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2011년 3월 30일 수요일

일본 원전 사고 - 방사능 오염, 제 2의 재난 우려 - 사고 원인 설명 등 : Japan's Nuclear Concerns Explained

Japan's Nuclear Concerns Explained

 
Workers in Japan are struggling to contain the crisis in Fukushima Daiichi Nuclear Power Plant, which was badly damaged by the earthquake and tsunami on March 11, 2011. Learn about how the Fukushima Daiichi Reactor work, what's causing the problem with these reactors and what's being done to get the nuclear under control.
Reactor Basics
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There are several different types of reactors, including boiling water reactor system and pressurized water reactor systems. The reactor of Fukushima Daiichi reactor systems are boiling water systems. Here's how they work:
At the heart of nuclear reactor is reactor vessel, a large steel tank filled with water. It's loaded with 13-foot -long fuel rod made of radioactive pellets encased in zirconium alloy. These ceramic fuel pellets are usually made of enriched uranium 235, which is incapable of nuclear explosion.

Reactor No.3 at Fukushima Nuclear plant used more toxic mixed-oxide(MOX) fuel rods, which contain a mixture of uranium and plutonium that is reprocessed from spent uranium.

The fuel rods are loaded vertically into the reactor vessel in a precise grid pattern known as the reactor core. Nuclear fission, or the splitting of atoms, is initiated. As each atom splits, it gives large amounts of energy in the form of heat and radiation. It also sends free neutrons toward other atoms, causing some of other atoms to fission. This can lead to a chain reaction.

Rods that contain neutron control this chain reaction. When the control rods are present, neutrons are absorbed and the reaction slows.  When the rods are removed, the reaction increases.

Normal Operation
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The speed of chain reaction and the temperature of the fuel are also controlled by water in the reactor vessel. If the reaction is properly controlled, it reaches a point of equilibrium known as "criticality", in which the amount of heat produced remains constant.

The heat generated by the nuclear reaction boils the water in the reactor vessel, turning the water into steam. This steam flows out of the reactor vessel through pipes to steam turbine. As the steam turns turbine, generators connected to turbines spin and generate electricity.

The steam then travels to a condenser unit, which cools the unit, turning it back into water. The water is pumped back into the reactor, where it is used again to control the reactor temperature and reactor speed.

Meltdown
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The reactor will operate at a safe temperature as long as the control rods are able to control the chain reaction and cool water is pumped into the reactor continuously. If, however, the nuclear reaction or reactor temperature races out of control, a meltdown can occur.

The fuel pellets normally operates at a temperature of 1,400 degrees Fahrenheit. If the temperature rises to 2,200 degrees for a sustained period of time, the fuel rods can become damaged. If the temperature continues to rise, the fuel rods can eventually melt. This melting can release large amounts of radiation and even vaporize the core.

The high temperature can also compromise the reactor vessel and surrounding the containment systems that may be present. If these systems are breached, radioactive material may be release into the surrounding environment.

In Japan and United States, a thick concrete and steel containment vessel surrounds the reactor vessel. These containment system is designed to keep any radioactive material from being released into the surrounding environment.

In 1986 Chernobyl nuclear power plant in Soviet Union suffered a partial meltdown, resulting in a explosion that killed 32 people. There was no concrete and steel containment system at Chernobyl, so radioactive material was directly released into atmosphere, causing widespread contamination and sickening dozens.

The Three Mile power plant in Pennsylvania suffered a partial meltdown in 1979. This plant did have a concrete and steel containment unit, which remained intact. So, release of radioactive material  in that incident was minimal. Nobody was killed in this incident.

The Earthquake and Tsunami
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Three of Fukushima Daiichi's six reactors were already shut down for inspection prior to earthquake and tsunami. The three reactors that were in operations shut down automatically in a matter of seconds when the quake struck.

Even though the control rods were fully inserted to slow the fission chain reactions to a stop, the reactor was still extremely hot. This is known as decay heat, which will be present for months or even years. Cool water is still needed to keep the reactor fuel from overheating and possibly melting.

When the earthquake struck and the Fukushima Daiichi reactors shut down, the water pumps lost power. Emergency diesel generators began to power the pumps. However, the tsunami that soon followed damaged the diesel generators. The pumps then operated on emergency battery power, which last for 8 hours before exhausted.

The water pumps shut down, and the workers left with no effective way to continue to cool the reactors and  prevent the fuel from melting.

The Explosions
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It's still unclear exactly what happened, but what is certain is that several explosions rocked the Fukushima Daiichi power plant in days after earthquake and tsunami.

The first explosion occurred in the reactor No.1 building on March 12, 2011. The second explosion occurred on Monday, March 14 at reactor No.3. A third explosion occurred on early Tuesday, March 15, at reactor No.2.

While these explosions caused significant damages to the exterior buildings that housed the reactors, it was unclear if the concrete and steel containment vessel suffered any damage.

Officials says in Japan the explosions at three reactors were likely caused by a buildup of hydrogen gas, which is the highly flammable, lighter-than-air gas used in the Hindenburg.


One CNN contributor suggests that some of the zirconium fuel casings in the reactor may have burst. The zirconium can react with water to produce zirconium oxide and hydrogen. When this occurs, the hydrogen gas needs to be vented. As the hydrogen is vented and encounters oxygen in the surrounding air, the two can recombine explosively.

Another theory suggests that radiation causes water in the reactor to separate into hydrogen and oxygen. Normally, these gases would be carried away to a re-combiner that would turn them into water. But, with the lack of water circulation, the hydrogen built up until it had to be vented into the area of the building above the reactor. When enough hydrogen built up, the explosion occurred.

Fire

Japanese officials say fire was discovered in a storage pond used for spent nuclear fuel rods in the reactor No. 4 building on Tuesday, March 15.

On Wednesday, March 16, officials with Tokyo Electric and Power reported the discovery of a second fire in the northeastern corner of the same reactor building.

Reactor No. 4 is one of the three reactors shut down for inspection when the earthquake and tsunami hit. However, nuclear fuel was still present in the building in the storage pond.

These fuel rods are not within the concrete and steel containment system that houses the reactor vessel. CNN contributors warn that a fire in the spent fuel storage pond could spread radioactive material directly into the surrounding environment.


Exposed Fuel Rods
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Helicopters began dumping massive amounts of seawater Thursday morning on Fukushima Daiichi's No. 3 reactor in order to cool its overheated fuel pool. Experts believe that boiling steam rising from that pool, which contains at least partially exposed fuel rods stored after being removed from the reactor, may be releasing radiation into the atmosphere.

How Bad is 'Bad'?
The International Atomic Energy Agency (IAEA) developed the International Nuclear and Radiological Event Scale to identify the severity of incidents involving nuclear energy.

Chernobyl was a level 7 accident, the highest level, denoting a "major accident."

Three Mile Island was classified a category 5, which is an "accident with wider consequences."

Japan's Nuclear and Industrial Safety Agency is rating the crisis at the Fukushima Daiichi nuclear plant a 5 -- putting it on par with the 1979 incident at Pennsylvania's Three Mile Island.


How Much is 'Too Much'?

Radiation is invisible. You cannot taste it, smell it or feel it. It's not possible to directly measure the amount of radiation exposure a person has had. When you see people with Geiger counters checking a site like Fukushima Daiichi, they're measuring contamination, which generally refers to actual radioactive particles.

There are four main types of ionizing radiation:

--Alpha particles: relatively heavy, cannot penetrate human skin or clothing, but can be harmful if they get into the body in another manner.

--Beta radiation: can cause skin injury and is harmful to the body internally.

--Gamma rays: high-energy invisible light that can damage tissue and is most dangerous to humans.

--X-rays: also high-energy invisible light that can damage tissue and is very dangerous to humans.

Levels of radiation exposure:

--An average person receives about 3.1 millisieverts per year from natural sources.

--A person in the U.S. typically receives a total of 6.2 millisieverts because of medical diagnostic procedures and other man-made sources of radiation.

--A chest X-ray delivers a dose of about .02 millisieverts of radiation.

--A CT scan to the abdomen delivers about 8 millisieverts of radiation.

--Japanese officials say they had measured radiation rates of up to 400 millisieverts per hour between reactor units No. 3 and No. 4, according to the IAEA.


Radiation Levels
The highest radiation levels detected at the main gate of the Fukushima Daiichi Nuclear power plant, as reported by the Tokyo Electric Power Company was 8.837 millisieverts at 10:15 am on March 15 while the lowest was 3.3538 millisieverts at 1:00 am on March 14.

These levels are millisieverts per hour. The main gate is located approximately one kilometer from the reactors.

For context, an average person receives about 3.1 millisieverts per year from natural sources.

A person in the U.S. typically receives a total of 6.2 millisieverts because of medical diagnostic procedures and other man-made sources of radiation.


Radiation in Food

Radiation exceeding legal limits also has been found in 11 types of vegetables and milk in prefectures surrounding the damaged Fukushima Daiichi nuclear power plant, prompting some prefectures to stop shipping these products. The United States is preventing the import of milk, milk products, fresh vegetables and fruit from four Japanese prefectures.

Radioactive particles released from the power plant bind to dust, traveling in the air for a distance before coming to ground, contaminating farm produce and water simply by falling on them. The main way these particles get into milk is when they fall on the grass eaten by cows.

Experts say little is known about how eating radiation-contaminated food affects people in the short- and long-term. But experts who have spoken with CNN say that the contamination levels reported so far appear to pose very little risk.

Even low radiation doses can damage or alter the DNA of irradiated cells, the NRC says. And the radiation protection community "conservatively assumes that any amount of radiation may pose some risk for causing cancer and hereditary effect, and that the risk is higher for higher radiation exposures."



How a Reactor Shuts Down and What Happens in a Meltdown: click the link under.




Radiation at Fukushima Daiichi






Plutonium found in Soil in Japan
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Nuclear Fallout Fears Grows in Japan
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Scope of Japan Crisis Continues to Widen
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Japan Nuclear Workers Flying Blind
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Radioactive Water Leak Escalates Japan Crisis
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Graphic: Stabilizing the Fukushima nuclear power plant



Four reactors at Fukushima nuclear complex will have to be scrapped, utility chairman says

Reporting from Tokyo— 
The chairman of the utility that runs the crippled Fukushima power plant on Wednesday said the facility's four tsunami-battered reactors would have to be scrapped, and he apologized to the Japanese public for the nuclear disaster.


Tsunehisa Katsumata, chairman of the Tokyo Electric Power Co., expressed his deep remorse for the accident at Fukushima in northern Japan, including explosions, the release of radiation and contamination of crops and tap water. Although Katsumata referred only to scrapping reactors No. 1 through 4, government officials and other experts have been saying for more than a week that the entire complex, including the less problematic reactors 5 and 6, eventually would have to be decommissioned.


Experts are also mulling whether radioactive water that has flooded parts of the facility could be sucked up and placed in a barge.

Radioactive material continues to seep from the plant. The government's nuclear agency said Wednesday that radioactive iodine-131 had been detected at 3,355 times the legal limit in seawater several hundred yards from the Fukushima plant. That's the highest such concentration recorded at sea to date, but specialists said the material's half short life and the diluting effect of the ocean meant there was negligible concern about the impact on human health.

Eventually, Katsumata said, the Fukushima plant could be entombed in concrete. Chief cabinet secretary Yukio Edano cautioned later that that was but one option being considered. "We should look at all the options," he said.       March 30, 2011.




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