the building housing the reactor containment was destroyed.
evacuation for up to 12 miles.
the Japanese nuclear agency still said a meltdown was possible.
in an AP news article right after the above statement it said:
'A “meltdown” is not a technical term. Rather, it is an informal way of referring to a very serious collapse of a power plant’s systems and its ability to manage temperatures. ’
so now a meltdown has been redefined to be somewhere between an “oops” and a Chernobyl-style meltdown.
the article also said:
‘Yaroslov Shtrombakh, a Russian nuclear expert, said a Chernobyl-style meltdown was unlikely.’
Lets take a very simple look at a typical Japanese, American, British or French nuclear reactor design.
Its a collection of fuel rods, control rods, cooling tubes in gigantic high pressure metal vessel containing water. That is housed inside an even bigger containment “dome”, that should the vessel rupture, the dome will most likely contain the leak.
The Russian design. A giant pile of graphite with fuel rods, control rods, and cooling tubes/water. Inside a high pressure metal vessel. No containment dome.
The bad thing about Chernobyl was there was no dome, so when it blew, lots of bad shit just went skyward. That left a massive pile of super hot graphite and highly radioactive stuff sitting out in the open air. Now don’t let that fancy word graphite mislead you. Graphite is just a fancy word for carbon which is a fancy word for very pure coal.
Who could have ever guessed that a giant pile of supper hot coal could catch fire and be very hard to put out?
You ask a very interesting question. One direct answer is that is exactly what caused the Chernobyl disaster. The Russian engineers were concerned that emergency cooling wouldn’t kick in for 2-3 minutes in the event of a shut-down. So they tried using the reactor generator run-down to generate enough electricity to cover that short period.
In other words they wanted to use the reactors residual energy to keep coolant water running until the diesels kicked in.
Good idea but earlier tests hadn’t worked. So another test was tried. The reactor was scrammed (shut-down) dropped too low, so the rods were lifted…and the reactor ran away…
Meltdown: very serious but only a public risk if the reactor core is exposed to the outside atmosphere. Even then you’d need an explosion and fire to throw radioactive material up into the air.
The problem right now is that the containment building appears to have been breached by the explosion. Even so, if the reactor vessel is still intact - and a meltdown doesn’t destroy it - then there is little risk of radiation release.
Power backup - there was triple redundancy to power emergency cooling so its difficult to know what actually went wrong. Maybe the water intakes were destroyed by the earthquake. All the pumps in the world can’t overcome that.
but was Chernobyl really a “meltdown” at all? Admittedly, what I know about it I found at the corner of Google and Wikipedia, but AFAIK the disaster there was two-part: 1) the rapid overpressure in the reactor due to the runaway reaction, causing the top of the vessel to blow off, and 2) the resulting burning of the graphite parts of the fuel assemblies. Once the reactor had blown apart, the reaction was over; it was the burning graphite which actually caused the spread of radioactive material.
From the articles I’ve just read, the word is that the explosion was in the pumping system they were using to try to cool the reactors. The explosion tore the wall and roof off the building the reactor was housed in, but that doesn’t imply damage to the reactor itself. Reactor buildings are just shells. Reactor containment domes inside the building are far, far studier than ordinary roofs.
The article also correctly says that a melt-down, as the public understands the term, is a breach of the containment dome of the reactor itself. That hasn’t happened and is still not likely to even if the rods themselves are still melting. Some coolant will be needed in the immediate future. Reports are that they will be using sea water. Presumably they have an auxiliary pumping system or can rig one. The plant doesn’t have the iconic cooling towers of Three Mile Island, so it’s likely they just run pipes out into the ocean they’re sitting on for all their water.
The plant was designed with redundancy, as noted above. It took a tsunami from the largest quake in Japan’s history to knock out all the redundant systems simultaneously. They’ll still get the blame for this, but it’s not a Chernobyl where the underlying design was flawed to the point of idiocy.
For those who are wondering, newer designs use a “dead man’s switch” for the reactor damping rods. When the power goes off, they fall automatically so that they don’t need to worry about backup systems coming to life in a disaster. I’m not sure whether that’s something that can be retrofitted into older designs, though.
Partly right but actually they put the rods back in.
However, there was a quirk of their design that would see a power increase when the rods were put back in making things worse. They were tipped with graphite which caused the reactor power to spike severely. This caused the thing to go mad and prevent the rods from being inserted further to stop it.
i think the public believes that melting of fuel assemblies is a meltdown. Three Mile Island had a meltdown. a meltdown can be less than a China Syndrome.
The China Syndrome is not considered a realistic possibility these days. Even at Chernobyl, where what remained of the core after the explosion did melt down, the melt didn’t make it past the reactor building basement. And Chernobyl didn’t have a containment vessel. Three Mile Island did. At Three Mile Island, the melt stopped the moment it reached the containment vessel wall.
It is also important to consider that Chernobyl and Three Mile Island were very different kinds of accidents. Three Mile Island was a failure to manage decay heat. When a nuclear reactor core is shut down it continues to generate heat for a while due to radioactive decay of fission products. At Three Mile Island the reactor was shut down safely, but then due to operator error and design flaws the core was allowed to become uncovered and partially melted from decay heat. Chernobyl was not due to decay heat, it was due to a reactor core that due to astonishingly unsafe design and operators doing things they really, really shouldn’t have was put into a state where it was generating power far beyond its design max without any cooling. Even then, if it had a containment vessel, the core might have just ruined itself and not ended up scattered across the landscape.
The Japanese reactor has been shut down and apparently still has an intact containment vessel, so there’s no way to get a Chernobyl type accident out of this. At worst, the core might melt down inside the containment vessel due to uncontrolled decay heat.
Why don’t plants like these have a water tower nearby, so that in the event of a power failure AND the failure of backup generators, they would still have a source of water/coolant that could reach the core without any active pumping necessary, due to the force of gravity? Of course, pumps are necessary to replenish the supply in a water tower itself, but that would give you a backup to a backup.
Many modern reactor designs, such as the AP1000, do have a gravity-fed coolant tank on top of the reactor. The six reactors at the Fukushima-Daiichi site are of an older design, built in the 1970’s, and don’t have some modern safety features. More modern reactors elsewhere in Japan handled the quake much better.
Experts that call something “unlikely” often mean the same thing that laymen mean when they call something “impossible”. Suppose (and I am not a nuclear expert; numbers are only an example) that I told you that this accident had a 1 in 100 million chance of being as bad as Chernobyl. If I were speaking sloppily, I might have just said “it’s impossible”. But that’s not entirely true; there is still a chance (albeit a very small one). So I say it’s unlikely, which is completely true.
Fukushima Unit 1 is a GE BWR 3. Started up in 1970. Before Three Mile Island, so it had no Hydrogen recombiners for containment. Diesel generators get hit with salt water, service water full of debris, no outside power. This is a bad day at work. Hydrogen starts building up in containment. WOW. Explosion in Containment.
There’s an interesting commentary from the engineering journal IEEE Spectrum.
He says that engineers generally prepare for what they consider to be the worst case scenario. However, they have a history of not expecting all the redundant backups to fail at once. In reality, most of the things that go really badly wrong happen because total system failure is more likely than they consider possible. Remember that little oil spill last year? That was a case of all the backups failing simultaneously in a way the designers didn’t anticipate.
That’s sobering. And in fact newer designs for nuclear plants do, as said, make better provision for total system failure than they did 40 years ago. But we live in a real world of time and budgets. No system, no matter how crucial or how important or how deadly, has an infinite set of fail-safes and backups. Even in a fantasy “screw the costs, do everything” scenario, we’re limited by the disasters the designers can imagine. Real life has a way of coming up with situations nobody ever thought of before.
After the fact, we blame everyone. Because humans are involved, the planning is never as perfect, the implementation never as designed, the maintenance never as thorough, and the checklists never as completely checked as those affected would have liked. Look around you. Do you see superhuman perfection in your co-workers? You’re lucky if you see mere competence.
I hate disasters and mistakes and screw-ups and general idiocy as much as anyone. Find what went wrong and fix it. But I also hate 20-20 hindsight. Yes, they should have known better and should have done better. But 40 years from now a disaster will happen in a way that nobody today would believe possible and hadn’t taken steps to prevent. If you can prevent it, speak up now. Just don’t say after the fact, well, sure, but if you had just done ***this ***it wouldn’t have happened.
Just a pet peeve of mine. I have a menagerie of them.
If the nuclear power plants were still operating, they could produce their own electricity. There is no such thing as any power generating plant “producing more [electricity] than they can handle.” Any electrical generator will only produce as much electrical power as there is electrical load.
This is the answer. Naval nuclear reactors are designed to be restarted very quickly in certain situations, but restarting commercial nuclear power plants is a far more involved process.
Also, a nuclear power plant needs a source of electrical power to restart the plant (analogous to a car needing a charged battery to crank the engine). If there is insufficient electrical power to cool the plant, there is almost certainly insufficient electrical power to restart the plant.
Finally, if parts of the plant were damaged due to the earthquake, or if there is core damage due to residual decay heat (and insufficient emergency cooling), they may not be able to restart the plant at all.
They were talking about pumping in sea water to the damaged reactor to cool it down. I guess (from listening to the report) that if they pump in sea water the reactor is basically a write off…it can’t be recovered later on. But since it seems that the stricken reactor is an older design (40 years old) and since the problem is dire enough they are going to go ahead and do it (or they might have already done it…the report was unclear on this point). Does anyone know if they went ahead and pumped in sea water? Also…true that pumping in sea water means that the entire reactor is a write off after that?
Sites that I’ve read are claiming that seawater is being pumped in to cool the core, but I haven’t seen a press release or anything else official.
Seawater is not only corrosive, but it also greatly increases the amount of secondary radioactive contamination. Normally the primary coolant is distilled water, and the only real radioactive element that’s produced is tritium. Seawater has sodium and chlorine, as well as trace amounts of many other elements, many of which can become neutron activated. Of course, if the fuel rods have ruptured as they appear to have, this is a moot point anyway.
Reactor #1 at the Fukushima site is 40 years old, of an obsolete design, and probably would have been decommissioned before too much longer even if this earthquake hadn’t happened. It’s going to be a complete writeoff. The other reactors at the site might be salvageable.
I came across a site that explains the whole situation well:
I thought they had undertaken this roughly 24 hours ago, in a process expected to take ten hours. If you’re citing a recent report, maybe it’s in reference to the second reactor… there’s three now-troubled reactors at one facility.