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.