TOKYO The stricken nuclear power plant at Fukushima has probably been leaking contaminated water into the ocean for two years, ever since an earthquake and tsunami badly damaged the plant, Japan's chief nuclear regulator said Wednesday.
In unusually candid comments, Shunichi Tanaka, the head of the Nuclear Regulation Authority, also said that neither his staff nor the plant's operator knew exactly where the leaks were coming from, or how to stop them.
The operator, Tokyo Electric Power, has reported spikes in the amounts of radioactive cesium, tritium and strontium detected in groundwater at the plant, adding urgency to the task of sealing any leaks. Radioactive cesium and strontium, especially, are known to raise risks of cancer in humans.
Tanaka's comments bring into sharp relief the precariousness of the cleanup at the Fukushima Daiichi Nuclear Power Plant, where core meltdowns occurred at three of the six reactors.
Until recently, Tokyo Electric, known as Tepco, flatly denied that any of that water was leaking into the ocean, even though various independent studies of radiation levels in the nearby ocean have suggested otherwise. In recent days, Tepco has retreated to saying that it was not sure whether there was a leak into the ocean.
Tanaka said that the evidence was overwhelming.
"We've seen for a fact that levels of radioactivity in the seawater remain high, and contamination continues I don't think anyone can deny that," he said Wednesday at a briefing after a meeting of the authority's top regulators. "We must take action as soon as possible."
A study released earlier this year by Jota Kanda, an oceanographer at the Tokyo University of Marine Science and Technology, examined Tepco's own readings of radiation levels in the waters near the plant's oceanfront site. The study concluded that it was highly likely the plant was leaking.
"If there was no leak, we would see far lower levels of radioactive cesium in waters off the plant," Kanda said last month. He said that natural tidal flushing of the water in the plant's harbor should have dispersed the initially released radioactivity by now, with a far more rapid drop in radiation levels than had been detected.
Tepco said Wednesday that it was not sure that any contaminated water was reaching the ocean. It has said in the past that the stricken plant was now having "no significant impact" on the marine environment.
"We can't say anything for sure," Noriyuki Imaizumi, a Tepco spokesman, said Wednesday at a news conference in Tokyo. "But we aren't just sitting back. We are first analyzing why there have been high radiation measurements in recent weeks."
The struggle to seal the plant has raised questions about the government's push to restart Japan's other nuclear power stations, which were shut down in the wake of the Fukushima disaster. To allay public fears, the government has promised that restarts will be authorized only for reactors that pass rigid new standards that took effect this month.