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Post by davidm on Apr 29, 2012 14:39:36 GMT 9.5
This is a subject I broached on another BNC forum and I never got full clarity on the matter so I'm trying again.
Imagine the best nuclear power system with the best passive system around to prevent a nuclear meltdown and if necessary protect against the consequences of a meltdown that has happened.
An extreme disaster happens, maybe a 9.0 earthquake. The electricity is knocked out, the water delivery is destroyed and the situation is such that human beings can't meaningfully intervene for a month.
What would be the probable consequences of such a situation?
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Post by David B. Benson on Apr 29, 2012 15:33:00 GMT 9.5
With a passive safety design (Gen 3), nothing bad escapes into the environment. The only requirement is that the control rods insert. This happens in case of a power failure as the control rods are held up by electromagnets. The control rods also insert in the case of excess ground acceleration.
So now the reactor is shut down and the only remaining safety matter is cooling the nuclear rods. The passive coolant design ensures that occurs.
Over on the steam side however, the steam turbine needs to be shutdown. Ideally the automatic control circuitry detects the ground acceleration and accomplishes that. Otherwise operators would need to do it. This is not a safety issue but rather one of protecting an expensive (and big) piece of equipment.
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Post by sod on Apr 29, 2012 21:16:53 GMT 9.5
What happens when the earthquake does direct damage to the structure?
possibly even before control rods could be inserted?
what if flood water enters the structure, reaching nuclear material?
Fukushima demonstrated that without access to the site, we can t even control the old spent fuel!
all of these scenarios are simply slightly worse examples of the fukushima accident. but what if terrorists gain access or even worse a high ranking insider does active sabotage???
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Post by anonposter on Apr 29, 2012 22:03:24 GMT 9.5
What happens when the earthquake does direct damage to the structure? An earthquake strong enough to cause a safety problem at a nuclear power plant would give you much more important things to worry about. possibly even before control rods could be inserted? The control rods get inserted before the earthquake hits unless the power plant is right above the epicentre. what if flood water enters the structure, reaching nuclear material? Do you have any idea how many barriers it'd have to get through to do that? The reactor pressure vessel would probably need to be punctured for that. Fukushima demonstrated that without access to the site, we can t even control the old spent fuel! Yet there was always enough water in that spent fuel tank. all of these scenarios are simply slightly worse examples of the fukushima accident. How am I to know they aren't just figments of your imagination. but what if terrorists gain access That's why guards at some nuclear power plants have MP5s. But even so, they can't do anything worse than what they could do at a much less protected oil refinery or chemical plant (that oil refinery fire near Tokyo killed ore people than Fukushima Daiichi). or even worse a high ranking insider does active sabotage??? The worst they could do with any western reactor is write off the reactor and needlessly scare the public (and the rest of the staff at the power plant aren't going to just stand by and let them). |
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Post by LancedDendrite on Apr 29, 2012 22:07:34 GMT 9.5
but what if terrorists gain access or even worse a high ranking insider does active sabotage??? Terrorism can't do much more than what the Tōhuko Earthquake and subsequent tsunami did to Fukushima Daiichi units 1-3 without resorting to air-dropped bunker-buster bombs and nuclear weapons. |
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Post by davidm on Apr 30, 2012 3:05:35 GMT 9.5
With a passive safety design (Gen 3), nothing bad escapes into the environment. The only requirement is that the control rods insert. This happens in case of a power failure as the control rods are held up by electromagnets. The control rods also insert in the case of excess ground acceleration.
So now the reactor is shut down and the only remaining safety matter is cooling the nuclear rods. The passive coolant design ensures that occurs.
Over on the steam side however, the steam turbine needs to be shutdown. Ideally the automatic control circuitry detects the ground acceleration and accomplishes that. Otherwise operators would need to do it. This is not a safety issue but rather one of protecting an expensive (and big) piece of equipment. The passive coolant design ensures the nuclear rods stay cool for how long? Remember the electricity is out, outside water sources are cut off and human intervention is not available. So after the water that is dumped in from the reservoir above has steamed off what keeps the nuclear fuel from eventually burning through its containment and contaminating the wider environment? In addition what about containment of the spent fuel rods? It would be interesting to get a blow by blow of what happens in these extreme circumstances. |
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Post by anonposter on Apr 30, 2012 7:39:17 GMT 9.5
The passive coolant design ensures the nuclear rods stay cool for how long? Long enoughh for the decay heat to drop to a level that doesn't require all that much cooling (of course it would depend on the design, though modern passive designs are designed to be able to remove decay heat without any pumps running and the S8G can even generate a significant amount of power with the pumps off). Remember the electricity is out, Passive reactors don't need the pumps to run to remove the decay heat. outside water sources are cut off and human intervention is not available. Some reactors could survive indefinitely without any human intervention while some others would eventually need some human intervention (being able to buy more time and delay the intervention until there is a lot less decay heat is not a bad thing). So after the water that is dumped in from the reservoir above has steamed off what keeps the nuclear fuel from eventually burning through its containment and contaminating the wider environment? That it would break the laws of physics (even in an old design reactor like TMI-2 or the Fukushima reactors). For you see while the decay heat is enough to melt the fuel, it isn't actually enough to melt its way through the reactor pressure vessel. In addition what about containment of the spent fuel rods? It would be interesting to get a blow by blow of what happens in these extreme circumstances. There's enough water to last quite a bit of time and the swimming pools they're stored in don't have any drain holes (if you even needed to drain one for some reason you'd have to pump the water out). |
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Post by David B. Benson on Apr 30, 2012 12:56:37 GMT 9.5
The Gen 3 passive design I know best is the Nuscale 45 MWe PWR. The reactor is in a concrete lined pit filled with fairly ordinary water. The reactor has a steel liner around the pressure vessel, only there because NRC says so. I assume, without really knowing, that the space between the liner and the pressure vessel is filled with treated water, but NRC might require air; it doesn't matter. Inside the pressure vessel is the reactor itself, below, and the steam generator, above. The working fluid is demineralized, deionized water which never leaves the pressure vessel. Heat moves up from the reactor to the steam generator by convection and the now cooler water drops back down around the outside of those two units, but within the pressure vessel. There is no pump.
In ordinary operation the feedwater pump pushes water into the steam side of the steam generator and hot steam comes out the other end to turn the Rakine cycle turbine. This of course removes heat from the pressure vessel. But this is not required to operate when the control rods are dropped into the off position. When that happens convection continues but the heat is removed through the liner to the water in the concrete pool. That water is at standard pressure and never heats enough to loose much by evaporation. Ideally there should be a small source of makeup water to keep the pool full, but even that is not required to cool the nuclear rods.
The control rods begin inserting as soon as the ground motion accelerometer detects enough ground motion. The control rods are designed to be never fail units and the technology is fully mature in over 60 years of continued development.
Everyone now puts below grade level the so-called spent fuel pool [better is just the wet pit or once-through nuclear rod pool]; earthquakes will not highly trouble this unit. Some wet pit designs require cooling the water, but the best design does not. That means the rod density needs to remain low so after about 5 years of residency the rods should be removed to dry cask storage. However, any wet pit will require some makeup water but by putting about 12 meters of water above the tops of the nuclear rods one can delay for quite some time doing the makeup.
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Post by davidm on Apr 30, 2012 18:53:29 GMT 9.5
A little dated but interesting. In 2009 a German nuclear safety expert offers a report on the safety of Gen. 3 reactors. He doesn't seem all that impressed. |
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Post by LancedDendrite on Apr 30, 2012 20:37:26 GMT 9.5
A little dated but interesting. In 2009 a German nuclear safety expert offers a report on the safety of Gen. 3 reactors. He doesn't seem all that impressed. Well, he offered an interesting critique of the EPR design - not much else though. I note that he made the presentation in 2009 and dismissed the AP1000 as existing only on paper. The 8 reactors currently being built in China and the US would beg to differ with his argument. Essentially his argument seems to be that, because the EPR (and although he mentions other Gen III designs, he only focuses on the EPR) relies on engineered safety systems it is little better than existing Gen II designs he cites such as Konvoi and N4 (of which the EPR is their progeny). As a result, he argues through (IMO flawed) logic that all Gen III reactor designs are of little worth compared to their forebears. Because obviously if a new reactor is both more economical and safer than older ones by 'less than a factor of two' it is not good enough. |
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Post by David Walters on May 1, 2012 5:45:49 GMT 9.5
The EPR is considered 'safer' because it has 4, not 2, 'cooling trains' that is feedwater pumps to the steam generators. I do not know what makes an EPR "Gen III" other than this, which I always thought was based on station-black out conditions and not simply pump failure alone.
The AP1000, EBWR, APR1400, VVER1200 (AES-2006) all have passive, non-human-intervention, safety systems for passive cooling for 72 hours minimum.
I should point out that the EPR has been 'endorsed' by the Union of Concerned Scientists.
To the poster, if the plant blows up, the plant blows up. I assume one can keep adding bigger and bigger earthquakes, taller and taller tsunamis until you get catastrophic failure. What's the point of question?
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Post by davidm on May 1, 2012 7:40:45 GMT 9.5
What's the point of question? To explore nuclear power safety. Do you find that to be an illegitimate concern? If not, what is the point of your question? |
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Post by David B. Benson on May 1, 2012 12:27:18 GMT 9.5
The largest possible earthquake, barring an assist from a massive bolide impact, is about moment magnitude 9.6. Chile fairly suffered such off the coast and yet Chile plans on building 3 NPPs. While these will be well inland Chile is riven with faults, making Southern California quite the piker by comparison.
Earthquake resistant design is now quite well understood.
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