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More than three months after a powerful earthquake and 14-meter-high tsunami struck Japan, the Fukushima Daiichi nuclear power plant remains flooded with a salty mix of ocean and fresh water that is contaminated with the radioactive residue of three reactors and four spent fuel pools' worth of nuclear fuel. Every day an additional 500 metric tons of seawater is poured onto the still hot nuclear fuel in the stricken reactors and fuel pools. More than 100,000 metric tons of such water now sits in the basement and trenches of the reactors—or evaporates inside the hot reactor buildings, making for a radioactive onsen (hot bath).
Thus far, neither the nation's 75 aftershocks of magnitude 6.0 or greater—the latest of which struck on June 23—nor inclement weather has halted ongoing efforts to cool the stricken nuclear power plant. With the start of any decommissioning process still at least year away, cooling the fuel with water remains the focus—as well as a potential source of additional problems as contaminated water threatens to overwhelm the plant and its environs.
In early June Tokyo Electric Power Co. (TEPCO) installed a series of devices—from nuclear equipment manufacturers Kurion and Areva Group—meant to filter radioactive material from the contaminated cooling water and enable it to be reused on the hot nuclear fuel rods. Without such filtration, radiation levels in the reactor buildings can climb too high to permit workers to advance their efforts to control and clean up the damaged power plant. But a trial run of the new filtration system was halted on June 18 in less than five hours when it captured as much radioactive cesium 137 in that span as was expected to be filtered in a month.
Massive tanks have been delivered to store some of the excess radioactive water, given that spraying must continue due to leaks in the reactors themselves that prevent restoration of the normal cooling system. "The most important thing is to keep the reactor fuel cool. If the only alternative is to use saltwater then that's the best thing to do," said Bill Borchardt, executive director for operations at the U.S. Nuclear Regulatory Commission (NRC) at a talk in May. "Given the situation that existed and that there were very few options available, I think injection of saltwater was clearly the appropriate thing to do," although TEPCO has switched to using fresh water more recently.
With just centimeters remaining before the radioactive water overtops its storage, however, another release of contaminated water into the ocean looks ever more likely. Already, pools of this water burned at least two workers at the plant when they stepped in the puddles, and TEPCO was forced to dump more than 11,000 metric tons of such contaminated water in early April.
"The reactors weren't designed to have water poured in the top, pour out the bottom and pool in the basement," says nuclear energy advisor Arnie Gundersen of Fairewinds Associates consultants. "What TEPCO should be doing is building a trench around the reactors down to bedrock, 20 meters deep and 1.5 meters wide, and fill that trench with zeolite." Zeolite minerals capture radioactive particles, and are used in the reprocessing of spent nuclear fuel.
The normal equipment for such cooling is inoperable thanks to the complete meltdown and corrosion from seawater, so spraying new water on the hot nuclear fuel remains the only option. Debris and detritus—radioactive and inert, alike—continue to impair human and robot workers' attempts to achieve so-called "cold shutdown," which would allow the real work of tearing down and cleaning up the contaminated site to begin.
"It's going to be very complicated to decommission this thing," physicist Arjun Makhijani, president of the Institute for Energy and Environmental Research notes. "The handling equipment has been destroyed, it was a complete meltdown, it's a highly radioactive environment and there's radioactive water."
The rising level of used cooling water is just one of the challenges at the plant, more than three months after the crisis started. Radiation levels continue to spike at times as high as 4,000 millisieverts an hour, impairing repair efforts, even with robots. (A sievert is a unit of ionizing radiation equal to 100 rems; a rem is a dosage unit of x-ray and gamma-ray radiation exposure.) Despite the restoration of electric power in April, cooling systems remain inoperable. And radioactive material—cesium 134, cesium 137 and some 50 other longer-lived radioactive isotopes—continue to be found farther and farther afield from the site itself, concentrating in hot spots as far as 225 kilometers from the stricken complex. For its part, Japan's Nuclear and Industrial Safety Agency now estimates Fukushima has released some 770,000 terabecquerels worth of radiation—or roughly 15 percent of the amount released by the catastrophic 1986 fire at Chernobyl in Ukraine. (One becquerel represents the rate of radioactive decay—or radiation emitted by a substance—as one disintegration, or count, per second.)
"When you have an accident for months and certain patterns of rainfall, you get hot spots," Makhijani says. As a result, entire towns, such as Date, Iitate and Iwaki City, may have to be permanently abandoned and roughly 80,000 people have lost their homes to radioactive contamination.
In addition to the failed water filtration system, TEPCO has proposed enshrouding in plastic the reactor buildings torn apart by hydrogen explosions to prevent further releases from that source of radioactive material. The good news is that the heat from the melted down nuclear fuel and still intact fresh fuel rods continues to decline. "As time goes on, the decay heat gets less and less," the NRC's Borchardt noted. "Around 90 to 100 days the problem becomes much less severe"—a time period Fukushima Daiichi has now entered.
More than 3,700 workers continue to attempt to control and contain the crisis. Nine of those workers have already reached the legal "emergency limit" of 250 millisieverts of cumulative radiation exposure, and 124 have received more than 100 millisieverts, the prior limit. In the U.S. annual exposure for nuclear power plant workers is limited to 50 millisieverts per year, and it is estimated by some that their risk of cancer increases by 4 percent per sievert. (This risk figure remains controversial as either too high or too low, by scientists who study the impact of radiation on health, primarily based on data collected after the 1945 detonation of the atomic bombs over Hiroshima and Nagasaki during World War II.)
In the end TEPCO plans call for a cold shutdown of the stricken reactors by April 2012—more than a year after the crisis began—and that means some kind of treatment plan for hundreds of thousands of metric tons of radioactive cooling water will be needed as soon as possible. Beyond that lies the challenge of hundreds of thousands of metric tons of soil contaminated with radioactive isotopes across at least 600 square kilometers of northeastern Japan.
The challenge is not insurmountable, just costly. "You can clean up almost anything if you're prepared to spend enough money on it," adds Peter Bradford, a former member of the NRC.