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A gravidade do acidente nuclear em Fukushima comparada ao de Chernobyl

15 de abril de 2011

Scientific American
Apr 12, 2011

Is Fukushima really as bad as Chernobyl?

By David Biello 

satellite image of Fukushima Daiichi explosionOne month to the day after the devastating twin blows of a magnitude 9.0 earthquake and subsequent 15-meter tall tsunami, Japanese officials have reclassified the accident at the Fukushima Daiichi nuclear power plant at the highest possible level. The partial meltdown of three reactors and at least two spent fuel pools, along with multiple hydrogen explosions at the site now rate a 7 on the International Nuclear Event Scale—a level previously affixed only to the meltdown and explosion at Chernobyl.

Fukushima is now officially a “major accident” per the scale—roughly 100 times worse than the worst civilian nuclear accident in the U.S.: the partial meltdown at Three Mile Island—constituting “a major release of radioactive material with widespread health and environmental effects.”

The elevated level doesn’t mean that anything further has gone wrong with the stricken nuclear power plant, although efforts are ongoing to cool the nuclear fuel and prevent any further radioactive material from escaping. It simply means that the accident—one of 12 to occur at nuclear power plants since the dawn of the civilian Atomic Age in 1957—is far worse than anyone cared to admit during the past few weeks.

“Right now, the situation of the nuclear reactors at the Fukushima [power] plant has been stabilizing step by step,” Japanese Prime Minister Naoto Kan said April 12 in a televised address. “The amount of radiation being emitted is falling.”

Yet, it has become clear that for hours on March 14 and March 15 following hydrogen explosions on those days, the stricken nuclear power plant spewed tens of thousands of terabecquerels of radioactive material—one becquerel (Bq) is the measure of a material’s radioactive decay equal to one nucleus disintegration per second, so a terabecquerel is trillions of such decays.

The Nuclear Safety Commission of Japan estimates that 1.7 x 10 ^17 Bq of iodine-131 has been emitted by the three stricken reactors as well as 1.2 x 10^16 Bq of cesium-137—although the Japanese Nuclear and Industrial Safety Agency (NISA) announced lower estimates for cesium. The radioactive cesium will persist in the environment for decades, potentially causing cancer or other health effects if ingested.

The cumulative release of radioactive material now equals at least 1.8 million terabecquerels—enough to merit the 7 designation on the INES scale. The releases have created regional “hotspots” of radioactivity in northwestern Japan, such as the village of Iitate, 30 kilometers northwest of Fukushima Daiichi.

The stricken nuclear power plant has also dumped 11,500 metric tons of contaminated water into the surrounding sea, and high levels of radioactivity have been detected in fish pulled from coastal waters between Fukushima Daiichi and Tokyo to the southeast.

Chernobyl, however, was far worse than the current accident because it actually burned, a graphite fire with smoke that spread radioactive material 30 kilometers around the reactor as well as injecting it high into the atmosphere where it wafted at least 500 kilometers to Europe and beyond. All told 14 million terabecquerels of radioactive material are estimated to have escaped during what remains the world’s worst nuclear accident.

That means roughly 10 percent of the radioactive material released by the Chernobyl fire has spewed from the overheated Fukushima Daiichi reactors, according to NISA.

At the same time, those reactors—and surrounding spent fuel pools—continue to emit radioactive material and could, in time, surpass Chernobyl, as Tokyo Electric Power Co. conceded April 12 in a press conference. As it stands, the nuclear fuel rods in the three stricken reactors at Fukushima Daiichi remain “partially or fully” exposed, according to the Japan Atomic Industrial Forum.

Ultimately, Fukushima will resemble Chernobyl in another way: final containment will likely be achieved by entombing in it concrete and surrounding it with an exclusion zone to prevent visits by humans.

Image: Reactor No. 3 exploded March 14 at 9:01 A.M. local time. Smoke rises from the damaged building in this

photo taken at 9:04 A.M. Credit: DigitalGlobe

http://www.scientificamerican.com/blog/post.cfm?id=is-fukushima-really-as-bad-as-chern-2011-04-12&WT.mc_id=SA_WR_20110420 
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