Japan's nuclear crisis worsened on Wednesday, with workers ordered to withdraw briefly from the stricken power plant after radiation levels spiked, just hours after smoke was seen rising from the quake-crippled nuclear facility.
Q: What does the smoke mean?
A: The smoke is most likely to be steam, a natural byproduct of pouring water into the reactors to cool them down and keep the fuel rods covered. Authorities are also trying to maintain water levels at a spent-fuel storage pool at the plant's No. 4 reactor, which experts now view as the real threat.
The steam is an issue because it contains radioactive particles, but experts say these particles may not be as serious as, say, an explosion within a reactor core. So far, it appears the radiation is mostly escaping in the form of steam and some experts believe the radioactive particles could merely be dust or from rusted structures within the reactor buildings -- known as "crud" within the nuclear industry. Radiation levels have not risen significantly in Tokyo, about 240 km to the south.
Q: Why is the situation at the spent-fuel pool a worry?
A: Unlike the reactors themselves, which are inside two containers -- a massive steel container as well as one made of concrete -- the fuel pools are not, with meters of water on top of the rods providing safety only under normal circumstances. Another concern with reactor No. 4 is that the water levels in the pool could be falling, perhaps because the water is or has been boiling and is evaporating as steam.
While nobody knows exactly how much damage Tuesday's explosion did to the building, the structure does have holes and some experts have said that proposals to drop boric acid particles on the reactor suggest substantial holes in the roof.
"That's got to be protected. I'd hate to see another explosion there," said Murray Jennex, a professor at San Diego State University in California.
Q: What can be done?
A: Dropping boric acid could be helpful since it is both a fire suppressant and also absorbs radiation, while keeping water levels up in the pool is also essential. Finally, a fresh crew of workers should be sent into the plant since those who are in there are likely exhausted and may be making poor decisions, which could be adding to problems.
Q: What about the spent-fuel pools at the other reactors?
A: There are six reactors at the Daiichi complex. Three of them - no. 4 through No.6 - were offline for maintenance at the time of the quake. There are signs that water temperatures are also rising at No.5 and No.6 spent-fuel pools.
But the spent fuel at these two pools, as well as at the three reactors that were online at the time of the quake, is all older and thus less likely to pose an overheating threat.
Q: What about the other reactors?
A: There have been explosions at reactors No.1, No.2 and No.3, and suspicions that the suppression pool at No. 2 may have been damaged. But experts say that current low radiation levels suggest that the container vessels at these reactors are by and large holding, and that they're actually more optimistic about this situation than they were on Tuesday.
Doubt does remain about possible damage to the suppression pool at No. 2, but these are "overdesigned" to cope with a certain amount of damage.
Q: What do we need to watch out for?
A: Steadily rising radiation levels are the main worry, along with a fall in the water level in the spent fuel pool. Radiation levels at the plant's main gate spiked on Wednesday morning but have since fallen back, which is a good sign.
"If we have the water level doing down and rising radiation, that's a bad sign, because this means more and more of this fuel (is) exposed," said Najm Meshati, professor of civil and environmental engineering, at the University of Southern California at Los Angeles.