By Elisabeth Rosenthal and William J. Borad, The New York Times, March 28, 2011
The announcement by Japan’s Agency for Natural Resources and Energy that high levels of radioactive cesium have been detected in seawater near the crippled nuclear reactors raises the prospect that radiation could enter the food chain.
Cesium 137 levels were 20 times the normal level about 1,000 feet from the effluent at the Fukushima Daiichi nuclear power plant. That is far less than the level of the other main radioactive isotope spilling from the plant, iodine 131. It was found in concentrations of more than 1,150 times the maximum allowable for a seawater sample a mile north of the plant.
Still, scientists say, cesium 137 poses the greater long-term danger to the marine food chain.
Iodine 131 degrades relatively fast, becoming half as potent every eight days. So the radioactive risk can be combated by banning fishing and the consumption of seafood for a period of time, as the Japanese have already done.
Cesium 137, on the other hand, has a half-life of 30 years. Worse still, it is absorbed by marine plants, which are eaten by fish and — like mercury — tends to become concentrated as it moves up the food chain.
“It’s worrisome in that CS 137 is leaking, although the levels are still low,” said Paul G. Falkowski, a professor at Rutgers University’s Institute of Marine and Coastal Sciences. “At some point this water that is pooling in various places is ultimately going to make its way out to the sea.” And if there is a lot of cesium 137 over an extended period “then you’ll have to worry.”
The exact source of the cesium 137 is unclear, although some scientists have speculated that the seawater dumped on the overheating reactors to cool them picked up radiation and then washed back out to sea. But Japanese officials said highly radioactive water in several tunnels is threatening to overflow and may also contain cesium 137.
Even so, the ocean has remarkable power to dilute radioactive effluents, because of its sheer volume and depth. And the ocean is already slightly radioactive.
Over the eons, elements like uranium have washed into it from rivers. More recently, humans have dumped radioactive materials into the marine environment, including dozens of nuclear warheads and reactors that are slowly decaying, as well as many thousands of barrels of radioactive waste.
In October 1993, a Russian ship dumped hundreds of tons of low-level nuclear waste into the Sea of Japan, touching off a diplomatic row between Tokyo and Moscow.
Oceanographers have monitored the areas around the dumps for dangerous levels of radioactivity but typically find little of consequence because of the sea’s powers of dilution. Even so, in 1994 most countries gave up the longstanding practice of dumping radioactive materials into the sea.