant in six months.

The operator, the Tokyo Electric Power Company, said the leak, discovered on Wednesday and stopped on 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 some previous spills. Still, the leak 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 flow unchecked into the Pacific every day.

The company, known as Tepco, 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 Tepco has reported in the aftermath of the March 2011 disaster at the Fukushima Daiichi plant, 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 that calcium is, 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 Tepco announced earlier this month. That reading — five 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 that it has been slow to acknowledge problems at the stricken plant and that it 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 seeping 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, the company must pump it out of the reactor buildings and store it in rows of huge tanks it has erected on the plant’s grounds.

So far, Tepco said, about 340,000 tons of water have accumulated in the tanks, enough to fill more than 135 Olympic-size swimming pools. A ton of water is equivalent to about 240 gallons.