Yes, there has been a partial meltdown a long time ago, there has to be because there's lots of hydrogen in the pressure releases, which, in turn - is causing explosions. (certain amount of oxygen + hydrogen + a spark (high temperature) = boom). If the core reaches very high temperatures it causes the cooling water to disassociate and split into hydrogen and oxygen, the higher the core temperature the more disassociation happens. To avoid a large buildup of both steam pressure and these gases, they have to open up the pressure valve once in a while.
Though, due to the large amount of hydrogen, every time they do this they risk an explosion. Those are the booms you're seeing. The core itself is not compromised as pressure release happens in another area.
In fact I believe most of the cores are stable, as they reported initial emergency shutdown procedures were largely successful - getting rid of the excess heat is a problem. The emergency shutdown happens via SCRAM (as kmar pointed out, thanks - I read that this is actually a safety precaution in almost all reactors these days) and injection of boron with the water circulation. SCRAM are moderator rods held outside of a reactor core with either electromagnets or electric motors. Once the power shuts down at an unexpected moment, these rods will instantly fall into the reactor because the electricity keeping them up vanishes. They cause neutrons and delayed neutrons (which cause nuclear reactions if they bump into the nuclear fuel, which in turn will create more neutrons because of that -> chain reaction) to decrease in speed, and thus have a much higher chance of reacting with other (non dangerous) substances than the fuel used in a nuclear reactor.
Once the emergency power supply is on line Boron, which is in a separate tank connected to the cooling system, is injected with the water circulation used to cool the core. Boron absorbs and attracts any remaining neutrons, there were problems in the first reactors that got in trouble because the emergency supply was washed away by the tsunami.
So the core is still very hot as neither SCRAM nor Boron act as a coolant, and perhaps a couple of neutrons are left reacting with the nuclear fuel because Boron injection stopped due to failure of the emergency power system. This is a problem because if Boron is not injected constantly or they ran out, the remaining neutrons may restart the nuclear chain reaction, and cause the core to once again heat up more.
So, that's basically what happened at the first nuclear plant that failed. They restarted the circulation process and by that time the core must have heat up enough to turn quite a bit of water into hydrogen. Furthermore, this plant was already 40 years old and probably didn't have the most up to date safety mechanics installed and this reactor was intended to go out of service later this month, the earthquake really struck at an unfortunate moment.
The other, newer plants that are in danger though - the story is a bit different there. Usually, there are two different types of nuclear fuel, UOX and MOX. The older plant used UOX fuel which is uranium dioxide. The newer plants used MOX, which is a mixture of uranium dioxide and plutonium. MOX fuel is more reactive than UOX fuel, and as such it's a lot harder to keep the nuclear chain reaction under control. These plants must have experienced the same problem as the first one, or even a very minor power outage, which caused the whole hydrogen production process by the core to get out of hand - and there's your explosive formula.
Hydrogen production can start up before a (partial) meltdown happens, yet due to the sheer amount and some rather large explosions, I think it's probably OK to assume that partial meltdowns have actually occurred.
Now, these explosions and sudden pressure releases also carry some radioactivity with them. But I wouldn't be all too worried about that either; nuclear reactors generally use filters to filter out the most of the radioactive isotopes. I don't think I'll have to explain the rest as you all already know that the released radioactivity isn't actually anywhere near lethal or even damaging.
Though, due to the large amount of hydrogen, every time they do this they risk an explosion. Those are the booms you're seeing. The core itself is not compromised as pressure release happens in another area.
In fact I believe most of the cores are stable, as they reported initial emergency shutdown procedures were largely successful - getting rid of the excess heat is a problem. The emergency shutdown happens via SCRAM (as kmar pointed out, thanks - I read that this is actually a safety precaution in almost all reactors these days) and injection of boron with the water circulation. SCRAM are moderator rods held outside of a reactor core with either electromagnets or electric motors. Once the power shuts down at an unexpected moment, these rods will instantly fall into the reactor because the electricity keeping them up vanishes. They cause neutrons and delayed neutrons (which cause nuclear reactions if they bump into the nuclear fuel, which in turn will create more neutrons because of that -> chain reaction) to decrease in speed, and thus have a much higher chance of reacting with other (non dangerous) substances than the fuel used in a nuclear reactor.
Once the emergency power supply is on line Boron, which is in a separate tank connected to the cooling system, is injected with the water circulation used to cool the core. Boron absorbs and attracts any remaining neutrons, there were problems in the first reactors that got in trouble because the emergency supply was washed away by the tsunami.
So the core is still very hot as neither SCRAM nor Boron act as a coolant, and perhaps a couple of neutrons are left reacting with the nuclear fuel because Boron injection stopped due to failure of the emergency power system. This is a problem because if Boron is not injected constantly or they ran out, the remaining neutrons may restart the nuclear chain reaction, and cause the core to once again heat up more.
So, that's basically what happened at the first nuclear plant that failed. They restarted the circulation process and by that time the core must have heat up enough to turn quite a bit of water into hydrogen. Furthermore, this plant was already 40 years old and probably didn't have the most up to date safety mechanics installed and this reactor was intended to go out of service later this month, the earthquake really struck at an unfortunate moment.
The other, newer plants that are in danger though - the story is a bit different there. Usually, there are two different types of nuclear fuel, UOX and MOX. The older plant used UOX fuel which is uranium dioxide. The newer plants used MOX, which is a mixture of uranium dioxide and plutonium. MOX fuel is more reactive than UOX fuel, and as such it's a lot harder to keep the nuclear chain reaction under control. These plants must have experienced the same problem as the first one, or even a very minor power outage, which caused the whole hydrogen production process by the core to get out of hand - and there's your explosive formula.
Hydrogen production can start up before a (partial) meltdown happens, yet due to the sheer amount and some rather large explosions, I think it's probably OK to assume that partial meltdowns have actually occurred.
Now, these explosions and sudden pressure releases also carry some radioactivity with them. But I wouldn't be all too worried about that either; nuclear reactors generally use filters to filter out the most of the radioactive isotopes. I don't think I'll have to explain the rest as you all already know that the released radioactivity isn't actually anywhere near lethal or even damaging.
Last edited by Shocking (2011-03-14 11:25:20)
inane little opines
