Tokyo: About 100 tonnes of highly radioactive water leaked from one of the hundreds of storage tanks at the devastated Fukushima nuclear plant, its operator said on Thursday, calling it the worst spill at the plant in six months.

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 previous spills.

Review mission members of the International Atomic Energy Authority at Fukushima No.1 nuclear power plant late last year. Credit:AFP

Still, the incident 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 tonnes of contaminated groundwater that still flows 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.