– Meanwhile, the Disaster at Fukushima Continues (The Atlantic):
As Nature News’ Geoff Brumfiel reports, workers went into the unit recently “to recalibrate some of the sensors on the reactor.” Much to their surprise and dismay, they found that the core experienced a total meltdown. The zirconium alloy tubes that hold the uranium fuel pellets during normal operation all warped and the uranium is now lying at the bottom of the pressure vessel, or possibly even outside of it in the basement below or outside the concrete containment building. With all the fuel piled up at the bottom, there is some danger that the nuclear reaction could have restarted. As of now, engineers on the scene aren’t sure what happened.
– Understanding the complete meltdown at Fukushima unit 1 (Nature):
Reactor restart?
Some have theorized that with all the fuel at the bottom of the vessel, unit 1 may have actually restarted its nuclear reactions. If that had happened, the fuel would be pumping out some portion of its normal 1380 megawatts of thermal power—probably enough to melt through the thick steal reactor pressure vessel. It would have dropped onto a concrete slab below (the basemat), where it would have hopefully been spread out, effectively diffusing the chain reaction.
There’s some reason to think that this “China syndrome”, as it is informally known, didn’t happen. Nuclear engineers I’ve spoken to say that reactors like unit 1 are finicky beasts. Their fuel needs to be carefully configured to work, and they won’t restart if the stuff is just a gloop on the bottom of the vessel. In addition, workers injected boric acid into the reactor just before the restart. Boron is a neutron absorber and would spoil any nuclear reactions. Moreover, temperature sensors at the bottom of the reactor vessel are continuing to function, suggesting it wasn’t completely destroyed.
That doesn’t mean that portions of the fuel weren’t briefly producing power during the accident. Nor does it mean that the reactor’s fuel has remained wholly inside the pressure vessel. In fact, a note from the Japan Atomic Industrial Forum (JAIF) quotes Banri Kaieda, the nation’s Economy, Trade and Industry Minister, as saying that it is “a fact” that there were holes created by the meltdown. That would likely mean at least some of the uranium fuel is now lying on the basemat below, or perhaps even outside the concrete containment.