What exactly is the nuclear situation in Japan right now?

There’s already dispute about whether anyone has suffered radioactive exposure. It is reported that the government has said zero, but that is contradicted by reports of nuclear worker exposure (maybe 4 individuals). There’s also reports of attempts to cool reactor(s) with seawater, which seems to be regarded as a last resort measure due to the corrosive effects of seawater.

I would guess that their evacuation radii are probably the most reliable indicators of risk/damage, but that they are minimizing to some degree other internal reports and descriptors of the risk at hand.

I’m not sure that anyone knows. I’ve found some commentary, and I’ve seen at least one reference that states that the explosion seen was the collapse of a wall, and not anything to do with the containment unit.

But what is going on inside the containment unit? In one report it was mentioned that one of the pipes inside the reactor was damaged by steam and that maybe one of the rods is beginning to melt.

I think the authorities are being fairly honest but attempting to cool the reactors with seawater is untried and no one knows how successful it will be. I have heard that emergency cooling has failed at a second reactor at Fukushima. If they can’t get the reactors cooled there is still the potential for disaster and the aftershocks that are still being experienced don’t make things any easier. I’m hoping it works but it is not good for the nuclear power industry or the people who live near the power station.

Is it possible to determine the temperature development inside the reactor?

Not directly, and if it’s all the same with you, I would rather that we not open up the containment vessel to take a look either. We can take educated guesses based on what is going on outside though. Think of it as the difference between listening to a heartbeat with a stethoscope and ripping a person’s chest open (with attendant blood loss) to watch the heart beat with your own eyes.

Most of what I’ve seen says it was a hydrogen explosion outside of the containment vessel. They are venting hydrogen, but the systems that are supposed to ignite it safely before it gets to the point where it could make a Hindenburg-style explosion rely on electricity, so there was a lot of hydrogen in the building when it went up.

I take the evacuation radii with a grain of salt. Having grown up near a nuclear plant, I know that they tend to be over-cautious to the point of paranoia. Probably not a bad idea considering the stakes. Given how tough the containment vessels are, I am not terribly worried. Concerned? Definitely! But not worried.

@flutherother Considering the magnitude of the quake and how well the plants held up despite that, I am not so sure. I wonder how Hoover Dam would fare against an 8.9 quake and a couple of >6.0 aftershocks.

I just learned when watching tv that the Fukushima rods have been used a long time, so levels of iodine 129 and 131, strontium 90 and so forth are very high compared to Chernobyl. Winds are expected to change too.