Yesterday, TEPCO, the company responsible for running the Fukushima nuclear reactors, released a provisional analysis of the events that occurred in Unit 1, one of the reactors that was active when the tsunami struck. In five stark pages, the report lays out the impact of the tsunami on the facility: water levels that plunged below the bottom of the fuel, a complete meltdown of the nuclear fuel, and extensive damage to the reactor vessel.

The new analysis was enabled by the recent installation of air purifiers that let personnel reenter the reactor control room for the first time. Once inside, they were able to recalibrate some of the instruments that have been monitoring the reactor core; the revised numbers have enabled TEPCO to better understand what happened in the wake of the tsunami.

Things moved fairly quickly: by four hours after the tsunami hit, the levels of cooling water had dropped enough that the top of the fuel stack was exposed to the air. Shortly after that happened, the temperature in the core reached nearly 3,000°C, and the cooling water boiled off the bottom of the fuel stack. Melting of fuel rods started at 4.8 hours after the earthquake hit, and a partial meltdown had already occurred by 5.1 hours. According to TEPCO, any residual integrity in the fuel rods was gone by 15 hours after the quake, and the reactor core was emptied of fuel by 16 hours.

If there's a bright side here, it's that, by melting to the floor of the reactor, most of the fuel was resubmerged in cooling water. Temperatures there are now in the area of 100-120°C. The temperatures still fluctuate based on the rate of cooling water injection, suggesting the majority of the fuel is still inside the reactor vessel. The temperature and pressure readings indicate that the vessel is generally intact, as well. But the continued loss of cooling water indicates that it almost certainly has been leaking water that is heavily contaminated with radioactive isotopes (though not necessarily reactor fuel).

As Nature's Geoff Brumfiel puts it, "Nobody really knows where all the water is going—but it can't be anywhere good." The best hope is that it largely leaked into the basement areas of the reactor building but we currently don't know for certain where the water escaped to or how far it has spread. Complicating matters further are reports that TEPCO thinks that the two other reactors that were active at the time of the quake suffered similar damage. Confirmation will have to wait for access to their control rooms.

All of this will make the cleanup efforts extraordinarily complex, as the fuel can no longer be lifted out using the structure it was incorporated into, since all that supporting material has melted away. Workers will also have to contend with leaks of highly radioactive water during the process; that water will have to be removed or contained before it leaches into the surrounding soil. There are positives here—temperatures are relatively low now, and the fuel appears to have remained in the reactor vessel—but Fukushima is likely to be a long-term worry.