Key context: thermodynamics dictates that heat engines waste a lot of energy.
Credit: Karin Kirk
this post was submitted on 11 Nov 2024
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How much of your EV charging money goes out the power plant smokestack, into the river/cooling tower, or heats up the air around the electrical wiring though?
i’m a proponent of EV’s, even when they charge from fossil powered grids, because of the thermodynamic efficiency gain.
But ragememes, no. Let’s not be like that.
In most places, at most times of day, a lot less.
Why? First, because a lot of electricity is generated using wind, water, solar, and nuclear. Those don't have that problem (ok, nuclear wastes a lot of heat, but really, who cares). The second reason is that power plants that burn stuff tend to be a lot more efficient than internal combustion engines; the best case is combined-cycle gas turbine power plants, which turn over 60% of the energy available into electricity, as compared with a gasoline engine which turns about 20% of the energy in the gas into motion.
So this made me wonder: How do nuclear plants produce the heat? Like, I know they're using nuclear materials to boil water and generate steam to turn turbines, but how is that accomplished? Are the fuel rods just naturally hot (in terms of thermals not just the radiation) or are they running current through them to make them hot enough to boil water? I always assumed the former, but maybe I've been wrong this whole time.
The reaction for which a nuclear reactor is named is the atoms of unstable substances rupturing on a subatomic level.
Every substance is made of atoms.
Atoms that share the same number of protons in their nucleus are the same element. The protons are all 'positively' charged and want to repel each other and fly apart, but they cannot because neutrons got them stuck together. The combined positive charge of the neutrons, though, attracts and captures electrons (which are negatively charged) in their orbit.
Sidebar: it is the interactions between the electron shells of atoms that allow atoms to stick together to form molecules. For instance, water is one hydrogen atom and two oxygen atoms.
Atoms with one proton in the middle are hydrogen. Atoms with two protons are helium. Atoms with three are lithium, beryllium has 4, boron has 5, carbon has 6, atoms with seven protons are nitrogen, atoms with eight protons are oxygen. And so on. The entire list of all known atoms is the periodic table of elements, and the atomic number of each element is how many protons it has in its nucleus.
Another sidebar: atoms can sometimes have an extra electron, or be missing an electron. These are "negative" and "positive" ions. Lithium ion batteries, for instance, operate on a principle of chemical reactions that can store extra electrons when charged, and strip those electrons off and release them when discharged.
Less of a sidebar because this bit is getting relevant to nuclear/atomic energy: atoms can have a varying number of neutrons too. Hydrogen only has one proton so it doesn't even necessarily NEED a neutron. If it has a neutron, it is significantly heavier than a hydrogen atom that doesn't have a neutron, and we call it deuterium. It can even have TWO neutrons, and be nearly three times as heavy as a result of the extra particle, and we call it tritium. the varying numbers of neutrons in an atom's nucleus are isotopes of an element.
Recap:
Now I can finally tell you what nuclear fission and nuclear fusion are about.
Fusion is when atoms (usually very light ones) under titanic, gargantuan, nigh incomprehensible pressure are forced together so close, under so much force that it overcomes the negative-to-negative electrostatic repulsion of their electron shells, that the nuclei of the atoms get close enough that they suddenly stick together, merging their assemblages of neutrons and protons into a single nucleus and the electrons all sharing that orbit.
Very light atoms such as hydrogen and helium can have an easier time fusing if there are more neutrons present in their nuclei, assisting with the 'stickiness' (not a technical term) of each atom's nucleus to stick to each other. When we do fusion here on earth, we can't achieve the pressures necessary for regular hydrogen or helium to fuse, so we use deuterium or tritium to do it instead.
Meanwhile, Fission is when atoms (usually very heavy ones with lots of extra neutrons) break apart. Isotopes of very heavy elements with abnormally high numbers of neutrons behave differently from their more stablely balanced 'not too many neutrons' related isotopes. The nucleus can become 'unstable' and prone to breaking. You could imagine this, metaphorically speaking, as a physics engine that's having to deal with too many rigidbody collisions between too many objects in a tight space, with the objects clipping into each other and building up incredible amounts of un-accounted-for forces which, when crossing an escape threshold, cause the pile of objects to break apart.
If you have a relatively stable isotope that will become a very UNSTABLE one if you just add another neutron, then you can cause it to break apart (fission) by shooting a neutron at it. And actually hitting. Now, if you have a whole crapton of these relatively stable atomic isotopes collected together (refined into nuclear fuel), you can shoot a neutron at that blob of atoms and statistically ONE of them is gonna get hit with that neutron and break apart.
When an atom breaks apart, it basically explodes very fast and that's a lot of kinetic energy. Kinetic energy on an atomic level, well, it hits other atoms which hit other atoms and they all vibrate and that's what we call heat.
But that's not all. When the atom breaks, it will release extra neutrons that it can no longer hold onto. IF it releases more than 2.1 neutrons on average when it breaks, those two neutrons will go flying off and statistically at least one of them will hit another atom of the same substance, the same isotope, with the same 'just on the cusp of blowing apart' situation, causing IT to fission too, and ALSO shoot off a few neutrons. Those also hit barely stable atoms that become unstable and fission releasing neutrons which then destabilize other atoms which fission and shoot off neutrons which then fission other atoms that fission other atoms... This is called criticality and it's the tipping point at which a nuclear fission reaction can sustain itself.
In order to sustain this reaction, we build a structure that we put the fissile fuel into, a structure specifically designed--with specific materials specifically shaped--to reflect the neutrons back into the fuel so that the reaction can keep going. This is a nuclear reactor core. By inserting substances, meanwhile, that will absorb neutrons and slow the effect down OR by withdrawing the fuel rods from the 'sweet spot' in the reactor core, we can control the intensity of the reaction so it doesn't blow up EVEN BIGGER, and therefore we call these Control Rods.
And that's the essential fissile chain-reaction that is core to the operation of a nuclear power plant. Every single one of those fissioning atoms releases a bunch of heat and that heat adds up. A thermal transfer fluid of some kind surrounding the core will absorb allllll that heat, and carry it to a heat exchanger that dumps all that heat into yet another working fluid, this one whose job is to boil FURIOUSLY when it gets hot enough and generate a crapton of vapor pressure, which then is allowed to blow through and thereby push turbines.
That's fission nuclear reactor power!
Bro, one of us is confused
HOLY SHIT FUCK YOURE ABSOLUTELY RIGHT JESUS CHRIST I REMEMBERED THE ORDER COMPLETELY FUCKING WRONG
I should have looked up the periodic table to reference it >_< It goes Hydrogen, Helium, LITHIUM, BERYLLIUM, BORON, CARBON, NITROGEN, and THEN oxygen Fuck my life I skipped five whole fucking elements Goddammit THANK YOU for pointing this out, I will fix it IMMEDIATELY
Holy exposition, Batman!
The fuel becomes hot because the nuclear reaction in it is producing both light (eg: gamma rays) and fast-moving subatomic particles. These both interact with the rest of the fuel to heat it up.
It's natural radioactive decay producing heat except it is artificially concentrated and accelerated.
If left in nature they would not produce anything near the heat they produce when we mess with them
The atoms of plutonium or whatever break down into other atoms. This process is called fission. When it breaks down it also lets neutrons loose, which then attach to other plutonium atome, destabilizing it, then the next one and so on in a chain reaction that produces heat. That heat is transferred to water, which the eventually powers steam turbines. Electricity does not come directly from the rods.
This whole process is regulated via neutrons(or maybe protons, not 100% on that). Put the rods of plutonium closer together, get more fission, more heat. I think they use graphene to regulate things too.
The gist of it is that the fuel rods are designed in such a way as to maintain about the right level of neutron emission, with further refinement using a neutron moderator* (which slows down neutrons and this increases the reaction rate, this is generally water or graphene) along with adjustable control rods, which can be inserted to slow down the reaction.
There are two kinds of decay leading to neutron emission: prompt neutrons are emitted immediately, while delayed neutrons take time to be emitted because of the decay path that the excited atom has to take. In order to maintain control of a reactor, the number of prompt neutrons must be lower than the level needed to reach criticality, with the additional delayed neutrons being enough to push it over the edge. The delay is what gives you time to control the reaction; if the reactor becomes prompt critical, then it begins to melt down.
*Not all reactor designs use neutron moderators, but fast reactor designs are generally military or research reactors, while moderated ('thermal') reactors are typical for civilian power generation. Here in Canada, we use the CANDU reactor design, which is moderated using heavy water
It's basically the former: the radiation given off by the atoms breaking apart triggers more fission when it bumps into other unstable atoms, which just keeps compounding if you have a bunch of fuel in one place until it gets REALLY hot and melts.
To prevent the meltdown you put control rods throughout the fuel which absorb the excess radiation and keep the fuel at useful temperature, roughly like how a fuse is built to burn at a consistent speed rather than simply exploding.