I was reading up on how some nuclear reactors work etc, but does anyone know/understand why the thing is still hot after 5 days? I mean wouldn't it auto shut off so that the fuel stops reacting or is the problem that they couldn't shut it off? If its not being fueled to burn, I dont see how it could still be burning for this long even after pouring water and whatever else they have done.

You can't just stop the thing from reacting, it's not like a machine that you can unplug or a fire you can just douse. The fission chain reaction will continue, so heat will continue to be produced. You can dump water in, but the chain reaction will still continue, plus you'll have radioactive water. I'm not 100% clear on the Japan situation, but I believe that the #2 reactor's fuel rods are completely exposed, meaning that the carbon rods aren't there to soak up the radiation. They can't necessarily just stick them back in; they tried that at Chernobyl, if the reactor's hot enough the rods will just catch fire.

I didn't really know anything either until I did some wikipedia'ing on the weekend (must've slept through nuclear physics in school -- I had The Simpsons' vision of a mushroom cloud in my mind)

From what I've read, the chain-reactions that generate heat during operation were shut down almost immediately by inserting neutron-absorbing control rods into the reactors.

The heat that's being generated now seems be from the regular decay of unstable elements, and the cooling down of the incredibly hot was-just-undergoing-nuclear-reactions fuel rots. This is normal, but it's usually cooled fast enough to not be a problem.

The danger is that it could melt the container, making a clean-up operation impossible, which would mean they'd have to permanently seal off the containment building like 3 mile island in the US.

Even if the reactor does melt(down), the public should be safe as long as the containment building remains intact.

If normal cooling had occurred, then the reactors could've been cleaned up and repaired, and put back into operation. Now that they've pumped them full of impure sea-water, they're going to have to write-off the reactors completely now. The only question is whether the broken reactors can be removed from the site (and new ones installed), or if the site will become a tomb.

the public should be safe as long as the containment building remains intact.

You mean this containment building?

[quote name='Hodgman' timestamp='1300159713' post='4785897']
the public should be safe as long as the containment building remains intact.

You mean this containment building?[/quote]Yes, that's the thin flimsy outer wall of the building that was blown apart by the hydrogen explosion. The inner concrete bunker is still intact, and the actual reactor itself (inside the concrete bunker) is also still intact.

At Chernobyl, the actual reactor itself was breached, which started an air/graphite fire. Also, Chernobyl did not house the reactors inside concrete containment buildings, which meant the smoke went straight into the atmosphere. Even if this happens in Japan, the concrete bunker should contain the pollutants.

Ohh, I was under the impression that the cube was the concrete bunker, and that the core vessel was the only thing left holding in the radiation. That's more optimistic if there's another layer still inside the building.

Yea I just read how the neutron reaction is controlled and timed on/off when running, so I wasnt sure if or why they couldnt just stop it as it seems they do in timely increments under normal operation. I guess it was just too late and the reaction couldn't stop then. Why cant they continually fly over and drop ocean water on top of it from helicopters though? I guess I just dont see why they only tried putting water into the thing for what seemed like an hour until their water pumps ran out of fuel. Japans a tech country, they should build a giant one of these: http://www.newegg.co...N82E16835103055 and drop it in w/ some thermal paste.

At Chernobyl, they were operating the reactor outside of it's designed safety margins (they had created extremely unstable conditions, with multiple warning indicators being ignored), and when they finally decided to shut it off by fully inserting all of the control rods, the reactor started to break apart, and the rods got stuck after only having been 1/3rd inserted.
In Japan the shut-down procedures went to plan - the reactions were stopped. The problem was the failure of the 4 independent (and internally multiply redundant) backup cooling systems per reactor, which meant the decay heat had nowhere to go...

They can't drop water on it because it's inside a building to protect the environment from radiation
I did read though that they're considering setting up 'spraying' equipment inside the building to shower the outside of the reactor in water. I'm guessing this would make the building pretty uninhabitable though.

Pumping in sea-water seemed to be a last-resort for the operator, because it means they'd have to write-off the reactors afterwards. If they'd managed to cool it using their normal methods, they'd be able to continue operation after everything's repaired. Delaying the use of sea-water could be seen by a cynic to be profit motivated due to this... but the use of sea-water has also been described as "completely not by the book" (I think it was an IAEA rep who said that), so it does really seem to be a last resort after all other cooling options failed.

Yea I just read how the neutron reaction is controlled and timed on/off when running, so I wasnt sure if or why they couldnt just stop it as it seems they do in timely increments under normal operation. I guess it was just too late and the reaction couldn't stop then. Why cant they continually fly over and drop ocean water on top of it from helicopters though? I guess I just dont see why they only tried putting water into the thing for what seemed like an hour until their water pumps ran out of fuel. Japans a tech country, they should build a giant one of these: http://www.newegg.co...N82E16835103055 and drop it in w/ some thermal paste.

Well, you can't just "turn it off" anyway. Like it was mentioned above, the reaction will continue. You can only reduce the speed that it takes place at because it is a self-sustaining ramp up. The more neutrons are floating around, the more fissions take place. The uranium will always release neutrons as it decays at a constant rate. But, since we pack all the fuel in together, we are increasing the density of the neutron flux, resulting in more flux up till the point that we moderate the reaction to. Now, the other thing to keep in mind is that neutrons collide with everything. Control rods adsorb more neutrons than they reflect. Reflectors reflect more than they adsorb, and materials like water slow neutrons down to "thermal" speeds, or "moderate" them. Some isotopes are easier to hit with slower neutrons, others are harder to hit (based on the wave functions of the particles involved). So, in a regular water cooled reactor, the water is slowing the reaction by taking neutrons out of their thermal range and converting that energy into more heat in the water. Let that water boil away, and boom, you have a Chernobyl incident as the reaction almost instantly speeds up to meltdown temperatures. Thus, they are trying to keep as much water on there as possible to help everything stay cool. But it is always going to try to get hot until the fuel is gone (dozens or more years), or the reactor is taken apart.

We use the fact that these reactions constantly produce heat to power long term space satellite too. From Voyager to Cassini we've powered these ships plutonium base Radio Thermal Generators that will stay at temp for dozens of years.

Other random fact of the day... Unlike a "fast fission reactor", some fuels work in the "thermal reactor" range, where the water slowed neutrons actually have a better chance of hitting the isotopes. A non-water cooled reactor in the thermal range could actually melt down if placed in water, as the water would result in additional neutrons being at the right speed to react.

A simplified explanation about what is going on can be found here. The site has an obvious pro-nuclear bias, but given the catastrophising in the general media it may provide a bit of balance.