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Nuclear Meltdown
Mar. 12th, 2011 11:50 pmOr not.
Great primer on the Fukushima Daiichi Unit 1.
If you look at the cutout, and compare it to what exploded in the dramatic images, you can see it was the secondary containment up top that was destroyed, and the rest of the building looks pretty much OK. The explosion was pretty impressive, so we have little clue how much damage, if any, it did to either the primary containment or reactor; the party line is that the top lid blew off because of hydrogen that was being released from the primary containment ignited. Primary containment is the thick concrete or metal barrier surrounding the reactor vessel, and it's supposed to contain leaks from the reactor itself. If that's true, then things are still pretty safe.
The nasty stuff is inside the reactor vessel. The radioactive fuel (uranium or MOX) is contained within metal rods (fuel rods). The idea is that this way water inside the reactor does not come in contact with the nuclear material itself. Inside the reactor vessel are also control rods that are used to control how much of the fuel rods is "bared" to the other fuel rods, which determines (partially) the pace of nuclear fission (a.k.a reaction). If you put a lot of radioactive fuel close together, fission occurs. If you insert dampening material (control rods, boron etc.) or reduce the amount of fuel, fission stops. When the quake happened, by all indications, in all of the reactors the control rods were fully inserted, and fission was halted; it's the fission that produces heat, which then is converted to electricity via turbines.
The problem with these kinds of reactors is that even when there is no fission, you have a lot of mass that is very hot from before, and the nuclear fuel, just by virtue of decaying, produces heat (called decay heat), and so even a subcritical reactor has to be actively cooled. It's that cooling that failed after the tsunami hit.
The reactor vessel is a thick metal container. In addition to the nuclear fuel and control rods, it's also filled with water -- that's how the heat gets to the turbines. This water is what cools it. If the water doesn't circulate, it begins to boil, which increases pressure inside the reactor vessel. This pressure can be vented, in an emergency, to the containment space. This venting supposedly happens through a filter to trap nasty stuff from within the reactor, and should be relatively harmless to the environment. However, if the water boils off and isn't replenished, the reactor can heat to the point where the metal that encases the uranium, as well as other things within the reactor, melt. This means that the radioactive nasty stuff can now escape with the steam or water from the reactor, if the reactor vessel is breached or the filters are overwhelmed. It can also, in the worst case, mean that the radioactive sludge melts through the bottom of the reactor vessel (China-syndrome). In either case, if the concrete/steel primary containment shell is intact, between all and most of the nasty stuff is still kept away from the environment. The Chernobyl reactor didn't have a proper containment shell for one, and the reactor had graphite which ended up burning/exploding and contributed to spreading the toxic mess into the atmosphere. The Japanese reactors do not have any graphite, and supposedly there cannot be a fire, so nothing should be able to exacerbate things in that way.
So, to sum up: the radioactive, truly dangerous material is in a reactor vessel, which is in a primary containment shell, which may be surrounded by a secondary containment building (part of which exploded in Fukushima.)
So, going back to the dramatic pictures in the news: If the top lid blew off because the primary containment failed, things are pretty dire, because really nothing stands between a breach of the reactor vessel and releasing nasty stuff into the environment. If the reactor vessel and primary containment are still intact, things are still a long way from a major catastrophe.
Going by official word, there has been radioactive cesium and iodine found outside the plant, but in non-health-affecting quantities. If accurate, it really isn't a huge disaster, especially if these substances came through venting the reactor / primary containment spaces in a controlled manner. If the quantities are higher, however, it's a good signal that something in the reactor has melted and the material inside the fuel rods has a way of escaping.
The three major radioactive substances that can be released are:
Great primer on the Fukushima Daiichi Unit 1.
If you look at the cutout, and compare it to what exploded in the dramatic images, you can see it was the secondary containment up top that was destroyed, and the rest of the building looks pretty much OK. The explosion was pretty impressive, so we have little clue how much damage, if any, it did to either the primary containment or reactor; the party line is that the top lid blew off because of hydrogen that was being released from the primary containment ignited. Primary containment is the thick concrete or metal barrier surrounding the reactor vessel, and it's supposed to contain leaks from the reactor itself. If that's true, then things are still pretty safe.
The nasty stuff is inside the reactor vessel. The radioactive fuel (uranium or MOX) is contained within metal rods (fuel rods). The idea is that this way water inside the reactor does not come in contact with the nuclear material itself. Inside the reactor vessel are also control rods that are used to control how much of the fuel rods is "bared" to the other fuel rods, which determines (partially) the pace of nuclear fission (a.k.a reaction). If you put a lot of radioactive fuel close together, fission occurs. If you insert dampening material (control rods, boron etc.) or reduce the amount of fuel, fission stops. When the quake happened, by all indications, in all of the reactors the control rods were fully inserted, and fission was halted; it's the fission that produces heat, which then is converted to electricity via turbines.
The problem with these kinds of reactors is that even when there is no fission, you have a lot of mass that is very hot from before, and the nuclear fuel, just by virtue of decaying, produces heat (called decay heat), and so even a subcritical reactor has to be actively cooled. It's that cooling that failed after the tsunami hit.
The reactor vessel is a thick metal container. In addition to the nuclear fuel and control rods, it's also filled with water -- that's how the heat gets to the turbines. This water is what cools it. If the water doesn't circulate, it begins to boil, which increases pressure inside the reactor vessel. This pressure can be vented, in an emergency, to the containment space. This venting supposedly happens through a filter to trap nasty stuff from within the reactor, and should be relatively harmless to the environment. However, if the water boils off and isn't replenished, the reactor can heat to the point where the metal that encases the uranium, as well as other things within the reactor, melt. This means that the radioactive nasty stuff can now escape with the steam or water from the reactor, if the reactor vessel is breached or the filters are overwhelmed. It can also, in the worst case, mean that the radioactive sludge melts through the bottom of the reactor vessel (China-syndrome). In either case, if the concrete/steel primary containment shell is intact, between all and most of the nasty stuff is still kept away from the environment. The Chernobyl reactor didn't have a proper containment shell for one, and the reactor had graphite which ended up burning/exploding and contributed to spreading the toxic mess into the atmosphere. The Japanese reactors do not have any graphite, and supposedly there cannot be a fire, so nothing should be able to exacerbate things in that way.
So, to sum up: the radioactive, truly dangerous material is in a reactor vessel, which is in a primary containment shell, which may be surrounded by a secondary containment building (part of which exploded in Fukushima.)
So, going back to the dramatic pictures in the news: If the top lid blew off because the primary containment failed, things are pretty dire, because really nothing stands between a breach of the reactor vessel and releasing nasty stuff into the environment. If the reactor vessel and primary containment are still intact, things are still a long way from a major catastrophe.
Going by official word, there has been radioactive cesium and iodine found outside the plant, but in non-health-affecting quantities. If accurate, it really isn't a huge disaster, especially if these substances came through venting the reactor / primary containment spaces in a controlled manner. If the quantities are higher, however, it's a good signal that something in the reactor has melted and the material inside the fuel rods has a way of escaping.
The three major radioactive substances that can be released are:
- Nitrogen-16, which has a very short half life and is only of concern to people at the plant;
- Iodine-131, which can be very dangerous if the body picks it up, because iodine is used by the thyroid, and accumulating iodine-131 is a good way to get thyroid cancer. Taking non-radioactive iodine keeps the radioactive variety from being absorbed, and it has a relatively short half-life, so it's not a huge long-term concern.
- Cesium-137, 134, and some other materials. This is the nasty stuff that poisons a region for a long time and wafts around the continent.
Regardless, the idea that not just one but several reactor units lost cooling ability, and cooling hasn't been restored despite a day or two of work means that worst case engineering wasn't nearly worst case enough -- not to mention that the quake was actually a fair distance from the reactors to boot.
