日本核爆炸的过程分析

Japan Nuclear Crisis

(March 15 2011) (March 16 2011) (March 17 2011)

(1)Nuclear Reactor Basics:

The nuclear reactors have a thick inner steel containment vessel and a thick

outer concrete containment shell (the big cube you see on TV). Inside the steel vessel there are radioisotope fuel rods (uranium or plutonium) for the nuclear fission reaction. This reaction releases a huge energy and this energy is used to heat up the water and generate the steam to drive a steam turbine for generating electricity.

The most important protection is from the steel containment vessel and concrete containment shell. All radiation materials should be kept inside.

(2)Nuclear Accident Scale:

There are 7 levels of nuclear accident scales.

▪Three Mile Island accident (1979) was rated as level 5. That accident was

a partial reactor meltdown that led to “very small” releases of radioactivity.

It caused no casualties, but was rated 5 on the scale, corresponding to “an

accident with wider consequences.” The radiation leak was held inside the

containment shell – thick concrete armor around the reactor.

▪The explosion in Chernobyl (1986) was rated a maximum level 7– was the world’s worst nuclear disaster. The Chernobyl reactor had no shell and

was also operational when the disaster struck.

▪The accident at Japan’s Fukushima plant was estimated at level 4 by Japan’s nuclear safety agency earlier. However, the authorities think it

should be rated as level 6by 3/15. Level 6 is a “serious accident.” All

reactors automatically shut down when the quake hit and are encased in

containment shell.

(3)Japan Nuclear Crisis Update:

▪Japan Nuclear Power Plants:

There are two major nuclear power plants are at the heart of the crisis, both of which were hit by the quake and tsunami.

The first one, Fukushima Daiichi – Fukushima Number 1- has six units, each housing its own nuclear reactor. Only the first three units were working when the quake struck.

The second one, Fukushima Daini – Fukushima Number 2- has four units, and all were working at the time of the quake.

Update Situations for both Plants:

All reactors in both plants were automatically shut down when the quake hit. However, the inside temperature was still very high after the shut down. If the cooling system fails then the reactor is overheated, turns the water into steam. The buildup of steam will cause the pressure inside the reactor increased. The fuel rods are tubes of zircoloid stuffed with uranium dioxide. When these are not cooled enough, they swell up and can crack. At that point, radioactive cesium (Cs-137) and iodine (I-131) gases can escape. Radiations from both radioisotopes can be detected by gamma sensors.

Fukushima Number 2:

All four reactor units are in “cold shutdown”, which means that the temperature and operation of the reactors are fully under control.

Fukushima Number 1:

On 3/17 a nearly completed new power line could restore cooling systems in this nuclear power plant, raising some hope of easing the crisis that has threatened a meltdown and already spawned dangerous radiation surges.

Reactor Unit 1:

On 3/12, engineers began venting the contaminated steam to relieve the pressure within the reactor. Later in the day, an explosion occurred at the reactor, as hydrogen produced from water vapor detonated. This destroyed the roof and outer shell of the reactor unit, but left the reactor itself intact.

On 3/14, engineers decided they had to take further steps to cool the reactor vessel down, and so began pumping sea water into the vessel along with boron, an element that dumping nuclear reactions by soaking up the neutrons which drive them. Later that day, a residual heat removal system was revived, and work towards a “cold shutdown”.

Reactor Unit 2:

On 3/12, engineers decided that the water level in the reactor was so low that it, too, needed sea water and boron injected into it. Early on 3/15 there was an explosion in the unit. The explosion had damaged a doughnut-shaped supporting vessel called a suppression chamber which recycles cooling water. However, the IAEA is concerned that the containment vessel itself may also have been damaged. Reactor Unit 3:

Venting of vapor to relieve pressure began on 3/13. The next day, engineers began pumping water and boron into the containment vessel to cool down the fuel. They couldn’t fill it u p enough to cover the fuel rods completely, however, and so decided to relieve growing pressure by venting contaminated steam. Then, on

3/14, there was an explosion in unit 3. There was said to be no damage to the containment vessel.

Unit 3 is the only reactor at that uses a mixture of uranium and plutonium (others use uranium) in its fuel mix. Plutonium, once absorbed in the bloodstream,