The radioactive water leaking into the sea from the Fukushima No. 1 nuclear plant has developed into a scandal, drawing media attention from around world in the past few weeks.

Despite the screaming headlines, however, a critical question remains unanswered: Just how much danger does the contaminated water pose to human health?

Jota Kanda, a professor at Tokyo University of Marine Science and Technology, has some insight to offer.

“Compared with the release of radioactive materials in the initial stage (of the crisis), the amount of material now is overwhelmingly small,” the expert on maritime movement of radioactive substances said in a recent interview with The Japan Times.

“This is not something that has a big impact on fish in the sea,” Kanda said.

The real threat is not the 300 tons of toxic water flowing into the Pacific each day, but the highly radioactive water sitting in the more than 1,000 huge tanks on land, Kanda argued.

“I understand it’s quite important to try to stop the groundwater (from flowing into the sea). But I’m far more concerned about the tanks,” he said. “We still have extremely contaminated water in those tanks. In that sense, we are in a crisislike situation.”

Kanda was one of the scientists who first pointed out that contaminated water was entering the sea, which Tokyo Electric Power Co. continued to deny until the day after the Liberal Democratic Party won the Upper House election in July.

According to an estimate by Daisuke Tsumune, a researcher at the Central Research Institute of Electric Power Industry, 3.5 petabecquerels, or 3.5 quadrillion becquerels, of cesium-137 was released into the sea in the first three months after the three reactor cores began melting on March 11, 2011. Tsumune’s estimate is based on his simulations of how the radioactive isotope spreads in the Pacific.

According to Kanda’s estimates, 17.1 trillion becquerels of cesium-137 was released into the sea in the period between June 2011 and September 2012.

Kanda asserted that this means most of the material contaminating the fish and other sea life largely came from the massive environmental release in the first part of the crisis, particularly from April 1 to April 6 of 2011.

“When you hear ‘17.1 trillion,’ it may sound awful, but this is overwhelmingly small” when compared with the initial release, he said.

Tests performed on fish caught along the Tohoku coastline, including Fukushima, have shown that seafood contamination has been declining since the first few months of the crisis, even though the water spill continues.

Although Kanda’s estimate covers only cesium-137, he said the other radioactive substances, including strontium, which causes bone cancer, and tritium should be following a similar pattern.

“Radioactive materials have been leaking into the sea, but the amount is very small and the contamination in fish has been steadily declining,” Kanda said.

According to sampling data published by the Fisheries Agency in August and September, 35 of the 2,211 fish taken from the sea and from rivers along the Pacific coast had been tainted with cesium exceeding the government’s limit of 100 becquerels per kilogram.

Of the 35 tainted fish, 26 were caught in places off Fukushima, where all fishing is banned. The other nine were freshwater species caught in rivers and ponds not only in Fukushima, but in Gunma and Chiba as well.

While downplaying the sea contamination, Kanda said he is deeply concerned about the massive amount of radioactive material in the huge water tanks at Fukushima No. 1.

According to Kanda’s estimates, which are based on official data from Tepco, the cesium-137 in the highly contaminated water in the basements of Fukushima No. 1’s flooded reactor buildings was giving off as much as 160 petabecquerels as of the end of May 2011.

This is nearly twice the cesium-137 released by the 1986 Chernobyl disaster, which was estimated at 85 petabecquerels by the United Nations Scientific Committee on the Effects of Atomic Radiation.

Tepco filters out most of the cesium-137 from the cooling water before storing it, but huge amounts of strontium, tritium and other dangerous materials remain in the roughly 340,000 tons of water still sitting in 1,060 tanks, Kanda pointed out.

If several of the tanks were breached in an accident or new natural disaster, all of that water would go directly into the sea, with catastrophic results, he emphasized.

Tepco and the government have announced an expensive experimental plan to create a massive wall of frozen soil around the plant to prevent groundwater from entering the reactor buildings, which are generating toxic water of their own.

Kanda, however, said the top priority for now should be to strengthen the tanks and remove the other radioactive materials in the water they’re holding.

Tepco had by now planned to start using ALPS — a high-tech filtering machine that can remove everything radioactive but tritium from tainted water — but the utility halted test runs after finding corrosion holes in the equipment. It is set to reactivate ALPS later this month.

Kanda, no defender of Tepco or the nuclear industry, was among the first scientists to sound the alarm about the radiation risk to the ocean, despite Tepco’s denials. He published his findings on probable cesium contamination in an academic thesis in English in February this year. When NHK and other media reported on his findings, Tepco disputed them.

“I was surprised. I thought, ‘Wow, the Tepco people clearly deny this,’ ” Kanda said, asserting that the conclusion of his thesis was basically “common knowledge” among marine researchers who have been monitoring the density of radioactive substances in and outside the crippled plant’s artificial bay on a regular basis.

Kanda’s estimates on cesium-137 are based on earlier cesium density fluctuations in the bay. Given the rapid decline in cesium density right after the three meltdowns, Kanda concluded that 44 percent of the water in the bay is exchanged daily with water from the ocean. Using this data, Kanda inversely calculated the amount of cesium-137 that was reaching the sea from the nuclear compound.

Since around April 2012, the cesium density in the bay has leveled out and stayed basically constant despite the daily exchange between the bay and the wider sea. This indicates that the radioactive substances have been leaking into the bay at a constant rate that has reached a rough parity with the amount leaving each day, he said.

In August, Tepco reluctantly accepted Kanda’s theory and published a simulation showing that a maximum of 20 trillion becquerels of cesium-137 had reached the ocean, along with 10 trillion becquerels of deadly strontium.

Tepco’s denial and ensuing about-face helped the radioactive water spill become a scandal instead of a discovery, fanned by media reports that didn’t clearly explain the impact of the leakage, Kanda said.

“Tepco had some (communication) problems, as did researchers like us. And, of course, so did the mass media,” he said.