The Earthquake and Tsunami
The magnitude 8.9-9.0 Sendai earthquake was one of a handful of such powerful megathrust earthquakes to occur in recorded history. Its epicenter was 130 km (81mi) off the east coast of Miyagi Prefecture and it took place at a depth of 24.4 km (15.2 mi). The huge earthquake shook buildings on land while sending a 6.9-10 m (23-33 ft) tsunami toward the northeastern coast of Japan. It swept inland as far as 10 km (6 mi), causing immense destruction to the area, which can be see through satellite photos from the New York Times.
The waves excavated homes and factories and have dealt a second blow as two nuclear power plants were so heavily damaged by the earthquake that they may be in partial meltdown. More than 10,000 people are dead, hundreds of thousands are homeless, 4.4 million have been left without electricity and 1.4 million do not have water. If you would like to help, click here for a list of organizations that are assisting with the recovery efforts.
All current nuclear power plants are fission plants, involving the splitting of atomic nuclei. In most powerplants, this occurs when a neutron collides with a Uranium-235 (235U) nucleus and is absorbed by it. The 235U was relatively stable, but 236U is unstable and it will break into five pieces, a Krypton-92 (92Kr) nucleus, a Barium-141 (141Ba) nucleus, and three free neutrons. In a bar of Uranium, these free neutrons can collide with other Uranium atoms, which in turn give off more neutrons, and so on. This is what we call a chain reaction.
In addition to the neutrons, heat is given off as well side-reaction chains that generate a few electrons and some radiation. The heat is used to create steam, which drives a turbine that creates electricity. Electrons that escape the reactor core into a surrounding water tank can create a bluish glow, called Cherenkov Radiation, as they pass through water.
Despite the fact that an Uranium is a very heavy element, its nucleus is still rather small compared to the size of the atom itself (including the electron shells). That means that even a heavy rod of Uranium contains mostly empty space at the atomic level. As a result, the free neutrons do not necessarily hit the Uranium nuclei sitting right next door. They can rather travel a good distance through the material before the are absorbed by another nucleus. If the bar of Uranium is too small, then too many neutrons will escape before they can collide with other atoms and in that case, there will be very few nuclear reactions.
This can be remedied by putting a enough of fissile Uranium together to ensure that neutrons will be absorbed by the 235U, but one must be rather careful about it. If too much Uranium is placed together in a small, confined place, there could be a runaway reaction that releases a good deal of all of the energy that is stored in the atomic nuclei in a very short time (read: boom). This video gives a good demonstration of the chain reaction process.
Editor’s note: I once put together a similar demonstration with 1,000 mousetraps. At one point, we reached 500 only to have a ping pong ball go off. Not good. I also remember being a bit gun shy around snapping sounds after that. My fingers were very sore at the end of the setup.
To avoid a runaway reaction, most nuclear power plants use a combination of Uranium and Carbon rods to moderate the reactions rate. The Carbon rods essentially absorb the neutrons before they can reach a Uranium atom. With fewer neutrons reaching the Uranium, the reaction rate slows. If higher rates are required, the power plant simply removes some of the carbon rods to increase the number of neutrons that generate new reactions.
The Fukushima-I (Daiichi) plant runs on a variant of this process, which incorporates Uranium and Plutonium Oxide fuels. The same general principles apply, though the specific isotopes involved in the reactions are somewhat different.
In any case, the heat generated by the reactor core is used to boil water. Each of the plants have variations on this theme, but the general idea is depicted here. Water can be heated to very high temperatures if it is kept under pressure, these plants carry superheated water into a seperate chamber to create steam. As the steam is generated, the formerly superheated water cools, and is recirculated to the reactor core. This has the effect of carrying heat away from the reactor core and regulating the reaction rate (which increases as it heats up, causing more heating). If the temperatures of the reactor were to rise to high, the Uranium rods could melt and create a molten puddle of Uranium at the bottom of the chamber. This is a meltdown and carbon rods would no longer be able to regulate the reactions if a meltdown were to take place. In a sense, there is no “off switch” for a nuclear reactor, especially one that has melted down.
Status of the Power Plants
The events in Japan are taking place quite rapidly, and from what I can see, no one is completely certain regarding whether the plants have melted down.
- Fukushima-I (Daiichi):
Three of six reactors (1,2 and 3) at Daiichi were operating and went into automatic shutdown prior to the tsunami. However, the emergency generators used to cool the plant stopped in unit 1 after the tsunami. Units 2 and 3 were still alright by this point. Temperatures began to rise and steam was vented to release pressure on the containment structure. A hydrogen explosion, caused by a buildup of the gas, occurred and damaged the exterior of the building, though the reactor core containment structure seems alright. Seawater was pumped in to cool the cores.
By March 13, reactor 3 began having trouble and venting was necessary. The core was not completely covered by water, and there may have been some damage to the core. Nearly 200,000 people were evacuated from the area, and 22 residents have shown signs of radiation sickness. On March 14, the Unit 3 building was destroyed by a hydrogen explosion.
- Fukushima-II (Daini):
All four units of this plant were shut down automatically, but Units 1,2 and 4 were damaged by the tsunami and an evacuation was ordered due to possible radioactive contamination. By March 12, the temperature of the reactors had reached 100oC. No pressure release has yet occurred, however. Residence have been evacuated to a distance of 20 km from the plant.
A fire began in the turbine section of the plant after the earthquake. On March 13, radiation levels reached 21μSv/hour, causing a state of emergency to be declared – the International Atomic Energy Agency’s lowest emergency level. After a few minutes, the radiation levels dropped by half and have now returned to background levels.
Japanese authorities believe that the elevated radiation levels at Onagawa were due to the Fukushima I (some 60 km away) accident rather than from Onagawa itself. The radiation levels at Onagawa were not incredibly high, but their increase was cause for alarm. It is not known, however, how high a dose of radiation that the 22 people near Fukushima I had received. Only time will tell. 1,500 others are being tested for radiation exposure as a precaution.
Officials are saying that there is a 70% chance of a magnitude 7 aftershock by Monday. The situation is already a long way from being finished. A large earthquake could make things worse. At the moment, there are still reactors threatening to require venting. Venting is fairly safe, provided that the fuel rods are intact, however it seems there has been at least one partial meltdown. That means that vented gas may become more radioactive as time goes on. In addition, the sea water that is being used in desperation to keep the reactors cool will eventually corrode the metal containment chamber. The stakes are incredibly high – whether a significant fraction of the northern part of the densely populated country will be inhabitable.
Hopefully the earthquake will not arrive. The last thing anyone needs is another challenge at the moment. Good luck to everyone involved in bringing the reactors under control. Stay safe.
Here is that link again if you would like to help out.