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Depending on active cooling measures in a nuclear reactor emergency seems like a bad idea. What happens when they fail?
Having the reactor rods fall apart and away from each other when a strap holding them together melts seems like a more fail safe method of preventing a meltdown.
Picture a bundle
of straws that you're holding together with your fist. Like the strap that holds the rods together, when you let go they fall apart.
Likewise, in a reactor, a strap that melts at a certain temperature would hold the rods in a bundle that generated heat as long as that heat was kept below a certain level by coolant. If coolant was lost, the strap would melt and the rods would all fall apart and away from each other. They would cease to generate additional heat and residual heat would be absorbed by a massive water pool that the disintegrated core could falls into.
What happened in Japan
http://www.cnn.com/...xplainer.nhk?hpt=C2
Lots of active backup systems failing. A good argument for a failure dependent safety system like a deadman's switch. [doctorremulac3, Mar 14 2011]
Half-Baked
http://www.euronucl...news-17/nps-kth.htm
...except they eliminated the control rods as well, for good measure. [4whom, Mar 14 2011]
More info in direction nuclear can and should take
http://www.ornl.gov...1/3445603211254.pdf
Caveat: long and interesting ~ may take up several days of your time [4whom, Mar 14 2011]
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It sounds like a good idea, I can't imagine they didn't
think of it. Perhaps each rod has sufficient density to
generate heat in itself. |
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// Residual heat whould still be there but it would cease to generate additional heat. // |
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Once the reactor has scrammed, the neutron flux drops right down and the chain reaction stops; that happens within milliseconds. |
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The "residual" heat is almost all from the decay of short-lived isotopes; that's why fuel pins go into a cooling pond for a while once they're pulled from the core. Then they can be reprocessed. Fresh pins are physically cool. |
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"dumping" the core won't help. |
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Yea, that's the first thing I thought of. "All the world's nuclear engineers didn't think of this and you did?" Seems pretty doubtful. |
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Whether it's been thought of before or not though, still seems like a better meltdown preventer than a series of failure prone backup systems. |
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This is the concept, though not the execution behind a SCRAM. Which drops control rods in place, rather than letting the fuel separate. |
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Ok, so that's actively inserting control rods right?
So how come it didn't work in all these plants in Japan? Or did it? If so, what's the problem? |
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Not being smartass, I don't know. The news reports on this are pretty technically simple to put it politely. |
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The reactors scrammed succesfully, but that drops the core output by 90% - ish; say, from 100 MW to 10 MW. |
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You still need to get rid of 10MW of heat from short-lived isotopes and with the primary coolant loop down, that's not easy. It's about the power produced by 150 average car engines. |
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The marinised version of the Rolls-Royce Trent generates about 25MW. |
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I'm thinking physically separating them would deal with the bundled heat issue that control rods wouldn't. |
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So having a hundred rods together, they're all dealing with the residual heat of the surrounding rods as opposed to just radiating and dissipating their heat alone. Inserting the control rods would stop the reaction but not the residual heat of all the rods bundled together and heating each other. |
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In other words, it's like taking a pie out of the oven and setting it away from the other pies to cool as opposed to leaving them in a big pile. |
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Correct me if I'm wrong, but it's my understanding that scramming the thing doesn't get you out of trouble if your cooling system fails. And what if your control rods melt? |
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Again, I'm obviously not a nuclear engineer, I'm just asking, but looking at the link that shows what happened in Japan, this seems like this would be a good idea. |
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Safety systems that use failure to work are a better bet than active systems. For instance, you could have a deadman's switch on a locomotive where the engineer needs to keep his grip on it at all times, or active sensors that senses if he falls down. The advantage of the passive system, the deadman's switch, is that if it breaks, the train stops. The active sensing mechanism might already be broken, you won't know until it doesn't work and you may only find out when you need it. |
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If you can precision control how the rods fall, such that none fall across each other, spreading them out might help with cooling, but you're still going to need cooling water. |
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There are protoypes of designs that cannot melt down, but there are other concerns on these, and they are still in development. |
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I was thinking they would fall into a pool of water that had enough volume around each rod to absorb that rod's residual heat. They could slide down rails into a massive pool for instance. Picture the rails extending down from the tip of a cone shaped base so they fell down and away from each other and into the massive pool. Alternatively, they might splay apart like when you chop an onion to make those onion blossom things. The walls of the smaller water vessel surrounding them would fall apart as well exposing the tipping rods to the massive surrounding pool. |
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The very simplist way to do it would be to have a strap holding the rods together that melts at a certain temperature releasing and disbursing the bundle into the cooling pool. In fact, think I'll change the idea to reflect that. Simpler is better. |
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There would be a couple of ways to do it, but depending on pumps like we do now appears to be a bad idea. |
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I'm surprised to learn that the control rods alone aren't
enough to let the reactor cool passively. I didn't realize that
active cooling was necessary even after shutdown. Is this
true of all designs? |
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//Is this true of all designs?// No, only the designs we are currently using commercially, and the ones we are going to be using. It seems long term, safe usage designs get second prize. Must be no money in them. |
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// active cooling was necessary even after shutdown // |
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Some designs can self-cool purely by thermo-syphon (convection). |
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That's OK - as long as it's theoretically possible, I'm happy. |
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Oklo cooled down by itself, eventually... |
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So, if normal output (mainly from uranium fission, I
assume) is 100MW, and if short-lived isotopes output 10MW
for a few....what, days?... |
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Then it should be possible to design a reactors which cycle
between full and low power (full fission, and reduced
fission plus short-live-isotope-decay), so that the short-
lived-isotopes never build up to a high level. Average
power output would be less (and so you'd need more
reactors, working out of phase), but they'd be cooler in an
emergency shutdown. |
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If the Japanese reactors do meltdown, they'll come out in
the sea off Brazil. |
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Months; typically, pins will stay in a pond for a year before transport or reprocessing. |
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As to duty cycle issues .... no, wouldn't work. Reactors are most effective when they run close to full power as much of the time as possible, with brief shutdowns for maintainance and refuelling. It's a non-trivial problem. |
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//Months; typically,// Yes, for both the expelled rods and the actual reaction chamber of horrors. "Short lived isotopes", where short is a relative term and doesn't include species who think 4 score and ten is a good innings. |
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No, Fukushima won't end up near Brazil, nor will it join the great fusion reaction at the core of our planet...you might get a nice big plume of nasty stuff going into the gulf stream, and a little bit around the plant, but it ain't gonna melt. The nice thing about plants designed with light water as a moderater is that there is lots of light water. You don't run out of moderating fluid. This means that while your graphite fails in a Chernobyl or TMI and results in messy unmoderated, supercritical (almost) gloop, you can just flood a LWR. Even if it means a platoon of kamikazi bilge pump operaters. |
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Maybe smaller fuel rods? There must be a point at which
even air-cooling would be enough to stop a thin rod from
self-melting. |
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// the great fusion reaction at the core of our planet // |
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Hmmm. You might want to keep your voice down about that. |
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// going into the gulf stream // |
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... or more likely the Jet Stream ? |
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// You don't run out of moderating fluid. // |
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The water's the coolant, not the moderator ... |
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// Even if it means a platoon of kamikazi bilge pump operaters. // |
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Well, you've at least got the right nation for that ... |
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//Maybe smaller fuel rods?// Ah, yes the much maligned PBMR. Rods so short that they are, in fact, balls. Much like the entire idea unfortunately. |
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Ah yes, I wondered what happened to the pebbly things. |
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Jet stream, yes. Sorry about that. Bloody pilot, the captain of my submarine is probably going to name her Jetstream 4, and then where will I be... |
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Fukushima's design uses light water as a moderator and coolant, no doubt about that. |
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I thought it was common knowledge that 6000 years ago our planet was the size of a tangerine and expanded under the pressure of the 2nd greatest fusion reaction to give man opposable thumbs... |
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Rubbish. Get your facts right .... it was 4020 years ago, and the size of a fairly small pink grapefruit. |
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The opposable thumbs thing is just an urban myth. But strangely, the story about "holes in the poles" is entirely true, but it's been the subject of a massive cover-up by the Salvation Army. |
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When Grandpa made dynamite during the war, it was done atop a tower with a huge tank of water underneath to cool the mix in case things got hairy. He told the story (many times) of witnessing a rail accident at some distance from the tower, and a plume of gas released that floated over towards them. When they started to choke and cough they pulled the lever to drop the nitroglycerine into the tank and jumped on the slide that delivered them some distance away - fortunately out of the gas cloud too. |
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Those bloody Slavs...always thought they were abserb...<mooches off to "Deliberately misunderstand the last person"> |
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