Japan's nuclear concerns explained

Reactor basics

There are several different types of nuclear reactors, including boiling water reactor systems and pressurized water reactor systems. The reactors at the Fukushima Daiichi plant are boiling water reactor systems. Here's how they work:

image source:http://www.straitstimes.com/BreakingNews/Asia/Story/STIStory_646025.html

At the heart of a nuclear reactor is the reactor vessel, a large steel tank filled with water. It's loaded with 12-foot-long fuel rods made of radioactive pellets encased in zirconium alloy. These ceramic fuel pellets are usually made of enriched uranium 235, which is incapable of creating a nuclear explosion.

Reactor No. 3 at the Fukushima Daiichi 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 off large amounts of energy in the form of heat and radiation. It also sends free neutrons toward other atoms, causing some of those atoms to fission. This can lead to a chain reaction.

Rods that absorb neutrons 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.

See demonstration video :CNN Video 1 - Reactor Basics

Normal operation

The speed of the chain reaction and the temperature of the fuel are also controlled by the 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 turbines. As the steam turns the turbines, generators connected to the turbines spin and generate electricity.

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

See demonstration video :CNN Video 2 - Normal Operation

Meltdown

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 operate 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 temperatures can also compromise the reactor vessel and surrounding containment systems that may be present. If these systems are breached, radioactive material may be released into the surrounding environment.

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

In 1986, the Chernobyl nuclear power plant in the Soviet Union suffered a partial meltdown, resulting in an explosion. There was no concrete and steel containment system at Chernobyl, so radioactive material was released directly into the atmosphere.

The Three Mile Island nuclear 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.

See Demonstration Video : CNN Video 3 - Melt Down

The earthquake and tsunami

Three of Fukushima Daiichi's six reactors were already shut down for inspection prior to the earthquake and tsunami. The three reactors that were in operation 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 reaction 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 lasted for 8 hours before being exhausted.
The water pumps shut down, and workers at the plant were left with no effective way to continue to cool the reactors and prevent the fuel from melting.

The explosions

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

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

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

Officials in Japan say the explosions at the 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.
See Demonstration Video : CNN Video 4 - Explosion

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.
See Demonstration Video : CNN Video 5 - Exposed Fuel Rods
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.

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."

The IAEA is not yet rating the incident in Japan on the INES scale.

image source:http://www.straitstimes.com/BreakingNews/Asia/Story/STIStory_646025.html

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.


Sources:
http://www.cnn.com/interactive/2011/03/world/interactive.nuclear.japan/index.html?hpt=C2
CNN, Britannica, IAEA, U.S. Nuclear Regulatory Commission

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