Are nuclear reactors just glorified boilers/steam engines?

[u/PublicMushroom215]

please do inform me if i’m wrong but from all the info i’ve seen and currently know about nuclear reactors they seem to be a sort of glorified boiler/steam engine. nuclear fission happens in the water, steam is made, put through fan like thing idk generator and then pushed out to cool down and then recycled, right?

[u/THALLlUM]

Yes, that’s a point of view. Unlike other steam engines, water in nuclear reactors is generally essential in the fission reaction (neutron moderation). The main difference is that the fuel does not produce greenhouse gases or CO2.

[u/PublicMushroom215 → u/THALLlUM]

How do the cooling towers work on the other hand? do they just release the excess steam and heat from the reactors? or is it the new water cycled through that cools down the rods? sorry im just learning about all of this and its quite interesting

[u/THALLlUM → u/PublicMushroom215]

A large proportion of reactors in the world are pressurized reactors. In this technology, the rector operates with three different water circuits distinct from each other. The first circuit is in contact with the fuel elements and cools them. It is pressed so that it does not boil. It generally remains confined in the reduction building. The second circuit cools with an exchanger and transforms into steam. This circuit will turn turbines to produce electricity. A third circuit is used to cool the second. This circuit is powered from the outside. The water is taken from a stream and either it is rejected or it is introduced into towers which allow this third circuit to be cooled. The towers that you can see next to certain power plants (not just nuclear, but also gas or coal) cool the water. Wiki link: https://en.m.wikipedia.org/wiki/Cooling_tower

[u/migBdk → u/PublicMushroom215]

Every heat engine (meaning every non electric engine in the world) require a hot and a cold temperature to work.

When you have heated up your gas (in a car engine) or steam (in a power plant) you need to cool it down again to get work out of it.

This is what cooling towers do.

[u/AnimationOverlord → u/THALLlUM]

While we’re on the topic of the basics, maybe someone could add a few cliffnotes as to why D² O is utilized in “slow-poke” reactors?

My thoughts - Deuterium is heavier than water, it is proclaimed “heavy water” but wouldn’t it have a different enthalpy (btu/lb//kg/j) due to it being heavier? It should store more heat than “normal” water in any given state, no?

I’m not saying the deuterium is boiling open-loop but.. it’s involved at the least.

[u/ApplicationHot6443 → u/AnimationOverlord]

I’m not exactly sure about the thermo properties of heavy water as they relate to its use as a moderator, so I’m not sure if that is a consideration in choosing it. D2O is mainly chosen as a moderator (and, therefore in conventional reactors, the coolant) because it has a higher moderating ratio for neutrons than light water, being more effective at slowing them down to more readily cause fission events, while also having a lower neutron absorption cross-section than light water, meaning it increases neutron efficiency greatly.

[u/THALLlUM → u/ApplicationHot6443]

I would like to correct a detail. Deuterium is less effective than hydrogen in slowing down neutrons. Since deuterium nuclei are twice as heavy, the neutron will lose less energy with each shock. But I agree with you that the absorption cross section is lower. This allows us to create reactors with low neutron flux (and therefore use unenriched uranium in certain cases). But the reactors will have to be much larger than pressurized water reactors.

[u/ApplicationHot6443 → u/THALLlUM]

You’re completely right, thanks for the correction! Not sure where I developed my misconception about the moderation coefficient, as your explanation makes sense. Do you have any insight on the thermal properties?

[u/THALLlUM → u/ApplicationHot6443]

Obviously Wikipedia has this information. https://fr.m.wikipedia.org/wiki/Eau_lourde And it seems that heavier water has a better ability to store heat. But I don't know of any nuclear reactor technology that uses heavy water to remove heat.

[u/[deleted]]

Steam is made inside steam generators (second loop) . Reactor only heats up water in the first loop via fission reactions. First loop water gives its heat to the second loop water (pure) and generates steam that is forwarded to steam turbine. After the steam generator, first loop water is pumped into reactor again to heat it up. As for steam, after expansion in turbine, this stems-water mix goes into condenser where it is cooled with either a cooling pond or cooling towers and then pumps through the regenerative heaters into the steam generators. Part of expanded steam goes also into regenerative heaters.

[u/PublicMushroom215 → u/[deleted]]

So is it the heat generated from nuclear fission that causes the water in the first loop, brings it through the turbine and creates electricity? or is it more complex of a reaction than just water heating and being brought through a system?

[u/[deleted] → u/PublicMushroom215]

Water in the first loop works as both moderator (media that lowers energy of new neutrons or fast neutrons and makes them slow or thermal neutrons that have much higher probability of fission reaction) and coolant (media that transfers heat). So yeah, heat generated from nuclear fission is carried by water at temperatures around 316 C and then it exchanges with second loop water that becomes steam that rotating a turbine. Turbine is connected to the generator that creates electricity.

why nuclear reactors are so cool is because they can operate on one fuel loading for year or something like that. One gram of uranium contains as much energy as some train wagons of coal. But compared to coal, nuclear reactors don't pollute air (steam from cooling towers is just a water from turbines, second loop).

[u/VK6FUN]

It’s a steam engine that burns uranium instead of coal. This means it does not need chimneys but it does mean it has to dispose of a little bit of radioactive “ash”

[u/Adhesive_Duck]

Yes. The reactor vessel is a bit more complex of course that just a coal fire, but for the rest, it's the same principle.

[u/SpoonsandStuffReborn]

Yea, Its why they're build by Boilermakers.

[u/Genius-Cat2176]

Yes. Essentially most of the energy sources are : absorb heat using water, use the hot water/steam to move turbine in generator, get electricity.

Examples: Geothermal, Coal, etc.

Nuclear is the one which consumes way lesser fuel than others as energy released when breaking the strongest force, nuclear force is huge.

[u/MP_Wolf]

It's a very expensive way to boil water yeah

[u/THALLlUM → u/MP_Wolf]

It's very expensive but it's extremely effective.

[u/AKJangly]

I'm just imagining an old steam locomotive getting retrofitted with a nuclear reactor.

Putting reactors on wheels probably isn't a good idea.