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Why do some nuclear power plants have smoke stacks?

  1. Jun 24, 2015 #1
    There are alot of nuclear power plants around the world that have smoke stacks, the kind you'd usually see at a coal fired power plant. Are they to release steam in an emergency, or are they some kind of exhaust system the plant uses, or something else? Most nuclear power plants in America have them, and mostly all of them in Russia have them aswell. What are they for? Picture below No=311130064&Ref=AR&MaxW=640&Border=0&NRC-reviews-Lacey-s-Oyster-Creek-nuclear-plant-after-Sandy.jpg
     
    Last edited: Jun 24, 2015
  2. jcsd
  3. Jun 24, 2015 #2
    I'm pretty sure that their only purpose is to release unwanted heat from the coolent system after the superheated steam coming from the reactor has done it's job of spinning the generators.
    Note that the steam being released is not coming from the reactor coolent directly, but from a secondary heat exchange system.
     
  4. Jun 24, 2015 #3
    But isin't that the condensors job of cooling it down? Or is there alot of leftover heat that needs to be vented?
     
  5. Jun 24, 2015 #4
    Well I guess it's fair enough to say that the steam which comes out of the stacks is the end product of the condensing process.
    The job of the condensor is to remove heat from the reactor coolent before it is recycled.
    That heat has to go somewhere and the steam coming out of the stack is where it goes.
     
  6. Jun 24, 2015 #5
    But the nuclear cooling towers take the heat away from the condensor, the smoke stacks however are rarely used, I've never seen any gasses at all come from one of these smoke stacks, so what do they do.
     
  7. Jun 24, 2015 #6
    Maybe those stacks are for a different purpose which I don't know of.
    However I did at one time live nearby to a nuclear station and that one had cooling towers which look very similar to the cooling towers you would see at a coal fired station, and assumed their purpose was much the same.
    They were not permanently emitting steam, I guess something like 10% of the time
     
  8. Jun 24, 2015 #7
    So they could either be an emergency release for steam or just for dealing with the heat and steam. But if cooling towers are so huge at the base and have room for all the water cooling, do these smoke stacks do the same thing as cooling towers in a different way, or do they just take steam from the generators and thats it?
     
  9. Jun 24, 2015 #8

    QuantumPion

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    I think those are for releasing non-condensable noble gasses. I believe the reason why some plants have them but not others may be due to the proximity to populated areas although it may be just a difference in design of different plants.
     
    Last edited: Jun 24, 2015
  10. Jun 24, 2015 #9

    256bits

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    The cooling tower should be the rectangular building to the rear.
     
  11. Jun 24, 2015 #10
    But indian point nuclear power plant, which is very very VERY close to New york, has one of these smoke stacks aswell. But if true, where do these noble gasses come from?
     
  12. Jun 24, 2015 #11

    256bits

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    Any chance that is not the building housing the diesel emergency generator system for the nuclear plant.
     
  13. Jun 24, 2015 #12

    QuantumPion

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    Plants which are near populated areas need stacks to release the gasses high up so they spread out quickly and don't come back down over people. Plants in remote areas don't have that concern since there is no large population center to worry about. Note Indian Point is 40 miles away from NYC.

    The gasses come from radioactive byproducts in the reactor coolant system. The gasses are stored temporarily in a tank until they decay into non-radioactive gasses, and then are released.
     
  14. Jun 24, 2015 #13
    So what do rural area Nuclear power plants use to vent the gas? Also, Indian point is 40 miles away, but any gas released from the plant could easily travel the 40 miles downwind to the city, And if a decent amount of radiation was released from Indian point, it could easily travel the 50 mile radius around the plant to new york.
     
  15. Jun 24, 2015 #14

    russ_watters

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    That is my interpretation as well.
     
  16. Jun 24, 2015 #15

    russ_watters

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    Do you have any references to any of that? I live a few miles from Limerick nuclear plant, which is only 20 miles from Philadelphia and it has no such stacks. Beyond that, I don't think I've ever heard that nuclear plants release anything potentially radioactive.
     
  17. Jun 24, 2015 #16

    QuantumPion

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    Well I'm not a weather scientist but I'm guessing you aren't either and you are just making wild assumptions. You'll need to provide a link to a reputable source detailing such as unsubstantiated claims are frowned upon around here.
     
  18. Jun 24, 2015 #17

    QuantumPion

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    I do not know why some plants have tall stacks and others do not, it may just be differences in plant designs. Gas emissions are filtered and monitored for radioactivity and are shut off if radiation is detected. That is for normal operation, they might use the stack in case of a severe accident situation to vent noble gases like TMI to prevent contamination at ground level although I'm not sure if that is what that's for.
     
  19. Jun 24, 2015 #18
    Every nuclear power plant in the US has a 50 mile radius around the plant, every single one. Most emergency plans in the US only call for a 10 mile radius around the plant to be evacuated during a nuclear disaster. However, in a normal radiation release due to emergency, far more than 10 miles around the plant would be affected. Wind would easily carry it 10, or 20 miles away from the plant. Where would this noble gas come from, and how is it filtered from the regular nuclear system to keep from being recycle with the other water or steam?
     
  20. Jun 24, 2015 #19
    Also, found the reason. Thank you plantmap.jpg
     
  21. Jun 25, 2015 #20
  22. Jun 25, 2015 #21

    Astronuc

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  23. Jun 25, 2015 #22
    I'm pretty sure all plants have a vent stack. At least all the plants I've been to had them. Sometimes they go up the side of the containment structure so you may not notice them unless you look hard. The ventilation system in the auxiliary building (PWR) gets filtered and released thru the stack; some plants have a small vent line on containment (to relieve the build-up of instrument air inside the can) - that goes to the stack as well. As someone said, during normal operation there is typically some small leakage of one kind or another. The stack is monitored and the releases logged and reported as required by the regulations I linked to above. The dose limits are something like 10 mrem (0.1 Sv) per year at the fence (I admit, I didn't read the current Regs and it has been a long time since I last read these).
     
  24. Jun 25, 2015 #23
    That stack is an elevated release point.

    If a gaseous release is required post accident, an elevated release point results in lower impact to the public down wind, as well as lower impact on site.

    A gaseous release stack is not required, but it is one way to ensure compliance with 10CFR100 requirements post accident.

    10CFR20 does not apply for nuclear accidents. The 10CFR100 and 10CFR50.62 requirements are used instead.

    For BWR plants, like the one pictured above, this stack is the exhaust point for normal and standby gas treatment systems. The design of the BWR containment system involves a primary and secondary containment. The primary containment has an allowable leakage rate, known as La (or L-Sub-A), for leakage allowable. This is a tech spec (operating license) limit.

    Any leakage from the primary containment can either go into secondary containment (the reactor building), or outside. All leakage must be quantified. Any leakage outside of secondary containment is considered "secondary containment bypass leakage", and is only allowed to be a small fraction of your total La. This is considered a direct ground level release during any accident scenario.

    The remaining leakage goes into secondary containment. The secondary containment (reactor building for most plants) has a slight vacuum (0.25 to 1.25" of water vacuum) drawn on it by the standby gas treatment system. SBGT uses a combination of HEPA and charcoal filters to minimize gaseous releases, while also ensuring any releases that do occur are monitored, and pass through the stack (whether it's an elevated stack or not). Not all plants have elevated stacks, so any releases from their "lower" stacks is considered a ground level release as well, but as I said, it's all based on your 10CFR100 or 10CFR50.62 limits. If your population around the plant is low enough, you don't need an elevated release point. A plant like Columbia Generation Station has a stack that is nearly impossible to see because it doesn't go higher than the reactor building and is painted to look just like the building, all their releases are considered ground level, but they are on the Hanford Site so population really isn't a concern.

    The standby gas treatment system will automatically start on any reactor building isolation signal. Typically this is high rads in the exhaust stack, level 2 reactor water level and lowering, high drywell pressure, high rads in the reactor building ventiliation system or near the spent fuel pool. When SBGT auto starts, it also automatically isolates the normal ventilation system.

    A quick side note, vacuum in the reactor building is a tech spec requirement, and is also an Emergency operating procedure entry condition for the secondary containment EOP. If vacuum degrades, you enter the tech spec and secondary containment becomes inoperable. If pressure goes positive, you now have a potentially unmonitored release, and you enter the EOP to restore vacuum.

    These stacks are also used for containment venting, although that wasn't their original purpose.

    Hope this helps!
     
    Last edited: Jun 25, 2015
  25. Jun 25, 2015 #24
    http://wind-energy-facts.com/wp-content/uploads/2015/03/The-Future-Of-Clean-Energy-4.jpg [Broken]

    The small bump on the reactor building side is Columbia's vent stack. They are a BWR/5 Mark II containment.

    http://cryptome.org/eyeball/npp2/pict425.jpg

    You can clearly see Clinton's stack here. This is also considered a ground level release stack due to how low it is.
     
    Last edited by a moderator: May 7, 2017
  26. Jun 25, 2015 #25

    etudiant

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    Interesting discussion that seems to skirt the obvious real life example from Fukushima.
    The stacks are to vent the reactor space in case of problems and there was a lot of discussion at Fukushima about rupture discs failing to rupture appropriately, thereby causing more local contamination.
    The major takeaway at least for me was that there really must be a very capable filter before the stack, otherwise it just spreads massive radioactivity widely, instead of leaving it local. Such filters are now mandatory in Finland, at least according to expert comments on this site. They are not similarly mandated in the US, partly because the retrofit would be a very costly exercise, but also because industry has convinced the NRC that the likely safety benefit is only achieved so rarely that it is wasteful to proceed.
     
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