They're purposely venting it. Sealing the thing shut would be a recipe for disaster.

Yes, but that fuel is now in the spent fuel pool, generating heat, where it needs to be cooled. The pools need quite a bit of water in them, mainly to keep air away from the depleted fuel and to buffer the radiation.

We were granted access again today to the area of the CBD where our apartment is. The amount of destruction is very sobering when you see it for yourself rather than on camera. I can barely imagine what the areas wiped out by the tsunami are like.

It's looking almost certain now that they're going to demolish our building. They had thought it could be saved, but the estimate to fix it so far is at 10 million dollars, and the engineers haven't been to most of the upstairs areas yet. If the cost rises over 14 million, which it looks certain to do now, they're going to demolish.

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Yup. We moved from the apartment in the base of the left turret to the apartment at the very end of the left side of that photo.

What you can't tell from that image is that the building is L shaped, and there's a courtyard on the other side. The new apartment had an entrance on to the courtyard, with it's own smaller private court-yard, unlike the old one, which just had an interior entrance. The new apartment was a lot larger as well, and the lounge area was north facing, so got the sun. The old apartment was south-east facing, and only got sun for about an hour in the morning.

The apartment we're going to move to in Melbourne will probably be about half the size of the original apartment we had in that building, and cost nearly twice as much.

Do I have this right?

There are spent fuel rods outside the reactor containment vessel. These rods lie in pools of temperature regulated water. The temperature in these pools has risen, and water has evaporated to expose some of these rods. Due to increasing temperatures in the rods, some may already be damaged.

Light water doubles as a coolant and moderator in BWR (and PWR) reactors. The word "moderator" means that is it used to slow down fission. In technical terms it means it adds negative reactivity (though much more negative reactivity is provided by boric acid in PWR reactors and even more by control rods). Without a moderator fission would go out of control. In normal operation the moderator is used to keep the fuel in a state of equilibrium called "criticallity". If there's too much moderator (i.e. control rods are inserted) the reaction becomes subctirical and eventually dies out and if you remove the moderator, the reaction becomes overcritical and the rate of fission increases. As I said the main source of negative reactivity is not the water itself, but it does provide some. The explanation is this: when a water molecule is hit by a neutron, it breaks up into hydrogen and oxygen, the oxygen receives this extra neutron and becomes a radioactive isotope of nitrogen with half-age of about 7 seconds. The nitrogen then decays (beta decay) into oxygen which then recombines with the hydrogen to make water again. This is actually a very lucky coincidence that makes it possible to use ordinary water as a moderator and coolant.

By the way, ordinary light water is a relatively good absorber of neutrons which means that in order to maintain criticallity, the fuel must contain a high enough concentration of U235 (uranium with higher than natural concentration of U235 is called enriched uranium). Some reactor designs (e.g. CANDU) actually use natural uranium (about 0.7% of U235) or only slightly enriched uranium (0.9-1.0% of U235), but in order to be able to do that, the moderator has to be heavy water. Heavy water already has an extra neutron and so can't absorb any more.

And all this talk is irrelevant when it comes to the current events at Fukushima because the reactors were shut down the moment the earthquake struck and there is now absolutely no fission whatsoever taking place.

The fuel that has been spent can't be used in the reactor anymore (because it has been spent), so it has to be removed to make way for fresh fuel. This is typically done once every 12-18 months and the whole process is called refueling. The spent fuel still generates significant amounts of heat, so it needs to be artificially cooled. The spent fuel is put into a deep pool of water (3m of water above and below), the heated water is pumped through a heat exchanger where it is cooled and then pumped back into the pool. Typically the temperature in the SFP is around 30°C and is pumped at around 300m3/h. In Fukushima they lost electrical power so the pumps stopped pumping, the temperature of the water in the SFP went up to 100°C and eventually boiled away. The spent fuel was left in the dry and consequently melted (most probably).

Maybe I didn't express myself properly. Yes, all reactors use control rods of some kind. They're the principal means of providing negative reactivity for the reactor. The water is only used to provide some more negative reactivity (especially with some additives). I don't know what is used in BWRs but in PWRs we use boric acid (boron is a very good absorber of neutrons). This is used for very fine adjustments of reactivity inside the reactor during normal operation. Maintaining criticallity is actualy a very fine balancing act. The reactor tends to want to shut down as the fuel "burns" and its reactivity goes down, so operators need to constantly adjust the control rods and the concentration of boric acid to put in exactly the right amount of negative reactivity. In case of emergency all control rods are quickly inserted and the reactor shuts down within seconds.

But again, the problem at Fukushima isn't in shutting down the reaction. The problem is in cooling down the reactor. In normal circumstances it takes about 5 days to cool down the reactor with very intensive cooling, but if that fails as it has at Fukushima the reactor still generates so much heat that it would take several years to cool down on its own (as it did at TMI). Again, there is no fission or uranium going on at Fukushima anymore. This was stopped the minute the quake struck.

The uranium in nuclear power plant fuel is enriched to 3-5% of U235 (refined and dense aren't the proper terms, the right word is "enriched"). This is easily enough for the reaction to go out of control. At TMI (in 1979) the power output of the reactor went up about 1000x nominal power before it was shut down. It couldn't have exploded though, because the automatic security system dropped the control rods before that could happen.

A nuclear bomb is actually nothing but an air tight container inside which fission is allowed to quickly go out of control, the pressure inside the container very quickly increases to an insane level and the container explodes to spread all of its contents over a large area.

Ok, well, I'm not an expert on nuclear weapons...

Yup, that's right. Even if the reaction inside the reactor got out of control the fuel would just melt and eventually become subcritical again, though releasing large amounts of radiation in the process.

To create a nuclear explosion the criticality must be achieved really fast. For uranium based weapons this is done via a gun assembly as you've mentioned. Basically you have two pieces of highly enriched uranium in subcritical amounts and quickly smash them together to form a critical mass (this was the case of the Little Boy bomb).

Other possible way is using sphere of plutonium and applying enough pressure to change its density so it becomes critical via a shaped charge (principle used in the Fat Man bomb). But getting close two pieces of enriched uranium to achieve criticality by hand would end up with you dead by radiation poisoning and two molten pieces of uranium together.^[1]

Releasing large amounts of radiation could be temporarily bad. But releasing some amounts of radioactive stuff is much worse. The radiating chunk itself is of course dangerous. If you get exposed to the radiation, it will do you bad. It would be ok, if the chunk just could be isolated. But problems arise, when radioactive stuff vaporizes, explodes or whatever to get all over the environment and the atmosphere. And when this stuff gets into your body, you are really screwed. It stays there and continuously radiates your fresh tissues, eventually causing malicious mutations to your DNA.

Johan: yes, that's why aforementioned Chernobyl was that bad. Hopefully nothing more than slightly contaminated steam won't get out of the Fukushima plant.

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