mardi 15 mars 2011

The Chernobyl Disaster

The Chernobyl disaster was a nuclear accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in the Ukrainian SSR (now Ukraine). It is considered the worst nuclear power plant accident in history, and it is the only one classified as a level 7 event on the International Nuclear Event Scale.
The disaster began during a systems test on 26 April 1986 at reactor number four of the Chernobyl plant, which is near the town of Pripyat. There was a sudden power output surge, and when an emergency shutdown was attempted, a more extreme spike in power output occurred, which led to a reactor vessel rupture and a series of explosions. This event exposed the graphite moderator components of the reactor to air, causing them to ignite. The resulting fire sent a plume of radioactive fallout into the atmosphere and over an extensive geographical area, including Pripyat. The plume drifted over large parts of the western Soviet Union, Eastern Europe, Western Europe, and Northern Europe. Large areas in Ukraine, Belarus, and Russia were evacuated, and over 336,000 people were resettled. According to official post-Soviet data,[1][2] about 60% of the fallout landed in Belarus.
The accident raised concerns about the safety of the Soviet nuclear power industry, as well as nuclear power in general, slowing its expansion for a number of years and forcing the Soviet government to become less secretive about its procedures.[3][notes 1]
Russia, Ukraine, and Belarus have been burdened with the continuing and substantial decontamination and health care costs of the Chernobyl accident. Fifty deaths, all among the reactor staff and emergency workers, are directly attributed to the accident. Estimates of the total number of deaths attributable to the accident vary enormously. Despite the accident, Ukraine continued to operate the remaining reactors at Chernobyl for many years. The last reactor at the site was closed down in 2000, 14 years after the accident.[4]

Source: Wikipedia

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Japan suspends work at stricken nuclear plant

FUKUSHIMA, Japan – Japan suspended operations to prevent a stricken nuclear plant from melting down Wednesday after a surge in radiation made it too dangerous for workers to remain at the facility.
Chief Cabinet Secretary Yukio Edano said work on dousing reactors with water was disrupted by the need to withdraw.
The level of radiation at the plant surged to 1,000 millisieverts early Wednesday before coming down to 800-600 millisieverts. Still, that was far more than the average
"So the workers cannot carry out even minimal work at the plant now," Edano said. "Because of the radiation risk, we are on standby."
Experts say exposure of around 1,000 millisieverts is enough to cause radiation sickness.
Earlier officials said 70 percent of fuel rods at one of the six reactors at the plant were significantly damaged in the aftermath of Friday's calamitous earthquake and tsunami.
News reports said 33 percent of fuel rods were also damaged at another reactor. Officials had said they would use helicopters and fire trucks to spray water in a desperate effort to prevent further radiation leaks and to cool down the reactors.
The nuclear crisis has triggered international alarm and partly overshadowed the human tragedy caused by Friday's double disaster, which pulverized Japan's northeastern coastline, killing an estimated 10,000 people.
Authorities have tried frantically since the earthquake and tsunami to avert an environmental catastrophe at the Fukushima Dai-ichi complex in northeastern Japan, 170 miles (270 kilometers) north Tokyo.
The government has ordered some 140,000 people in the vicinity to stay indoors. A little radiation was also detected in Tokyo, 150 miles (240 kilometers) to the south and triggered panic buying of food and water.
There are six reactors at the plant, and three that were operating at the time have been rocked by explosions. The one still on fire was offline at the time of the magnitude 9.0 quake, Japan's most powerful on record.
The Nuclear and Industrial Safety Agency estimated that 70 percent of the rods have been damaged at the No. 1 reactor.
Japan's national news agency, Kyodo, said that 33 percent of the fuel rods at the No. 2 reactor were damaged and that the cores of both reactors were believed to have partially melted.
"We don't know the nature of the damage," said Minoru Ohgoda, spokesman for the country's nuclear safety agency. "It could be either melting, or there might be some holes in them."
Meanwhile, the outer housing of the containment vessel at the No. 4 unit erupted in flames early Wednesday, said Hajimi Motujuku, a spokesman for the plant's operator, Tokyo Electric Power Co.
Japan's nuclear safety agency said fire and smoke could no longer be seen at Unit 4, but that it was unable to confirm that the blaze had been put out.

Japanese plant poses little threat to US  for now

AP/Gregory Bull
It's a big ocean between northeastern Japan and the United States and thousands of miles from the crippled nuclear power plant to much of Asia. That means there's little chance — at least for now — that radiation from the shattered reactors could pose a serious threat to the wider world.
Experts say the amount of radioactivity emitted by the facility has been relatively minor and should dissipate quickly over the Pacific Ocean.
"Every mile of ocean it crosses, the more it disperses," said Peter Caracappa, a radiation safety officer and clinical assistant professor of nuclear engineering at Rensselaer Polytechnic Institute in Troy, N.Y.
The only people at immediate risk are workers inside the plant and the people living closest to it. The danger of radiation exposure elsewhere is minuscule — unless the plant sustains a complete meltdown, which would sharply escalate the dangers.
Japanese officials told the International Atomic Energy Agency on Tuesday that a fire had broken out in a fuel storage pond where used nuclear fuel is kept cool and that radiation had been "released directly into the atmosphere."
If the water level in such storage ponds drops to the level of the fuel, a worker standing at the railing looking down on the pool would receive a lethal dose within seconds, according to a study by the Millstone nuclear plant in Connecticut.
Such intense radiation can prevent workers from approaching the reactor or turn their tasks "into suicide missions," said David Lochbaum, a nuclear engineer who heads the nuclear safety program of the Union of Concerned Scientists.
Next in the line of danger would be those who live within a 20-mile radius. Areas around the plant have been evacuated for that reason.
"The odds of someone outside the plant getting an acute injury — sick in the next couple of weeks — is close to zero," said John Moulder, a professor of radiation oncology at the Medical College of Wisconsin in Milwaukee who studies the effects of radiation exposure.
The radioactive particles probably contain materials linked to cancer in high doses, including cesium and iodine. The long-term cancer risk for nearby residents will depend on exposure and cleanup efforts, Moulder said.
Radioactive cesium and iodine also can combine with the salt in sea water to become sodium iodide and cesium chloride, which are common elements that would readily dilute in the wide expanse of the Pacific, according to Steven Reese, director of the Radiation Center at Oregon State.
Click image to see photos of quake, tsunami damage

Reuters/International Red Cross/Handout
Winds in the area are currently blowing toward the coast because of a winter storm. But that will change to a brisk wind blowing out to sea at least through Wednesday, he said by telephone.
Still, the forecast offered little comfort to those living in the area — and in nearby countries such as Russia.
The Russian Emergencies Ministry said it was monitoring radiation levels and had recorded no increase.
Many Russians, however, distrust the reassurances, perhaps remembering the Chernobyl disaster 25 years ago and how long it took the Soviet government to reveal the true dangers of the radiation.
"The mass media tells us that the wind is blowing the other way, that radiation poses no threat. But people are a mess," Valentina Chupina, a nanny in Vladivostok, said in a comment posted on the website of the newspaper Delovoi Peterburg. "They don't believe that if something happens we'll be warned."
The news portal Lenta said that in addition to potassium iodide and instruments used to measure radiation, people in the Far East also were stocking up on red wine and seaweed, which they believed would offer protection from radiation.
Even so, many experts here say that this emergency is nowhere near the level of Chernobyl, the worst nuclear disaster in history.
For one, that reactor's core contained graphite that caught fire, which blasted radiation high into the air and into wind currents that carried it long distances. The Japanese core is metal and contains no graphite, experts said.
The Chernobyl plant also lacked a heavy shell around the reactor core. And the incident there happened quickly, with little time to warn nearby residents.
So far, the radiation released in Japan has not reached high altitudes, said Kathryn Higley, director of the Oregon State University Department of Nuclear Engineering and Radiation Health Physics.
"In addition, radioactive material is sticky. It has a static charge," she said, so it will stick to the sides of buildings, and "rain is going to knock it down."
As a precaution, the World Meteorological Organization has activated specialized weather centers to monitor the situation. Those centers, in Beijing, Tokyo and Obninsk, Russia, will track any contaminants.
Meanwhile, the International Atomic Energy Agency said a single reading at one location in the Japanese plant recorded levels of 400 millisieverts, or 40 rems, per hour.
"You start getting radiation sickness at around 100 rems" — nausea and vomiting. Damage to blood cells can show up two to four weeks later, said Dr. Fred Mettler, a University of New Mexico radiologist and adviser to the United Nations on radiation safety. He led an international study of health effects after the Chernobyl disaster.
Levels were much lower at a plant gate, and "if you get further away from that, the population got a very small dose if anything," said Kelly Classic, a radiation physicist at the Mayo Clinic and a representative for the Health Physics Society, an organization of radiation safety specialists.
The U.S. Nuclear Regulatory Commission says doses of less than 100 millisieverts, or 10 rems, over a year are not a health concern.
By comparison, most people receive about three-tenths of a rem every year from natural background radiation, according the U.S. Environmental Protection Agency. A chest X-ray delivers about .1 millisieverts, or .01 rem of radiation; a CT scan of the abdomen and pelvis is about 14 millisieverts, or 1.4 rems.
If a full meltdown occurs at the Japanese plant, the health risks become much greater — with potential release of uranium and plutonium, said Dan Sprau, an environmental health professor and radiation safety expert at East Carolina University in Greenville, N.C.
"If that escapes," Sprau said, "you've got a whole new ball game there."

Source: Yahoo News

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