This discussion is in the science genre rather than current events as there was often just too much delay between events at the Fukushima nuclear plant #1 and cogent reporting. This topic is more of an overview of events, and a list of suggested vetted, reliable sources of information.
During the crisis' first week, I heard misinformation and inappropriate speculation every day on every news channel, and in at least half of all stories and analyses. Even the best experts that media could find didn't have adequate information or direct access to information about events at the reactors. The Japanese government wasn't much help, either. As "collective" as the Japanese people are and concerned about each other, they seem to be sometimes intentionally naive about events that they trust other organizations or interests to manage. (Well known examples are the slaughtering of whales up until recently, and ongoing slaughtering of dolphins.)
The plant operator TEPCO has a history of being unreliable as a source of information, even though they are the primary witnesses and managers to the crisis. TEPCO did not adequately assess and report the current crisis, partly because of incompetence and/or stoic silence, but mostly because of inadequate resources since they lost all power and backup power.
Hydrogen explosions, releases of radioactive material and fires garnered headlines. Yet ironically, these are actually minor events, in the context of both the recent tsunami disaster and the potential lethality of a future, unstoppable meltdown.
There are two nuclear power plants in Fukushima: Dai-Ichi (#1) and Dai-Ni (#2). All four reactors at plant #2 were shutdown successfully, and will not be mentioned here again.
Before the earthquake, plant #1 reactors 1 through 3 were in operation while reactors 4 through 6 were undergoing routine maintenance. Loss of backup power from the earthquake and tsunami led to serious damage at reactor cores and/or spent fuel in reactor buildings 1 through 4.
All fuel rods at Fukushima use uranium, but reactor 3 is fueled by a combination of uranium and plutonium called MOX, incurring higher risks. (Since a portion of uranium in fuel rods actually becomes plutonium, all fuel rods accrue plutonium, but to a much lesser degree than in MOX.) The half-life of plutonium is 24k years.
Reactors 1 and 2 have significant core damage, meaning "partial meltdown" due to loss of coolant (water).
In addition to risk from core damage is the risk of damage to separately cooled, spent fuel rods held outside of the reactor vessel in a storage pool (in the same reactor building). Even though this fuel is no longer radioactive enough to be used for generating power, it is still radioactive enough to require constant submersion in water in order to stay cool. All spent fuel also contains plutonium.
At least one of the fires (and perhaps one or more of the hydrogen explosions) was caused by used fuel losing its cover of water and subsequently overheating. The used fuel pools were what the helicopters tried to keep flooded with water, not the fuel in any of the reactor vessels. Only the reactor buildings with exploded rooftops had spent fuel pools accessible to the helicopters.
Excellent, detailed wikipedia links:
When adding to this discussion, please limit any speculation, or at least indicate when you or your source is speculating.
I'll occasionally fine tune the text above, and hope no one minds if I don't display corrections as errata. I may even delete posts here or there if they seem superfluous, so feel free to add or delete your own. (Sorry for late links. I have less time and luck with this than I expected.)
MIT Nuclear Science and Engineering set up a website that may be the best, overall link for vetted news and scientific analysis. Although the news topics there are in chronological order, you can scroll freely to find articles that interest you the most. It provides wide range of depth of information, from basic to advanced.
Here's a blog run by Union of Concerned Scientists. Of course it's a bit more left-leaning than the pro-nuclear industry, but seems to have extensive and current information.