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radioactive_dude t1_j5v61b5 wrote

It depends what you mean by coming out of the plant. In most nuclear reactor designs, the fuel is cooled by water under pressure being pumped over it. By the time the water exits the core, it is typically something over 300C and 15 MPa, but under the critical point of water at that given pressure.

This water is not used to drive the steam turbine directly. It goes through a heat exchanger to heat a secondary loop of water to create steam for the turbine. The steam needs to be condensed back to water in order to be reused. Some reactors use bodies of water, some use cooling towers. The steam coming out of cooling towers is considerably colder at less than 50C and at atmospheric pressure.

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MTGamer t1_j5vapv3 wrote

Wait, can steam exist at 50C, 1atm? Doesn't it have to be at least 100C to not just be a liquid?

Or in this case is it not really steam and just water vapor like breath in the winter?

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Ausoge t1_j5vy30m wrote

"Steam" is evaporated water - that is, water in gas form. It is colourless and invisible. What you see as a steam cloud is actually cooled, recondensed, liquid water droplets.

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BuffaloBoyHowdy t1_j5wfkzm wrote

This. You can't really see steam itself. All that white stuff is water vapor. Look closely at a kettle with boiling water. As the mist comes out of the spout, there should be a clear space at the opening, that's the steam. It quickly condenses into water vapor, which is what you see. Don't stick your finger in it.

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ramriot t1_j5x57pm wrote

Yup, this is why Live Steam is so damn dangerous. If a high pressure steam line develops a crack the escaping steam can sound like a banshee on heat but is also completely invisible. The jet is just hanging out there waiting to slice off your limbs or scald you do death.

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Rangoras t1_j5yyang wrote

If we ever had a super heated steam leak in the engine room we were trained to grab a broom or other stick and wave it in front of you from floor to above your head while moving slowly. When the broom was cut in half by the invisible steam jet you found the general leak location.

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unclepaprika t1_j5wiewo wrote

2 steps forward, 1 step back. Didn't we just establish that steam IS water vapor?

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lurkandpounce t1_j5ym84p wrote

Steam is water in a gaseous form and invisible.

Water vapor is made up of water droplets that are so light they are carried by the air currents.

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BuffaloBoyHowdy t1_j5zc5xr wrote

Steam is water vapor; water vapor is not steam. A square is a rectangle; a rectangle is not a square. Both are parallelagrams; but a parallelagram isn't a square or a rectangle. A thumb is a finger but a finger is not a thumb. Things are just like that sometimes.

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[deleted] t1_j5wmz6o wrote

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TrappedInASkinnerBox t1_j5wy37c wrote

On the engineering side at least the gaseous phase is definitely referred to as "steam" not "water vapor"

Saturated steam, superheated steam, etc

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mightyn0mad t1_j5x4ezm wrote

Seconded. Steam is steam, we do not call it water vapour. Depending on conditions it can be saturated steam (the one that looks white) or superheated steam (the colourless one)

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Angdrambor t1_j5wob3l wrote

>Steam is droplets of liquid water suspended in the air which appears white due to light scattering.
>
>Vapor is colorless and transparent. You cannot see it.

When I took thermo in my country, we refered to it the other way around. Steam is the invisible gas phase, "vapor" is the white cloud of liquid droplets.

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lichlord t1_j5wrrfh wrote

Was it thermo taught by scientists or engineers?

Thermo in chemistry and physics usually simplifies steam into gaseous water.

In engineering thermo courses the focus is less in phases and equations of state, and more on work and transformations. Engineering thermo will often distinguish between wet, dry, and superheated steam.

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CYWNightmare t1_j5x2odw wrote

I thought rainbows were water vapour with light hitting it so wouldn't I see water vapour to an extent?

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Tarhish t1_j5x63jz wrote

I got started at a nuclear plant as a computer/instrumentation guy, and was a little surprised at first by the term, 'dry steam' that got thrown about, though it makes perfect sense in retrospect.

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Become_The_Villain t1_j5z553g wrote

>got started at a nuclear plant as a computer/instrumentation guy

Homer Simpson that you?

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[deleted] t1_j5yw5hf wrote

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Rangoras t1_j5yxvzt wrote

Sea water needs to be desalinated before being used in a boiler. If you only just filtered it for solids the salts in the water would form a nasty layer of scale on the water side of the boiler tubes quickly resulting in poor heat transfer and failure of the boiler tubes. When we make our boiler water on my ships we only use water where salinity is under 5 PPM

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crujones43 t1_j606k33 wrote

Even the highly filtered lake water has enough particulate to build up calcium and sludge inside the boilers after only 2 or so years. I have been on a few water lancing jobs where we install and operate remote tooling to reach between the hundreds of boiler tubes and use a 9000psi water jet to break all the sludge and calcium up.

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[deleted] t1_j5vbsyx wrote

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graebot t1_j5yj1aq wrote

Clouds are water vapour, but I wouldn't refer to them as steam. Technically, steam is invisible, and exists above 100 C at 1 atmosphere. Only once it drops below the vapour point (100C @ 1 Atm) does it start condensing into water vapour. Water vapour is not steam, it's just liquid water droplets suspended in air.
When steam is used to do work, it starts as high pressure, high temperature, and as it does work, the pressure reduces, and the temperature with it, and after doing work you're usually left with water vapor as the spent product.

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blanchasaur t1_j5yrprq wrote

No, that's not quite right. Water vapor is definitely a gas, you can't see humidity. Clouds are a suspension of liquid water in the air, not water vapor.

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bigbaltic t1_j5vg5s3 wrote

What you see coming out from stacks is not steam. It's more like a cloud of extremely humid air

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braize6 t1_j5vy5fo wrote

In power plants, water is under pressure. Increasing the pressure, also increases the boiling point. The water at my plant is around 800 degrees Fahrenheit, yet still a liquid. We do this through a series of enclosed heaters, which actually increases pressure as steam is released, which in turn makes the steam condense back to water at a higher temperature. Our plant does this 3 times. Which then the water goes into the boiler. This process is called superheating. Or known as "superheated steam."

Our throttle pressure is 3500psi. So you can imagine just how hot our water can get before it turns into steam (there's a chart out there if you're curious about pressure and temperature of liquid and steam.) Now let's add that water expands about 1600 times from liquid to steam. The result, is a massive amount of energy.

Edit- I appreciate the feedback, and yes, there are many different processes that are going on, in order to generate power, and the boiler process. I'm trying to simplify, because to explain the entire process I'd need about 50 more pages, as it seems many here also know.

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racer_24_4evr t1_j5wyztb wrote

Superheating is when you take steam that is at the boiling point for the given steam pressure, and heat it further. I believe what you described is preheating feedwater.

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Coomb t1_j5xc81d wrote

More likely it's a combination of preheat/superheat/reheat. If you follow a little packet of water through a plant with feedwater preheat, then superheat, then reheat, you get "three times" through the heater. But the description does leave something to be desired.

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racer_24_4evr t1_j5y59ny wrote

Yeah, in our plant, we have three boiler drums, each with preheating and superheating, as well as a reheat on exhaust steam from the high pressure side of the turbine, so there’s a lot of heat moving.

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braize6 t1_j5y905y wrote

Yes it leaves a lot to be desired, because well, there is. The entire process would take me the rest of the month to specifically explain. If someone is interested in all of that, the info is out there

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braize6 t1_j5y8vnx wrote

Basically, yes. With the result of the preheated feedwater going to the water walls, to the economizer, to the superheater to the..... Honestly I could go on for a while here as you seem to also know lol

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racer_24_4evr t1_j5yc8xx wrote

It’s cool to talk about, people don’t realize how much goes into operating a large high pressure version of a kettle.

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Angdrambor t1_j5wnycf wrote

What comes out of the towers, if it's visible, is water droplets suspended in air. Steam is clear in air, but it partially condenses into visible clouds of droplets if it cools down too much.

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lemlurker t1_j5ybumj wrote

It's not steam by the time it exits the tower. It's condensation like clouds

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DrunkenGolfer t1_j5ws4fu wrote

The boiling point is just the point at which a liquid can no longer remain a liquid. It can become a vapor at much lower temps, below freezing even.

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Old_comfy_shoes t1_j5za47o wrote

If you have some water and it touches something very hot, the water can't exceed 100C, because as soon as it does, it becomes steam. But the steam can be crazy hot. So with enough temperature difference, if you plunge something crazy hot into water, the vapour won't instantly escape, and the heat transfer could be very rapid creating larger pockets of really hot steam, only the outside of which would be cooled by the water. So, the steam can be much hotter. But the water must stay at 100C in order to be water.

Steam, should always be hotter than 100C, which is why it converted to steam. It's essentially water that is hotter than 100C at 1 ATM, by definition. But usually it would only be a little bit hotter in conventional use. 300C seems like a lot though. Idk anything about reactors, but I think they plunge massive insanely hot rods into water?

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Seraph062 t1_j63qqh0 wrote

> But usually it would only be a little bit hotter in conventional use. 300C seems like a lot though. Idk anything about reactors, but I think they plunge massive insanely hot rods into water?

You generally want steam for turbines to be pretty hot, the ones I'm familiar with usually run about 500°C. There are a few reasons for this, hotter steam is generally easier (and more efficient) to extract energy from, it's also better for the turbines because droplets can cause damage and higher steam temperatures help avoid droplet formation.

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Flannelot t1_j5xnts5 wrote

> This water is not used to drive the steam turbine directly.

Just to clarify, in Pressurised Water Reactors this is true, but in Boiling Water Reactors the steam forms directly in the reactor vessel and goes straight to the turbine.

This results in two different control strategies. Both are still popular designs.

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Krail t1_j61z616 wrote

So, to make sure I'm understanding this correctly - One form of reactor has superheated water kept under pressure so it won't boil, and that superheated water heats another body of water that does boil, and then a different type of reactor directly boils the water without that intermediary step?

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MacDugin t1_j5x26up wrote

So the correct question would be how hot does the steam have to be to run a steam turbine.

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Branchdressing t1_j5wycbu wrote

Good explanation though technically a dirty boiler does pass irradiated steam to the turbine. Such as those at River bend station in Louisiana!

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CoreMemory_ t1_j5x2o5y wrote

The only notable exception in commercial operation is probably the British AGR (Advanced Gas-cooled Reactor) fleet which uses carbon dioxide as the coolant, circulating at approx 640 °C (1184 °F) which goes through a heat exchanger that’s really more like a traditional power station boiler in many respects. It raises super heated steam, much more like the temperature profile seen in a conventional gas, coal or oil burning plant.

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colt61986 t1_j5yu5ll wrote

You seem to be someone to ask this question. Is it possible that the boiler feed water is filtered sea water? I know in Saudi Arabia they use oil fired power plants to also desalinate and generate electricity but I’ve always wondered if there was technical reason why this was never done with a nuclear plant. From my limited work in coal fired power plant that the boiler feed water is on a loop and economized ,and to use the water that was desalinated by the steam generation for public consumption would make it a 100% make up system which would lead to a much heavier filtering/chemical treatment process and there would have to be engineered process to clear the debris from the bottom of the boiler system but it seems like this would be a great solution to some water problems in arid/desert places that are close to an ocean like Texas maybe. It’s an answer I’ve been looking for for years and nobody has ever given me an answer.

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radioactive_dude t1_j5z03ka wrote

The boiler side of reactors is not my area of expertise, but I'll try to answer your question. My understanding is the water that is used to make steam for the turbine is a closed loop because the water chemistry and steam quality is very important to safely operating the fast spinning turbines with many blades. For that reason, the water needs to be fresh water.

For desalination, my understanding is there is still enough heat in the steam after it has run through the turbines that it can still boil water, but is not useful for generating further electricity. It is a way to use this waste heat. I believe such systems completely separate the turbine loop water from the desalination water. Essentially, it is seawater sprayed on hot piping to create fresh water steam. The turbine water (downstream of the turbines) flows through the piping in a closed loop to keep it hot.

As for desalination at nuclear plants, there is experience in Kazakhstan and Japan in doing so. I don't know why it hasn't seen more widespread use. I am guessing the need is not that great as lots of reactors are located near large bodies of fresh water by design. The nuclear physics in a reactor can also be complicated by changing temperature, so complicating the heat removal system is probably avoided unless it is a specific requirement.

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colt61986 t1_j5zc9af wrote

Thank you so much for the clarification. It’s been a question of mine for 20+ years. The most important part of your answer, to me, is that it can be done. I’m guessing the reason that it isn’t has more to do with nuclear energy fear than engineering. I’d imagine Arizona, California, and the Colorado river would greatly appreciate a couple of nuclear powered desalination/power generation plants on the Texas gulf coast piping in as much fresh water as they can make west to the desert, where people apparently could never have foreseen a water shortage…..in the desert.

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Ricksterdinium t1_j5z9nen wrote

Is that because the water molecules are irradiated? And the turbine needs to vent into atmosphere?

As in the cooling water heats up the turbine water?

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crujones43 t1_j60a3qe wrote

The turbines don't vent unless they are throttling down or in an emergency shutdown and would never do it if the system uses water that came from the reactor. They run closed loop systems where the reactor heat transport fluid (d2o in the candu reactors I work on. But can be regular water) this system circulates between the reactor and a heat exchanger. The heat exchanger passes the thermal energy into regular water (unless it is the type that feeds direct to the turbine but I have no experience with that type) regular water is then turned to steam and heads to the turbines. There is normally a high pressure turbine which is the smallest, after the steam passes through that there is still lots of energy in the steam so it is sent to a drying system (I have never worked on that so I don't know exactly what it does) and then the steam comes back into the low pressure turbines. The ones I work on have 1 hp and 3 lp turbines. The 2 steam pipes feeding into the hp are about 2.5ft in diameter and carry a total of about 1.6 million hp of energy. From the low pressure turbines the steam goes to a condenser. It is just another heat exchanger where they pump cold water through tubes surrounded by the steam. The steam condenses and drops to the bottom where it cycles back via pumps to the first heat exchanger to be turned back to steam in a closed loop. The cooling water gets warm from the exchange and gets pumped into cooling ponds were it can be released back into the lake once it reaches a certain temperature. If you see a cooling tower like in the Simpsons. That is like having an air cooled car vs a water cooled car. The steam goes to the inside of the tower in tubes. It stays in its closed loop but warms the air up. The warm air rises causing cooler air to be sucked in the bottom. The steam you see rising is not the steam from the turbines, it is from the air changing temperature.

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Ricksterdinium t1_j60hey7 wrote

I see, but then the coolers after the turbines Create a vacuum Effect so as to not let the turbines stagnate.

I was thinking that there had to be less pressure after the turbines for it to circulate.

Thanks 🙏

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crujones43 t1_j60sevh wrote

The condensers are massive. Many times larger than the turbines themselves. The water in the bottom is quickly pumped out maintaining low pressure. Steam is 1600 times more volume than the same amount of water. All this keeps the turbines from feeling any back pressure.

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Redwoo t1_j5wlw9v wrote

Steam is clear and colorless and is water vapor…water in gaseous form. Water vapor that condenses from its gaseous phase to its water phase is called condensed water vapor, or fluffy white clouds. Condensed water vapor consists of a multitude of tiny droplets of liquid water. Condensed water vapor converts to steam, invisible gaseous water vapor, if it is heated above 100 C.

The steam that enters a turbine at a nuclear plant is around 800 degrees F, depending on whether it is a pressurized or boiling water reactor. The white fluffy clouds of water droplets coming from the cooling towers at some nuclear plants is within several degrees of atmospheric temperature.

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ZLH-040 t1_j5z1wwb wrote

Reactors use a closed loop system that is prevented from escaping into the environment. There's a heat exchange process so clean outside water gets heat from the water exposed to the reactor.

After that, the clean water in the reactor turns the generator turbines just like gas or coal plants would. The steam escaping the cool towers is clean, non-radioactive water.

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TheKvothe96 t1_j5ypmph wrote

Temperature of the steam is not that important because it depends on the generator system each nuclear central has. Maybe a bigger nuclear central has lower steam temperature but higher flow so the important point is how many energy is generated in the reactor.

Even that number is not important because it depends again on the generator system. So the data to compare nuclear plants is not how many heat they generate but how many electricity they do generatei in a year (maintenance stops can be quite long).

Largest nuclear plant in 2019: "Tokyo Electric Power Co.’s (TEPCO) Kashiwazaki-Kariwa plant in Japan is currently the world’s largest nuclear power plant, with a net capacity of 7,965MW".

https://www.power-technology.com/features/feature-largest-nuclear-power-plants-world/

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Busterwasmycat t1_j5z9ok9 wrote

It isn't steam, it is condensed water vapor in air (fog or a cloud). The temperature depends on a few things but generally somewhere around 35 degrees C based on info I just looked up. The water being pushed out of the tower is not water from the steam-generation system used to power the electric turbines. That steam is not usually released except in emergency (the steam is recovered as liquid and reheated to make new steam, in a big circle of use).

Instead, what happens is that the used steam from driving a turbine (now only hot water) is pumped to a radiator of sorts (some sort of closed unit that has air pass over it) located at the base of the tower. Air passing over this radiator heats up (takes heat form the hot water in the radiator, cooling that water more). the cool water is sent back to be made back into steam.

To help cool this air taking the heat from the "radiator", a spray of cold water is typically used. This spray is released near the top of the tower and descends down over the upflowing hot air (cold water is used as a counter flow to the rising hot air to help cool it faster). The water is usually taken from a nearby river or other similar water body. it is "fresh" water, different water from the water used by the electricity-generating system.

Because of this water spray, the hot air gets saturated (maximum humidity), and because maximum humidity decreases as temperature of the air drops, the water vapor condenses into droplets, making a fog or a cloud. This is what we see leaving the plant tower. The water in the discharge fog is not radioactive and it is not all that hot.

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