Three of the six nuclear reactors at the Fukushima nuclear plant melted down in March of 2011. The Fukushima Unit One, Unit Two and Unit 3 reactors were destroyed in the disaster. The combination of melted nuclear fuel rods and the rest of the core structure produced by a meltdown are known as corium.
It is still not clear exactly where the melted cores are currently located. The big question is whether or not the melting cores are still within the containment vessels. There is speculation that the corium has sunk into the ground under the power plant. The facility is so radioactive that workers cannot get in to find out where the corium is. Current radiation in the ruins of Unit One would kill a human in an hour. A robot that was sent in to check Unit One ceased functioning in three hours. A muon detector system was set up to locate the corium and basically found out where the corium was not.
Once the corium has been located, then there is the question of how to retrieve it. Some analysts say that there is no technology that exists that could be used to get to the corium and remove it from the ruins of the Fukushima nuclear plant.
Now there are three proposals from the Japanese Nuclear Damage Compensation and Decommissioning Facilitation Corporation (NDF) for how to get the corium out. The NDF is developing a "road map" schedule for use by the Japanese government and TEPCO with respect to technical issues with the corium removal.
The first approach which has been the only one considered before the roadmap consists of flooding the containment vessels with water and removing the corium by going in through the top of the vessels. The water would help protect the workers from the radiation. This process is called the "water-covered method." If the containment vessel has corroded through or has cracks, water would leak out. In addition, filling the containment vessels with water would make them less able to withstand an earthquake.
The second approach would be to just go in through the top of the containment vessels with water only in the bottom of the containment vessel. This approach is called an "airborne method." The big danger here is the intense radiation that exists in the containment vessels is lethal. As mentioned above, even specially-designed robots cannot withstand the radiation for more than a few hours. The third approach would be to drill a hole in the bottom of the containment vessel to take out the corium.This is also an "airborne method." There is still the problem with the radiation in this process.
At this point, there are too many unknowns and dangerous knowns to really evaluate which of these methods offers the best chance of getting the corium out.
