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 on 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.
Mr. 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. A critical problem has been the groundwater that has been pouring into the basements of the damaged reactor buildings and becoming contaminated. Workers have been pumping the water out to be stored in dozens of tanks at the plant, but have not stopped the inflow.
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.
Mr. 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.
"That said, considering the state of the plant, it's difficult to find a solution today or tomorrow," he added. "That's probably not satisfactory to many of you. But that's the reality we face after an accident like this."
By acknowledging that the Fukushima Daiichi plant is not watertight, Mr. Tanaka confirmed suspicions held by experts that the plant has continued to leak radiation into the ocean long after the huge initial releases seen in the disaster's early days.
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," Professor 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.
"This suggests that water might be leaking out from the plant through damaged pipes or drains, or other routes Tepco doesn't know about," he said. "We need to find out where exactly these leaks are, and plug them."
Unexplained spikes since May in cesium levels detected in groundwater, coupled with higher strontium and tritium readings off shore, have added to the urgency.
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. Some critics have said that the work of certifying and reopening other plants will distract from the cleanup at Fukushima. 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.
Four utilities across Japan have applied to restart a total of 10 reactors, applications that must now be assessed by the nuclear regulator with a staff of just 80 people. Tokyo Electric has said that it intends to apply to restart two of the seven reactors at a power plant on the coast of the Sea of Japan. That workload may leave the agency with few resources to devote to monitoring the messy cleanup at Fukushima.
Tepco has taken some measures in the hope of keeping contaminated groundwater away from the sea, including fortifying an underwater wall that runs along much of the shoreline at the plant site. Mr. Tanaka said it was doubtful whether those measures would be effective.
"We don't truly know whether that will work," Mr. Tanaka said. "Of course, we'd hope to eliminate all leaks, but in this situation, all we can hope for is to minimize the impact on the environment. If you have any better ideas, we'd like to know."
This article originally appeared in The New York Times.