Why do some nuclear power plants have smoke stacks?

[Fearguy1234]

There are a lot 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

 

[rootone]

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.

 

[Fearguy1234]

But isin't that the condensors job of cooling it down? Or is there a lot of leftover heat that needs to be vented?

 

[rootone]

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.

 

[Fearguy1234]

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.

 

[rootone]

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

 

[Fearguy1234]

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 that's it?

 

[QuantumPion]

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.

 

[256bits]

The cooling tower should be the rectangular building to the rear.

 

[Fearguy1234]

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?

 

[256bits]

Any chance that is not the building housing the diesel emergency generator system for the nuclear plant.

 

[QuantumPion]

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?

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.

 

[Fearguy1234]

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.

 

[russ_watters]

The cooling tower should be the rectangular building to the rear.

[separate post]
Any chance that is not the building housing the diesel emergency generator system for the nuclear plant.

That is my interpretation as well.

 

[russ_watters]

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.

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.

 

[QuantumPion]

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.

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.

 

[QuantumPion]

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.

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.

 

[Fearguy1234]

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?

 

[Fearguy1234]

Also, found the reason. Thank you

 

[gmax137]

For the regulations on routine radiological releases, see 10CFR 50 Appendix I http://www.nrc.gov/reading-rm/doc-collections/cfr/part050/part050-appi.html

Also look at 10CFR20 http://www.nrc.gov/reading-rm/doc-collections/cfr/part020/

Implementation of the Appendix I requirements is explained in Reg Guide 1.109 http://pbadupws.nrc.gov/docs/ML0037/ML003740384.pdf

 

[Astronuc]

There is some background on off-gas treatment, at least for BWRs. As far as I know, the stacks are for containment venting. Some small amount of activity may be released, especially in BWRs if there is a degraded fuel rod(s) in the core.

http://pbadupws.nrc.gov/docs/ML1125/ML11258A351.pdf

Containment Structures - lecture at UIUC

http://www.nrc.gov/reading-rm/doc-collections/commission/secys/2012/2012-0157scy.pdf

http://pbadupws.nrc.gov/docs/ML1305/ML13059A267.pdf

 

[gmax137]

... 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.

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).

 

[Hiddencamper]

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!

 

[Hiddencamper]

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).

http://wind-energy-facts.com/wp-content/uploads/2015/03/The-Future-Of-Clean-Energy-4.jpg

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.

 

[etudiant]

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.

 

[Hiddencamper]

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.

One needs to remember that the Mark I/II containment was never designed with a containment vent system. These containments are designed for the transient plus decay heat only, with an expectation that a decay heat removal train be put in service anywhere from 10 to 30 minutes after the start of the event to maintain containment parameters within design limits. This is how it was licensed, and was considered completely acceptable at the time. However, between whistleblowing activities and further studies into decay heat and severe accidents, it was determined that the Mark I containment system did not have adequate capability to withstand severe accidents for an extended period of time, which is why a vent was required. In the US, Mark Is with vents don't utilize rupture discs, this was a Japanese idea. In all cases, the vent, even the rupture disc vents, were never intended to be opened and left opened, there are vent valves which also need to be opened and closed.

The stacks were never primarily intended for containment release. That was never their purpose. They are for standby gas release from the secondary containment only, which is a filtered release path.

Venting from the wetwell achieves a reasonable decontamination factor prior to release, especially if being vented to an elevated release point. Which is why even post Fukushima, wetwell vents in the US do not require filters. The cost of the filter does not justify the added decontamination factor. These studies are all available through EPRI by the way, with regards to severe accident venting in Mark I and II containment structures. The wetwell venting, especially when sprays are utilized, are as high as a dedicated external filter. Remember, BWRs have a wet pool to scrub fission products in. This is why BWRs are not required to vent their wetwell release paths. EPRI has demonstrated that decontamination factors of as high as 500 are easily capable using the wetwell gas space as a filter path.

As for drywell vents, not all BWRs need them, this is being done on a unit by unit basis. If a severe accident water addition strategy is employed to the drywell, the need for venting is minimized plus some level of scrubbing is now available to ensure releases are comparable to a vented setup. What the industry asked for, essentially, is credit for the fact that they would already be flooding containment and spraying containment per their severe accident strategies. Since these water additions to the drywell would be occurring to halt core damage, using FLEX equipment or other severe accident responses, it would mean that the drywell's heat capacity would be improved by the addition of water to the drywell, greatly reducing the time a release would need to be in progress compared to a case without water addition. This would result in a similar release rate to a dedicated drywell filter.

There are exceptions though, especially for some designs of Mark II drywell units, where there exists a higher potential for suppression pool bypass leakage, which would then result in unscrubbed releases. I believe some units will have to utilize a dedicated wetwell vent, along with a dedicated filter.

One of the big issues with regards to filtered release paths, at least a few years ago, was how decontamination factors greater than 1000 were kind of a "point of diminishing returns", and many/most studies pointed out that DFs > 1000 did not necessarily mean there was a 1 for 1 benefit for cost return, and later studies demonstrated that using lower DF strategies to minimize the overall venting required were just as effective in minimizing total radionucleide release as dedicated filters with > 1000 DFs.

(I'm not saying I necessarily support the US industry's approach, rather that I've read several reports and studies and followed this to the point of knowing the general logic/idea behind why filters were not required for BWR wetwell release paths)

 

[Hiddencamper]

Relevent to severe accidents

"Draft Regulatory Basis for Containment Protection and Release Reduction for Mark I and Mark II Boiling Water Reactors (10 CFR Part 50)"

http://rs.scientech.com/ScientechToday/80a444d0-12d4-4fce-80a2-9b4d7f7dc986.pdf

 

[insightful]

For BWR plants, like the one pictured above,...

Isn't the schematic provided by the OP that of a PWR?

 

[Hiddencamper]

Isn't the schematic provided by the OP that of a PWR?

The OP's picture is Oyster Creek, a BWR

http://www.exeloncorp.com/PowerPlants/oystercreek/Pages/profile.aspx

That later schematic thing is a PWR one, but I was going off of the picture from the first post.

 

[arydberg]

There are a lot 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

from: http://www.nrc.gov/about-nrc/radiation/related-info/faq.html#9

I found,

What kind and how much radiation is produced by a nuclear power plant?

An operating nuclear power plant produces very small amounts of radioactive gases and liquids, as well as small amounts of direct radiation. If you lived within 50 miles of a nuclear power plant, you would receive an average radiation dose of about 0.01 millirem per year. To put this in perspective, the average person in the United States receives an exposure of 300 millirem per year from natural background sources of radiation.

#BeginLibraryItem "/Library/images/navigation/05-gif-go-top-gif.lbi" #EndLibraryItem

What happens to radiation produced by a plant?

Nuclear power plants sometimes release radioactive gases and liquids into the environment under controlled, monitored conditions to ensure that they pose no danger to the public or the environment. These releases dissipate into the atmosphere or a large water source and, therefore, are diluted to the point where it becomes difficult to measure any radioactivity. By contrast, most of an operating nuclear power plant's direct radiation is blocked by the plant's steel and concrete structures. The remainder dissipates in an area of controlled, uninhabited space around the plant, ensuring that it does not affect any member of the public.
What kind and how much radiation is produced by a nuclear power plant?

An operating nuclear power plant produces very small amounts of radioactive gases and liquids, as well as small amounts of direct radiation. If you lived within 50 miles of a nuclear power plant, you would receive an average radiation dose of about 0.01 millirem per year. To put this in perspective, the average person in the United States receives an exposure of 300 millirem per year from natural background sources of radiation.

#BeginLibraryItem "/Library/images/navigation/05-gif-go-top-gif.lbi" #EndLibraryItem

What happens to radiation produced by a plant?

Nuclear power plants sometimes release radioactive gases and liquids into the environment under controlled, monitored conditions to ensure that they pose no danger to the public or the environment. These releases dissipate into the atmosphere or a large water source and, therefore, are diluted to the point where it becomes difficult to measure any radioactivity. By contrast, most of an operating nuclear power plant's direct radiation is blocked by the plant's steel and concrete structures. The remainder dissipates in an area of controlled, uninhabited space around the plant, ensuring that it does not affect any member of the public.btw The taller the smoke stack the more toxic the products that are meant to be diluted as much as possible before people are exposed to them.

There are new plants being built now in the US.

 

[jim hardy]

Common sense dictates that a nuke plant shouldn't just open its windows. All the ventilation air is collected and exhausted up the stack so it can be monitored. If activity goes up unexpectedly, dampers close to bottle things up while plant guys figure out what's happening.The stack is there as an ounce of prevention not a pound of cure.

A lighthearted yet sensible look at the subject here:
http://www.scientificamerican.com/article/coal-ash-is-more-radioactive-than-nuclear-waste/

The popular conception of nuclear power is straight out of The Simpsons: Springfield abounds with signs of radioactivity, from the strange glow surrounding Mr. Burn's nuclear power plant workers to Homer's low sperm count. Then there's the local superhero, Radioactive Man, who fires beams of "nuclear heat" from his eyes. Nuclear power, many people think, is inseparable from a volatile, invariably lime-green, mutant-making radioactivity.

Coal, meanwhile, is believed responsible for a host of more quotidian problems, such as mining accidents, acid rain and greenhouse gas emissions. But it isn't supposed to spawn three-eyed fish like Blinky.

Over the past few decades, however, a series of studies has called these stereotypes into question. Among the surprising conclusions: the waste produced by coal plants is actually more radioactive than that generated by their nuclear counterparts. In fact, the fly ash emitted by a power plant—a by-product from burning coal for electricity—carries into the surrounding environment 100 times more radiation than a nuclear power plant producing the same amount of energy.

Key phrase there is : "carries into the surrounding environment" .
We go to great lengths to keep our high activity stuff bottled up and out of the surrounding environment.
Some of Fukushima's got out.

 

[nikkkom]

Venting from the wetwell achieves a reasonable decontamination factor prior to release, especially if being vented to an elevated release point. Which is why even post Fukushima, wetwell vents in the US do not require filters. The cost of the filter does not justify the added decontamination factor. These studies are all available through EPRI by the way, with regards to severe accident venting in Mark I and II containment structures. The wetwell venting, especially when sprays are utilized, are as high as a dedicated external filter.

Which is a faulty logic because a filter on a vent line works in *addition* to any wet scrubbing in the primary containment. In particular, it would work even if, you know, an "unthinkable" happens and wet scrubbing fails to occur.

Remember, BWRs have a wet pool to scrub fission products in.

Remember, there are now thousands of square kilometers with their population evacuated around a BWR station in Japan which failed its wet scrub.

*This* attitude of not being capable of acknowledging the truth even when it is right into your face is what made me lose faith in the nuclear industry.

(This post is not directed personally at you, Hiddencamper.)

 

[mheslep]

Remember, there are now thousands of square kilometers with their population evacuated around a BWR station in Japan which failed its wet scrub.

Seems to me Fukushima makes HC's point, not yours, as in that case what additional safety is provided by filters whose function clearly would have become moot when the containment seal was blown at the time of the explosions? To my mind the lesson of Fukushima is the guaranteed removal of sufficient decay heat before it induces a destructive explosion, not the indeterminate use of what remains after an explosion.

 

[Hiddencamper]

A filter at Fukushima would not have been beneficial as the evidence seems to show that the reason for a lot of the release was due to containment system failures. An engineered filter only works if your containment is intact. Yet unit 3 it appears the dry well head "burped" and there were penetration failures. Unit 2 was speculated by ANS in 2012 to have large containment failure when there was an explosion sound in the vicinity of the suppression pool. Not as sure about unit 1. But a filter would not have provided additional scrubbing in those scenarios, while the pool would (up until vessel failure and hot debris ejection), and the containment, if water was added, would have been more capable of absorbing fission product releases.

Now there is an argument that Fukushima could have vented early if they had passive filters. Remember these units have active filters installed (standby gas treatment system). I argue that it wouldn't have mattered, there are noble gasses that do not hold up in charcoal adsorpers or hepa filters which still dictates you delay the release, because the noble gas inventory is a large contributor to the early release.

 

[etudiant]

A filter at Fukushima would not have been beneficial as the evidence seems to show that the reason for a lot of the release was due to containment system failures. An engineered filter only works if your containment is intact. Yet unit 3 it appears the dry well head "burped" and there were penetration failures. Unit 2 was speculated by ANS in 2012 to have large containment failure when there was an explosion sound in the vicinity of the suppression pool. Not as sure about unit 1. But a filter would not have provided additional scrubbing in those scenarios, while the pool would (up until vessel failure and hot debris ejection), and the containment, if water was added, would have been more capable of absorbing fission product releases.

Now there is an argument that Fukushima could have vented early if they had passive filters. Remember these units have active filters installed (standby gas treatment system). I argue that it wouldn't have mattered, there are noble gasses that do not hold up in charcoal adsorpers or hepa filters which still dictates you delay the release, because the noble gas inventory is a large contributor to the early release.

I'd thought the noble gases were not scrubbed from either nuclear plants or nuclear fuel processing facilities. Is this incorrect?
The document here: http://www.areva.com/EN/operations-2332/gaseous-releases-annual-statement.html suggests the release by the Areva La Hague processing facility is about 280,000 Terabequerels of noble gases, 3,000 times as much as the tritium and C14 releases.