As of 11:30 pm EDT Monday 3/14/11
Three of the six reactors at the Fukushima Dai-Ichi nuclear facility were operating at the time of the earthquake—Units 1, 2, and 3. The reactors shut down when the earthquake hit, but even after they stopped producing power, the reactor cores were very hot and required cooling systems to circulate water around the fuel to carry heat away.
In this case the cooling systems failed at all three reactors shortly after the reactors shut down. Once the cooling stopped, the water surrounding the fuel heated up and began to boil away. The water level dropped and for at least a period of time, the fuel in all three reactors was partially uncovered.
The part of the fuel that was uncovered became very hot. Several things can happen at that point.
As the fuel heats up, the hot cladding of the fuel rods can react with the steam around them and produce hydrogen. This is probably the source of hydrogen that caused explosions in Units 1 and Unit 3. That would indicate that the surface of the fuel rods is damaged.
Damage to the fuel rods can also release radioactive cesium-137 and iodine-131, which build up inside the fuel rods during the normal operation of the reactor. The detection of trace amounts of these elements outside the plant is also evidence that the fuel rods have been damaged, but not necessarily that the fuel inside them has melted.
If the rods, or parts of them, are uncovered for a matter of hours they can heat up enough that the fuel will start to melt. That is called a partial meltdown.
Because of the lack of cooling systems, over the weekend workers were forced to start pumping sea water into the reactor vessels to attempt to cool the cores and keep the fuel rods covered with water. However, for all three reactors, parts of the fuel rods were uncovered long enough that some fuel melting has occurred. The amount of melting, which is currently unknown, will affect the severity of the accident.
Early Monday (U.S. time), reports suggested that efforts to pump sea water into the cores of Units 1 and 3 were successfully keeping the rods covered with water. If this is true and they can continue to pump in sufficient water, the core will keep cooling. The goal is to reach “cold shutdown,” which means the core has cooled below the boiling point of water, so it is no longer boiling off the cooling water.
If that can be done, the situation will be more stable and the reactors will have a larger margin of safety in case something else happens.
However, by Monday night there were reports that efforts to continue cooling Units 1 and 3 might be running into problems. The situation at those two reactors is currently unclear.
Unfortunately, reports on Unit 2 suggest the situation is more serious. After the cooling systems for this reactor failed over the weekend, workers were unable to fill the reactor vessel with sea water. This appears to have been caused in part by human error, and the plant owner, TEPCO, has stated that the fuel in the core of Unit 2 was completely uncovered for a matter of hours. This could lead to very serious melting of the fuel. If that condition continues long enough, it could lead to a total melting of the fuel, which is called a meltdown.
If that occurs, the molten fuel can drop to the bottom of the reactor vessel, burn through the reactor vessel, and drop onto the floor of the primary containment. There is currently no indication this has happened at Unit 2, but if they are unable to get cooling water into the reactor vessel it is likely only a matter of time until this happens.
Even if this happens, as long as the primary containment structures surrounding the reactor remain intact, the release of radiation into the atmosphere may be relatively small.
However, there are two serious concerns:
The first is that Monday night TEPCO confirmed there is damage to the Unit 2 containment and that it may be leaking gases and or liquids. Normally the reactor building is intended to act as a secondary containment and capture radiation leaking from the primary containment so that filters can remove the radioactivity before it is released to the atmosphere. But the reactor buildings for all three reactors have been damaged by explosions and no longer provide this secondary containment.
So if the primary containment is leaking, then a core meltdown could lead to a very large release of radioactivity to the environment.
The second concern is that even if the primary containment is currently intact, the Mark I containment system used in these reactors has a known vulnerability to meltdowns. Molten fuel that enters the primary containment area can melt through the wall of the primary containment—a situation called liner melt-through—which would also allow the release of large amounts of radioactivity to the environment.
By late Monday (U.S. time), the situation is getting much worse. Larger radiation releases have occurred. And the New York Times is reporting that the radiation levels around the plants have gotten so high that TEPCO may withdraw workers from the plants. If this stops efforts to cool the reactors, the result may be meltdowns of the fuel in all three reactors.