Not only that, but the outer building was *designed *to give way in the event of an explosion so that pressure would not damage the containment dome proper.
Just as not understanding this blows away constanze’s credibility.
I agree with this point. No engineer at Chernobyl recklessly decided to see what would happen if they ran a trial emergency. They were in fact running a third safety test in order to ensure that the plant’s emergency cooling systems worked. The objective was safety and it went badly wrong.
No excuses for that but thinking Chernobyl must have happened because of those evil commies is xenophobic and completely dismissive of the integrity of the plant engineers.
Here in the West we are not perfect. Mistakes do happen. Embarrassing failures are obscured - but generally learned from. Consider the GFC and the part our financiers have played in bringing economic ruin to ordinary families.
However Constanz I can’t go along with the rest of your extrapolations. Three Mile Island was a reactor melt-down - why didn’t the core end up in China…???:smack: Oh - and just how many were killed in that accident?
I don’t understand how modern melt downs are even possible.
I get that the generator then backup generator failed, but why should it even have to come to that?
Why can’t the unpowered state of a reactor be inert?
Take air brakes for instance. Electricity is required to run the pump that fills the cylinder that keeps the pad off of the disc. If the electricity fails or the air hose is cut the pad slaps down on the disc and thats it, yeah full brakes on a highway are bad but not nearly as bad as heading down a steep hill with no breaks.
Chicago Pile-1, the worlds first nuclear reactor, was just a honeycomb of uranium and graphite. Cadmium rods were inserted to cool it down and when they were removed the pile would heat up. But even in this primitive barnyard reactor there were scientists stationed around it with buckets of cadmium and a massive amount of graphite suspended above it waiting for an ax swing to bring it down in case the worst happened.
No electricity required.
To expound upon the linked thread, the issue is not stopping the primary fission reaction. The primary reaction was stopped days ago when the control rods were inserted into the core. The problem is dealing with the residual products of the reaction decay chain, which produce heat at an asymptotically decreasing rate even after the primary reaction has stopped, referred to as decay heat.
Once the daughter nuclides have decayed away and the decay heat decreased to a negligible amount, then the unpowered state of a nuclear reactor is indeed “inert,” but not until then.
Also, the Chicago Pile-1 experiment most certainly did NOT have a “massive amount of graphite suspended above it” that would halt the chain reaction. The graphite was used as a moderator, which in nuclear parlance means that it slowed down the so-called fast neutrons produced so that the resulting slowed (or “thermal”) neutrons could continue the chain reaction. Adding graphite would have increased the rate of reaction, not decreased it.
Hey everybody, OP here…
Thanks for ALL the replies… some are above my head, some are below my knees.
Here’s where I am on this:
IIRC Japanese nuclear reactors SCRAM as soon as a significant earthquake is detected. Basically, the control rods are dropped in…all of them. This stops the fission reaction.
As noted the reactor stays hot from decay products. That is the issue here.
Fission has stopped but copious heat is still produced. Enough to melt things which is bad.
They need coolant to stop that and that has been a problem.
I could be wrong, but I think a lot of the byproducts of the intial reaction also fission?
Not sure what you are asking/suggesting here.
Not being snarky. Just missing your point to answer.
Byproducts can produce up to 7% of the energy of a full functioning nuclear power plant.
Why do “byproducts” also produce heat?
Are there not some unstable elements produced from the inital fision? I mean, don’t some of them break down further into smaller elements?
I am not a physicist.
All I can say is that 7% is still substantial.
We are talking about a lack of coolant. Even 5%, without cooling, can mess things up.
Remember the reactor in Japan SCRAMed. Basically it went into full shut down (the rods dropped in to stop the reaction).
Heat is still produced from the decay of fission products even though the rods are in.
The problem is a lack of coolant. Even when the reactor is “off” (e.g. rods are in) heat is produced. Apparently this is enough to melt stuff.
As noted before the Japanese reactor has a negative heat coefficient. This is safety measure (Chernobyl had a positive heat coefficient so the reaction increased as it got hotter).
Despite that the thing is hot and needs to be cooled. If the cooling fails you have problems.
I think I replied to the wrong post, but yes I agree with you. 7% is a shit-ton and requires significant cooling. I know that heat is generated from weird-ass X and Gamma rays, and weird particles decaying and shit… but I’m also pretty sure that like Uranium 238 decays to weird atoms with weird atomic numbers with wrong neutrons and shit like 155 or some shit that has to decay again and make something stable like gold or lead.
So like yeah, I’m a fucking atomic physicist or some shit.
Missed edit window, but seriously this is an awesome link:
http://mitnse.com/2011/03/13/why-i-am-not-worried-about-japans-nuclear-reactors/
That’s exactly right. The fission fragments from the chain reaction are unstable and undergo alpha and/or beta decays, releasing more energy as they go.
7% is the initial decay heat at the instant the chain reaction is halted. It decays exponentially from there to about 1% within a few hours, but exponential decay has a very long tail. According to this graph you’re at about 0.3% in ten days. Which for a 600 MW plant is about 1000 electric kettles worth. I’d have thought convection within the cooling loops would be enough to take care of that, but first you have to get to those ten days without melting the core. We’re on day 5 now. Not a lot in the press about how the problem gets smaller with time…
The term you are referring to is the “temperature coefficient of reactivity,” not heat coefficient.
In any event, since the main nuclear reaction stopped days ago when the reactors were scrammed (with all the control rods inserted into the core), it’s no longer particularly relevant, unless we’re talking about a restart accident. The negative reactivity due to the fully inserted rods is now the largest factor keeping the reactor subcritical (i.e. reactivity < 1).
The issue now is dealing with the decay heat, which is indeed due to the continued radioactive decay of the residual fission fragments.
Ok? So there is residual heat from the cores that dissipates over time.
No one has yet explained why its impossible to build a coolant system that doesn’t require electricity. A manually operated sluice gate…something, anything.
And why does this seem like a bigger deal than minor7flat5 and Exapno Mapcase let on.
Not just residual heat. There are short-lived radioactive elements in the core that continue to generate new heat for some time after the reactor is shut down.
It isn’t impossible, and many more modern reactors have completely passive emergency cooling systems built in. The reactors at the Fukushima Daiichi site were of a 40 year old design and did not have all of the safety features present in modern reactors. There are quite a few reactors elsewhere in Japan, some of them closer to the epicenter than Fukushima Daiichi, which shut down safely and have not had any significant problems since.
There are about five threads going on simultaneously, so I forget what I’ve said where. However, modern system design - Fukushima is 1971 - uses a “dead man’s” switch that automatically drop the control rods if the power goes out.
But that’s stopping the fission, not cooling the core. The core is always supposed to be cool. The normal procedure is a heat-exchange water system. That’s in continuous operation because so much heat is generated that water in a static system would immediately boil, turn into high-pressure steam, and threaten to blow open the containment.
That’s essentially exactly the problem now. It’s not just getting water to the fuel rods, even though they’ve been turned off. It’s getting a continuing supply of fresh cool water to the fuel rods, with the heat from the water moving away through the heat-exchange system. A manual sluice wouldn’t help a bit. It would be disastrous, in fact, because you don’t have anywhere for the water to go. You have to have the outgoing system. That’s why you need electricity. It is a long-term continuing process of pumping. Dumping won’t work. And they did have the electric backup system, which ran for eight hours. Whatever your idea is for a manual system would have to be better than an eight-hour backup. And it would have to work when all the systems have been damaged by the quake and tsunami that caused the original problem.
You mean the article that says:
That article?
Nobody is saying that the situation isn’t serious. But this isn’t like the gulf oil spill. They know what the problem is, where it is, and have equipment currently in place that is actively working to correct it. All those efforts may fail and I’ve said that would be an enormous danger. But it doesn’t help to start by jumping to the worst case scenario. That’s scaremongering and I won’t be part of it.
First, agreeing with Exapno, the mods might want to merge some of the 5 or so threads in GQ on this subject. The main problem described in your linked article, Sitnam, isn’t that the rods within a working reactor are melting. Those reactor rods are kept within multiple layers of containment, which I don’t believe have been breached. They can melt, a la TMI, and while that leaves one hell of a mess, the overwhelming majority of the radioactivity is kept within the reactor vessel or, at worst, the giant concrete shield at the bottom of the complex. It’ll be contained.
The article is describing another type of accident with fuel rods, kept within a Spent Fuel Pool (SFP).
I’ll disagree with the quote and say that an SFP fire is more dangerous than any likely meltdown of a working reactor, simply because the rods within an SFP aren’t as contained as the rods within a working reactor. The rods within a SFP aren’t within a reactor vessel. I’d really like someone from GE to step forward and explain how contained the SFP for this BWR is: whether it has more or less concrete surrounding it than the reactor vessel, whether it’s as hermetically sealed as the reactor vessel. I’m also a bit confused as to whether the fire mentioned is from hydrogen evolving from the coolant or whether the spent fuel has been exposed and the rods themselves are burning or have burned. (technically, the Zirconium cladding on the rods)
I suspect the former, but the latter would be a really bad thing. For more reading, I dumped a bunch of links on the subject in this thread, here. Found a .pdf link to the Alvarez paper that went into some of the worse-case hazards from a SFP accident.
Another unsettling thing from Sitnam’s linked NY Times article.
Now, they didn’t say where they took the measurement, but 400 mSv/hr is a really big f***ing deal. I hope somebody screwed up the translation and meant microSievert instead. The source of that measurement had better be somewhere within the reactor building. Or standing right next to the reactor vessel as they were actively venting activated steam.
Not being in the industry, and raised on too many Cold War dramas, I still think in terms of REM. 400 mSV is equal to 40 Rem. 40 Rem/hr is dangerous to life. It won’t instantly kill you, but you’d really want to be somewhere else, fast. The article mentions that 7 minutes exposure to that intensity will give you the max annual exposure for a U.S. nuclear plant worker. 75 minutes gives you acute radiation sickness. An 8 hour workday spent in that field stands a pretty good chance of putting you in the ground. Again, I don’t remember reading anecdotes about that kind of radiation exposure for TMI.