Last updated Mar 17, 2011 10:35 am local time (9:35 pm EST)
Current situation as of morning of March 17. Tokyo radiation level remains within normal range.
JSDF helicopters attempted four successive water-dumps over reactors #3 and #4. The condition of those reactors is most troubling, as the spent fuel rod cooling ponds have evaporated and without refilling them, there is a high chance the rods will overheat, melt, and release radioactive material. The results of the water drops have yet to be announced.
Current situation as of evening of March 16. Tokyo radiation level remains within normal range.
Fukushima Reactor #1: in spite of explosion, containment housing believed unbreached, sea-water injection ongoing to lower temp of fuel rods, situation stabilizing.
Fukushima Reactor #2: leaked some amount of material, causing radiation spike this morning around 10am. Cause unknown. Rad level has been dropping steadily since.
Fukushima Reactor #3: vented what appears to have been steam earlier today. Suspicions that containment housing may be breached. Conditions preclude direct observation. There is a very high chance the pool containing spent fuel rods has completely evaporated and rods are melting, irradiating the area. A planned water-drop by helicopter had to be called off due to radiation counts exceeding safety limits for the pilots.
Fukushima Reactor #4: fire broke out this morning, since extinguished. Theory is that evaporation of spent fuel rod storage pool caused hydrogen build-up that caught fire. Fire occurred in area where seawater pump was placed, and radiation has slowed but not stopped efforts to add water to pool.
It pains me to say this, but in the wake of the situation at the Fukushima nuclear power facility, there has been a lot of innuendo about the potential danger to the Tokyo metropolitan area. The following represents all of the current information released to date about the situation in Japanese press conferences and from other sources mentioned below.
I'm not a nuclear authority. I welcome all info, even contradictory info, so we can piece together a picture of what is happening without needing to sensationalize things. If anyone has data that contradicts any of the below, or sheds light on it, PLEASE comment so that I can amend it.
Point 1: A sievert (whether milli- or micro-) is a measure of exposure, expressed per unit of time (in the case of info coming from Japan, by the hour). The effect is cumulative. This means that at a 2 mSv/h (millisievert / hour) level, you would need to be exposed to the source for 100 hours to be irradiated to the point that is considered clinical radiation poisoning (200 millisieverts, according to this chart translated by @gakuranman). But the current background radiation in Tokyo is somewhere around 0.3 microsieverts. That's something like three ten thousandths of a millisievert.
Point 2: As reported by NHK and corroborated by datafeed from the Hino monitoring station, metropolitan Tokyo radiation levels rose from a normal reading of around 20-30 CPM (0.2 - 0.3 microsieverts per hour) up to to 89 CPM (0.89 microsieverts per hour) for an hour, then dropped to close to normal background levels again. They remain normal as of this writing.
Point 3: Tepco released a graph showing a spike of 12,000 microsieverts (12 millisieverts per hour) at front gate of the Fukushima nuclear power plant. It has since dropped to below 2,000 microsieverts (2 millisieverts per hour) This seems to correspond to the fire at reactor #4, which has since been extinguished.
Even still, again note @gakuranman's chart showing comparative radiation levels. Even one hour at 12 millisieverts (the max reading taken at the gate of the plant) is equivalent to less than 2 CT Scans. These readings were taken at the front gate of the nuclear plant in Fukushima. They decline rapidly with distance from the reactors. Again, metro Tokyo readings remain normal.
Point 4: The situation at the #1 and #3 reactors is under control, with seawater being pumped in now to keep the cores cool. There was no core breach. The situation at #2 is that water is being pumped in but the level is not rising. This needs to be watched closely. The fire in Reactor #4 was extinguished earlier today. A measure of radiation and some radioactive isotopes may have been released in this fire, but the release of radiation and material seems to have stopped with the extinguishing of the fire. The big issue at #4 now is a potentially dropping water level. But as stated above, radiation levels are corroborated by the Hino geiger counter datafeed.
Point 5: The biggest issue as of the the afternoon of March 16 is that the situation at the reactors has not improved, though it is not clear if it is worse. A fire broke out at #4 early this morning (since extinguished), while elevated temperatures were detected at #2 and smoke or steam issuing from #3. Elevated radiation levels were detected at the front gate of the plant but have since dropped. There was a brief rise in background radiation levels detected in Tokyo early this morning (apparently corresponding to the fire at #4) but it has returned to normal as of this writing. As of right now, the danger appears confined to the plant grounds itself. The repair/firefighting crews remain on-site and working. (Many foreign media are incorrectly reporting the plant was "abandoned." This is not true.)
Again, PLEASE comment if you have info that contradicts any of the above.
This is a fluid situation, and an environmental disaster around the reactor site, but Tokyo residents and those outside of the evacuation zone do not seem to be in immediate danger as of this writing. The people in the most danger are the workers inside the plant. By exposing themselves to higher than normal radiation levels, they are sacrificing their health to keep the rest of us safe, and are the unsung heroes of this potential crisis.
Thanks very much to Andy Szymanski for summarizing this data and co-authoring this article.