First of all, “meltdown” is not a technical term with a precise definition. It’s a slang term coined by opponents of nuclear power.
Here’s what happened:
1. Spent fuel rods had been removed from Reactor Number 4 a few weeks ago and set aside in a cooling pool outside the containment vessel. They produce decay heat, but can’t go critical.
2. Earthquake hits, reactors scram (control rods are inserted to halt the nuclear chain reaction).
3. Electric power is lost; shut down nuclear reactors need an external power source to keep cooling going in order to dissipate decay heat.
4. Generators kick in; however, when tsunami hits (40-foot tsunami trumps 30-foot sea wall protecting nuclear plant), some generators aspirate water, damaging them.
5. Working generators have only about eight hours of fuel on hand; due to earthquake and tsunami devastation, they can’t be promptly resupplied. Major planning failure here to account for both earthquake and tsunami at the same time.
6. Portable generators are trucked in, but are incompatible with electrical connections at the reactors—another planning failure.
7. Decay heat rises, hydrogen gas builds up in the reactor containment vessels, is vented to containment buildings.
8. Gas ignites, containment building explosions—but containment vessels aren’t breached.
9. Sea water is pumped into the reactors to cool them, but radiation of course exits the containment vessels as water/steam vents.
10. Local radiation levels rise, but not to the catastrophic levels implied by the media. In order to keep skilled personnel from exceeding their radiation badge limits, skilled workers are rotated out.
11. Media reports of radiation detected at sea by U.S. Navy fail to mention that we can detect very small amounts of radiation. No health hazard.
12. Nuclear plant workers are decontaminated by the simple expedient of washing them down with soap and water. Still have to watch cumulative badge limits, of course.
13. Radiation fluctuates up and down as water leaks out of the containment vessels, then evaporates. Even when it’s “up,” it isn’t the huge deal portrayed in the media.
14. As decay heat temperature levels are dropping, there is no danger of the nuclear fuel within the reactors heating up to the point where it melts the containment vessels and drops down into the soil/water table. This is the general sense of “meltdown.”
15. There is no potential for a Chernobyl-type explosion of any of the containment vessels flinging irradiated building materials and soil over a large area. These reactors use a very different design, as do all reactors in the USA and Europe.
16. Radiation levels will go up and down as more water leaks out and evaporates, but there’s no chain reaction taking place anywhere, no containment vessel explosions anticipated.
17. So, the whole thing is an unpleasant mess, but not some apocalyptic Chernobyl scenario—and Chernobyl was grossly exaggerated by the media, none of the doomsday predictions came true.
18. The biggest threat that this incident poses is the loss of electricity at a time when Japan is cold, when electricity is needed to pump and purify water, to pump out waste, et. al. In other words, the loss of electrical generating capacity and the attendant health hazards is the true danger, not the inconsequential radiation leakage.
This article goes into detail on the dosage stuff, no need for me to repeat it:
I wrote: “1. Spent fuel rods had been removed from Reactor Number 4 a few weeks ago and set aside in a cooling pool outside the containment vessel. They produce decay heat, but can’t go critical.”
It turns out that reactors 4, 5, and 6 were all shut down for maintenance, and fuel rods removed from all of them.
I wrote: “8. Gas ignites, containment building explosions—but containment vessels aren’t breached.”
And it’s likely that the gas that ignited actually came from spent rods in the cooling pools, as the water in the pools drained out due to cracks.
19. TEPCO are now trying to get electrical grid connectivity run back into the plant; if the cooling systems haven’t been damaged too much, they hope to restart some of the regular cooling systems. We’ll see.