I know (I think) that in the simplest, most basic terms a nuclear reactor is essentially just a big steam engine. However that’s about all I know (and I could be wrong).
Pretend I’m an idiot (which isn’t hard; I am an idiot ;)):
What exactly does “nuclear meltdown” mean?
Barring direct sabotage/attack, what’s the worst possible thing that could happen at a nuclear reactor (in respect to the general public)?
The nuclear reaction produces an tremendous amount of heat in a very small area. If that heat isn’t removed fast enough, the fuel rods and support structure can get hot enough to melt, which would create a big, glowing blob of radioactive material at the bottom of the reactor. Even if it didn’t get to that stage, it could get hot enough to warp components which would wreck the reactor core. The worst possible thing would be a “China Syndrome,” where the entire core loses all coolant, the reaction can’t be stopped, and the core melts into an uncontrollable white-hot blob, which then melts through the bottom of the containment building. Once this mass hits groundwater, enormous amounts of deadly radioactive steam would spew uncontrolled into the air. Not a good thing at all.
Nuclear fission reactors generate heat by nuclear “chain reactions”. The nucleus of atom A breaks apart, releasing energy and atomic fragments that break apart the nuclei of atoms B and C, which release more energy and more fragments that break the nuclei of atoms D, E, F, and G, and so on.
If it went on like this, eventually the whole thing would have heat up and melt or have some sort of explosion, so they insert “moderators” that absorb excess fragments and keep the reaction going at a steady level. It’s a bit of a balancing act: you don’t want too much moderation, so that the reaction goes out, but you don’t want too little moderation either, or the thing melts or goes up in flames. It’s kind of like running a message board. 
The reactor is also cooled by circulating some sort of fluid around it. This coolant serves to supply the heat that the plant was built to generate: heat that warms things directly or turns turbines that generate electric power.
In some reactor designs, if the coolant leaks away, the reaction continues, and, without adding moderation to slow the nuclear reaction down, the reactor melts. In other designs, the coolant is also necessary to sustain the reaction, and if it leaks away, the reaction stops.
So, a “meltdown” happens when the nuclear reaction continues without adequate cooling.
BTW, I have a somewhat perverse fascination with these scenarios. In the early days of reactor design, the cores were not engineered with the many of the fail-safe (or at least fail-less-catastrophically) features of current reactors. One experimental reactor, the SL1, failed (for reasons that will never be know precisely, but probably because of really bad design and human error) in such a way that it produced many, many times it’s rated power in a few thousandths of a second. This basically turned it into a “steam bomb” - it overheated so fast that it shot the reactor core out of the housing like a ball out of a canon.
I’m kind of fascinated by this as well, not least because I grew up in the vicinity of two large nuclear plants.
Power generation historically has been all about making steam (these days with wind and solar and hydro power that is not always the case but by far most power generation today is from boiling water).
In coal, gas and nuclear power plants the goal is to boil water into steam. Steam then is pushed through a turbine which turns the generators.
Obviously to make steam you need heat and coal, gas and nuclear processes all do that.
In the case of a nuclear reactor you have fuel assemblies.
These consist of fuel elements and gaps between them for control rods.
A nuclear reactor is, roughly, a controlled nuclear explosion (not quite right but cannot think of a better way to put it). The same principles are at work where a chain reaction produces energy.
In a nuclear bomb this reaction is very fast and releases massive energy in a very short time. That gets you incredible destructive power.
In a power plant you have the same reaction occurring but you are controlling it. Rather than a fast, near instant burst of energy you moderate the reaction to a manageable level. This produces a lot of heat and boils water that spins turbines.
To moderate the reaction the reactor has control rods. These are made of things like boron or graphite. In a fission reaction neutrons are produced which fly off and split other atoms which produce more neutrons that do the same (and so on). That is the chain reaction for a bomb. In a reactor the control rods absorb these neutrons thus controlling the reaction.
As mentioned all this produces copious amounts of heat. That heat needs to be managed. It is quite capable of exceeding the limits of the materials in the reactor taking them beyond their melting points.
In a melt down the reactor loses the ability to cool itself. Eventually the heat can rise enough to cause the fuel assemblies to melt. Once they are molten their properties to continue a reaction remain but there is no way to moderate the reaction. This is your “China Syndrome” worst case scenario (although for the reactor in Japan it’d be the “Argentina (or Uruguay) Syndrome”).
That is your melt down and it should be noted it does not have to be as bad as the China Syndrome or Chernobyl. It is still serious and I think the reactor is done for but the containment building should keep the worst of it in control without a disaster.
It should also be noted that there are designs now for “inherently safe” reactors that cannot do this. In those physics itself moderates the reactions so it is literally impossible for them to melt down…even if you tried you could not make them do it. The reactors we deal with today are often 20+ years old and older designs. Inherently safe reactors also have other issues (operation/cost/politics) so they have not been built.
Best I have seen lately is a Traveling Wave reactor. Hope politicians let it be built.
I should say this may be wrong.
I have been tutored here on the SDMB on how control rods work. I have forgotten most of it but have a vague recollection that “absorbing” neutrons is not what is happening.
Honestly I have almost completely forgotten how control rods do their thing except a vague sense that it is counter-intuitive to the layman (makes sense once you understand how it all works).
For this thread suffice it to say the control rods moderate the reaction. Put them in it slows down, take them out it speeds up.
Aren’t gas-cooled reactors (the kind the British like) inherently safer? They have graphite cores, that can withstand tremedous heat. Plus, there is no water involved, so no steam explosion possible.
Are the gas cooled reactors much safer than water-colled designs?
Quoth Whack-a-Mole:
This is misleading, since it seems to imply that if you fail to control the reaction in a power plant, it’ll blow up like a bomb. Actually, a bomb requires fuel that’s refined to a much greater purity than what’s used in a power plant, so even if everything goes wrong, what you get still isn’t a bomb: It might make a big radiological mess, but it won’t explode (at least, not any more than any overheated steam boiler).
It should be noted, by the way, that Three Mile Island was a worst case scenario for that design, and it still didn’t cause any deaths directly, and estimated at a single-digit death toll from increased cancer rates. Chernobyl, meanwhile, represented a truly ingenious piece of engineering, in that it would be very hard to come up with a design that was more dangerous, or which would cause greater devastation when it failed. First-world reactors could never even come close to Chernobyl if we tried.
It should also be noted that Three Mile Island was the second-worst radiological disaster in the US. The worst wasn’t caused by nuclear power at all, but by coal power. The coal ash spill in Tennessee a couple of years ago released significantly more radioactive material than did Three Mile Island (in addition to all of the other problems it caused).
Don’t forget the interesting part. One of the control rods (the type of rod differs by source) shot out vertically from the reactor and impaled a guy to the ceiling of the building. He an the two other fatalities were buried “in lead-lined caskets sealed with concrete and placed in metal vaults with a concrete cover.”(from Wikipedia)
Yea, if it goes badly in Japan, we probably won’t see another nuclear reactor built here for another 30 years. Politicians operate on knee-jerk reactions. They don’t let pesky facts get in the way of a good scare tactic.
Good point and worth noting.
Sorry if I insinuated that a nuclear reactor is really a nuclear bomb waiting to happen.
One factor overlooked by many is decay heat. Once you shut down, the reactor continues producing substantial heat for some time. This is a serious concern in a reactor that has lost coolant.
Here is post I wrote a few years ago on the topic:
When a nuclear power plant is running, the fissions generate a whole boatload of different isotopes. Back in high school, I remember having the distinct impression that a nuclear reactor followed a distinct set of deterministic fissions, producing a neat chain of fission products, mostly due to pictures like this.
It just ain’t so. Everything is smashing around so much that you are likely to find many different isotopes in the mess that is produced, though the products do follow a sort of nonrandom distribution, an interesting curve that some have called the Dolly Parton Curve.
Anyway, among all of that stuff that is generated are many unstable isotopes with vanishingly-small half lives and some with half lives on the order of billions of years.
The heat that runs the power plant comes mostly from the kinetic energy of the fission products being tossed about, with a small percentage (~7%) of full power coming from the decay of the fission products.
See the chart on page 6 of this document (Warning: PDF) for a little insight into the way decay heat works.
Once the reactor is shut down, meaning the neutron population goes below self-sustaining, the reactor still retains that steady-state decay heat of ~7-8%
Immediately, the fast-decaying isotopes start to peter out, resulting in the decay heat dropping, as shown in the curve in the PDF.
After a day, the power is < 1%. After a week, around 0.1%. After a year, around 0.05%. After ten years, .0025%
This means that if you were operating at 1000MW, immediately after shutdown you have to deal with 70MW of energy in the reactor. After a week, you still have 1MW of heat being generated. After a year, half a megawatt. This is why spent cores still need babysitting.
The decay heat is, in fact, being used during full power operations: it is always present and always around 7-8% of your total power output.
There are systems for preventing loss of coolant, typically referred to as the “reactor fill” system, but that brings its own problems: a reactor that is designed to automatically increase power when cooler water is introduced (meaning more steam is being drawn from the steam generator) is a good thing in most cases, but when you suddenly introduce a slug of cold water from the reactor fill system, this might just add enough reactivity to overcome the negative reactivity of the control rods and therefore cause bad things to happen.
And about those rods. They are indeed designed to absorb neutrons. They do not perform the role of moderator. In a typical pressurized water reactor (the only kind of plant I ever worked in), the rods are raised at startup and lowered at shutdown, with little movement during normal operations. The rods are not used to throttle power.
In these reactors (PWR), the water is the moderator. The water slows down the fast neutrons that come from fission so that they have a better chance of interacting with other nuclei. It turns out that slow “thermal” neutrons are better at causing fission than fast ones, so a moderator is used to bring the speeds down.
Thanks!
Learned something new (or in my case I think re-learned it…hope it sticks this time).
If the rods do not throttle power what does?
More water? (Can’t see how that would work but want to be educated on it.)
You are misunderstanding the term “moderator.”
The control rods do, indeed, control the power of the reactor, but that is not “moderating.” A moderator is a substance that slows fast Neutrons down, so that they can be more easily absorbed by the fuel nuclei.
This is wrong. It comes from a Scientific American article which claimed fly ash was more radioactive than nuclear waste. This conclusion was reached by testing the air around a coal plant and the air around a nuke plant and finding more radioactivity around coal. The average person living near a coal plant supposedly gets 1.9 millirems per year from coal. On average, people get 300 millirems per year from background radiation. There’s no radiation hazard from the fly ash spill. When pressed, Scientific American weaseled around with it by adding an editor’s note saying “ounce for ounce, coal ash released from a power plant delivers more radiation than nuclear waste shielded via water or dry cask storage.” Well yeah, I would hope so.
Steam demand.
Note: this is for pressurized water reactors—the ones the Navy uses. I don’t know jack about other kinds.
This is how it works:
U-235 fissions better with slow (thermal) neutrons.
Fissions typically produce fast neutrons.
The easiest way to slow down fast neutrons is to bounce them against like-sized things. They lose large amounts of kinetic energy with each collision. Imagine a billiard table: if you rolled a marble into a group of bowling balls, the marble would simply bounce about and never lose energy; if you rolled a bowling ball into a bunch of marbles, it would plow on through and keep most of its energy; but if you roll a billiard ball into a group of identical billiard balls, it quickly loses its speed by sharing energy.
One good thing to use for this purpose is the single proton in the nucleus of a hydrogen atom. And there are plenty of these to be found in water.
Now that you have the lay of the land, here’s what happens:
The Bridge asks for “Ahead Flank”
Throttleman wings open the throttles
More steam is drawn from the steam generator
Temperature of the boiler water in the steam generator drops (as boiling happens)
“cold leg” temperature of the reactor coolant loop drops, since it gives up more energy as the water passes through the steam generator
The reactor coolant is more dense, because it is cooler.
The hydrogen atoms are more closely packed.
More fast neutrons are slowed down.
More thermal neutrons are available for causing fissions.
More fissions happen.
Power output increases.
Now, the reverse:
The Bridge sends “All Stop” signal
Throttleman shuts the throttles
Less steam is drawn from steam generator, pressure and temperature increase.
Less energy is being removed from reactor coolant that is passing through the steam generator.
Reactor coolant returns to reactor with much of its heat still remaining.
Reactor coolant becomes less dense.
Hydrogen atoms are more widely spaced.
Neutrons travel further before striking a hydrogen nucleus
Population of “thermal” neutrons drops
Reaction rate decreases
Power level decreases.
This makes the PWR design inherently stable. Reactor power follows the steam demand, with no adjustment needed.
One indication of power output is the difference between output temperature and return temperature of reactor coolant. At high power, the coolant leaves hotter and comes back cooler. At low power, the difference is much less.
Ok…I can see that.
But you have a reactor with long fuel rods.
I presume they all need cooling, top to bottom.
So you pump in water to keep things in check.
Now you want to control the rate of reaction to control the power being generated.
If the fuel assemblies are already immersed in water how does “more water” change anything?
It was said above that rods are not used to throttle reactor output so to me that leaves the water to do it.
I am not arguing here, I am just laying out my confusion on the subject. Obviously the power plants work. I am just wondering at how they do it.
ETA: An answer was given above while I wrote this but still not seeing how water is everywhere in the core to keep things cool and can be increased or decreased. Either you have water cooling or an element is uncovered. Seems a binary choice. Water or no water. You can’t have “more” water between two points. It is there or it isn’t.
Please correct me if I’m wrong, but it seems to me that of all the possible and currently practical fuel sources, nuclear is different in that a stockpile of it is dangerous all by itself.
Coal, gas, other burnable materials: if they are stored safely – that is, without any special treatment other than not allowing them to ignite – they are not dangerous. In other words, without a fire or spark, coal is just a big black lump.
But nuclear fuel, if stored in a big lump, is inherently dangerous, and must have some kind of control to prevent it from doing what comes naturally, or overheating.
Is that a fair description?
Well, given that coal ash is allowed to float through the air willy nilly while radioactive waste is treated like nuns would treat used porn magazines, I think the fact the comparison of the effects of the two substances, even though they are contained differently, is quite fair.
You can hold uranium in your hands with no ill effect. Indeed it is still possible to buy jewelry or pottery made from uranium (has a nice, yellow color).
There are some natural “reactors” in the world where a low-level nuclear reaction occurs. Mostly though uranium is mined much like any other metal and does not require some special protective efforts.
Refined uranium sufficient for power production or bombs is dangerous. But then there are lots and lots of materials used in manufacturing that are dangerous to downright lethal. Often these things are shipped by rail or truck. Uranium is not uniquely dangerous. We just want to keep the refined stuff away from bad guys.
So sure, a truck filled with uranium is more dangerous if it spills than a truck load of coal but then one truck of uranium is equivalent to a few thousand (don’t know the real number…made this up but the difference is huge) trucks of coal. Not to mention the environmental impact of strip mining for coal and such.
