Do nuclear reactors eventually cool off?

How long does that take, or do the fuel rods (even spent ones) perpetually need cooling?

IIRC, spent fuel rods take about a year to cool.

IANA Nuclear Physicist, but…

This question kind of depends on what you mean by ‘nuclear reactor.’ I assume you’re talking about the issues currently going on in Japan.

See, the neat (and scary) thing about nuclear reactions is that we don’t have to do anything to make them happen other than bring a sufficient amount of fissile material together. The fuel rods contain the fissile material and are inserted together into the reactor, and provided you set everything up nicely and don’t smoosh them together too much, some of the radiation emitted from the splitting nuclei will go zooming off to find another heavy, unstable uranium nucleus to slam into, and every time they split, we get some presto-bango energy. The nucleus has settled into two slightly more stable ones, so we get the difference. That’s where we get the heat to boil the water, or melt the sodium, or whatever transfer medium we’re using.

Thing is, though, we don’t want ALL of those high-energy particles crashing into another nucleus and setting off another reaction. That’s the sort of thing we do when we want to scare the Other Guys or level their cities using nuclear weapons. If we smoosh too much of this radioactive material together, it’s going to go critical, which is the point at which the neutron hits nucleus > neutrons > which hit nucleus and so on in a self-sustaining positive feedback loop that continues until the critical lump of uranium, or plutonium, blows itself apart by releasing so much of that energy.

So, we use control rods and generally to some extent the heat transfer medium itself to keep all the neutrons from zooming into other reactive things. Stuff like boron and heavy water (D2O) suck up just enough of the neutrons to keep the whole thing from blowing itself apart. And ideally the reactor chugs along happily at a subcritical level, heating the fluid and spinning a turbine somewhere downstream. Once we set up this collection of fissile material, the reaction is going to continue whether we want it to or not unless we a) reduce the amount of material or b) absorb more of the neutrons to keep the reaction slow. So, in a sense, reactors DON’T ever cool off, unless we force them to.

Of course, this isn’t a free party. Eventually the highly radioactive, unstable isotopes in the fuel rods will have been pretty much spent and split into lower-energy elements. These are still plenty nasty and hot (in radiological and literal terms)…just not hot ENOUGH to make them worthwhile in a power plant.

Now, since we’ve got this big lump of fissile material, that’s going to stay hot as long as it’s a big lump of fissile material (which it will for quite a while) but isn’t hot enough to do anything useful… What? Basically we just chuck them in a pool. Maybe we’ll dope the water with a little boric acid for neutron-gobbling goodness or lay down some shielding, but more or less we’re just trying to keep cool water on top of it, blocking the radiation and absorbing the heat until we can run out the clock and the stuff’s cool and safe enough to dispose of in other interesting ways like vitrification and burying it really, really deep where nobody will notice.

So, yes, fuel rods need perpetual cooling. But so do reactors! In the latter case, the heat transfer medium generally acts as a coolant, doing its thing upstream and dumping its remaining energy into something like the nearby river to complete the circuit cool enough to do something useful in the reactor itself.

Japan’s mostly screwed right now because all of their active cooling systems have failed, and so the nuclear reactors are running rampant and hot, and the spent fuel rods (which are usually on site at nuclear plants since nobody really knows what to do with them) are boiling in their pools, and for sure have been exposed to air at least once. The state of their control rods is another another critical (hah!) issue - they were supposedly all inserted fully as an automatic safety measure during the earthquake, therefore modulating the reaction as much as possible. But who knows! We already know some of the fuel rods are damaged, and it’s hot enough in there to split water into hydrogen and blow the roof off.

Until power is restored or they figure out some way to get sustained active cooling going again, there’s really nothing they can do.

Again, IANANP.

ETA: OH! And, of course, since the roofs are being blown off and neutron-modulating pools are boiling off, radiation’s kind of shooting around from all the damaged sites right now and, as you can see, nobody really wants to get all that close.

Note that’s the requirement for how long your emergency cooling backup has to hold out, not the amount of time it takes for the spent fuel rods to be cool enough to no longer require cooling, or be exposed to air. Spent fuel rods can spend years in holding tanks, and those tanks must always be cooled.

Presumably, a month or so is long enough to get the power back on after a disaster, but the cooling must continue regardless.

Unless you slap a heavy sarcophagus over it and worry about it later.

OK, you were pretty close.

I am also not a nuclear physicist, but I designed and built a few instruments for them to use and learned a bit along the way.

ISoT’s Discribed a nuclear chain reaction (“criticality”) but left out a few bits. Natural Uranium won’t become critical no matter how much you have. It is the U-235 that chain reacts, but U-238 makes up about 99.3% of Uranium. To get criticality with just Uranium, you need to increase (“enrich”) the U-235 content to a fairly high level. Even though that level is widely available, it is classed as “Unclassified Nuclear Information” (UCNI) and may be one of the things people that have ever held a clearance could get in trouble for talking about.

There is a trick, though, and that is neutron moderation. U-235 fission results in neutrons that are moving really fast. These fast neutrons tend to whiz right past other nuclei and not cause a fission. If you slow them down,(“moderate”) then they are far more likely to react and so you don’t need near so much U-235 in the fuel to have a self sustaining (“critical”) chain reaction. If you use water as a moderator, then you only need to enrich the U-235 up to 3% or so. If you use graphite as a moderator, then you can get away with no enrichment at all.

One huge advantage of a moderated reaction is if the fuel melts, it loses it’s moderator and the chain reaction stops. With natural or low enriched uranium, you can’t get to criticality without a moderator. The though slightly lighter, U-235 won’t seperate on it’s own. In fact it is so wicked tricky and expensive to separate U-235 and U-238 that this is the main thing that prevents every Tom, Dick, and Husein from making their own A-bombs.

BUT even though you don’t have a nuclear chain reaction, you still get spontaneous decay of stuff that was formed by the heavy neutron bombardment when the chain reaction was happening. This is the source of the problem heat in the Japanese facility. Neutron absorbers won’t reduce it much at all. Those unstable isotopes are going to decay regardless they get hit by a neutron (though a neutron can indeed cause them to, so absorbers do make a slight difference).

The good news is that most of the heat is coming from stuff that has a short half-life. The short half life means it is decaying at a high rate and making lots of heat. This is why spent fuel rods are normally stored on site in a cooling pond for a year before reprocessing (Japan does, or at least did reprocess spent fuel…one of my instruments went into a Japanese reprocessing facility)

The above point is something the anti-nuke forces tend to either be ignorant of, or deliberately talk around. While it is certainly true that spent fuel will “remain radioactive for thousands of years,” only the most stable isotopes last that long, so the radiation is at very low levels after those eons because the stuff that is left is only left because it produces very little radiation. One of those isotopes would be unburned U-235…if U-235 wasn’t pretty stable it wouldn’t exist for us to mine. This and Plutonium are what reprocessing seeks to recover.

So important points are:

-If the fuel, spent or otherwise, is in one mass, there is no way to create criticality. That would require the fuel be spread out and a moderator introduced amongst it.

-If the fuel is still pellitized and in fuel rods, then the water used to cool it could make it become critical.(this is what normally makes the reactor “go”). This is why Boron is being added to the water. The Boron absorbs neutrons and ruins (“poisons”) the water as a moderator. This is why the Japanese engineers are trying to use borated water to cool things off.

-The spent fuel has much less U-235 in it (that is why it is spent) it does have some Plutonium in it (produced in the reactor) but it is still much less likely to become critical than the “good” fuel in the reactor. It for sure won’t happen without a moderator. Because Plutonium is toxic, and easy to refine to weapons grade stuff, Japanese reactors are designed and operated to minimize Pu production.

-In contrast to Chernobyl, the moderator in Japan was water, not graphite. Because the water is missing, the melted fuel is not critical. In addition, the graphite moderator made an excellent fuel at Chernobyl, allowing a huge smoky fire. Chernobyl was actually a runaway nuclear chain reaction that spiked the heat to 100X or more the design limit.

-The heat of hot isotope decay is nowhere near what the reactor creates when generating power. Yes it is a problem, but you don’t have the GigaWatt or so of heat (heat, not electrical power) that these reactors produce at full output. That is what would be required for the fuel to “melt it’s way to china”.

-The isotope decay could create a fire that will generate a radioactive smoke cloud. That is the outcome that has enough of a chance to happen that it is worth worrying about. That is why people are being evacuated and told to stay indoors.

-The more time that passes, the less heat is being produced. The really hot stuff doesn’t last long. While it is hard on nerves, the longer this drags out, the less there is to worry about from a heat standpoint. Of course it seems like the cooling ponds boiled dry after the first few days, and that was bad, but over a longer time scale the risk is declining.

-In spite of the failures, the fundamental safety feature of a water moderated reactor seems to have worked well. You lose the cooling water, you lose the moderator, and you get a hell of a mess, but the reactor goes sub-critical and stays that way.

Why don’t they build a dike around it and flood the place? I know that’s simplistic, but dropping water via helicoptor has got to be like spitting in the wind, literally. Dropping ice cubes would be better than that.

Boy, my old writeup on Decay Heat is getting a workout with recent events.

Read all about it.

Not that my post is more descriptive than those above, but I do have some links to a couple of graphs and documents in there.

The short form: decay heat is ~7% of operating power at time of shutdown and it decreases logarithmically over time, with a quick drop over hours and days, but a lower level of heat that dribbles out over decades.

And it’s nothing to sneeze at: if you have a gigawatt reactor operating at full power at time of accident, then you now are dealing with 70MW of heat that must be discarded somehow.

That’s seventy million watts of heat—quite enough to melt things nicely.

Fortunately, this level of heat drops off quickly, and the actions of the first few hours after the incident have handled this heat load.

And I’ll repeat what gets repeated in every nuclear power thread:
The moderator in a reactor does not restrain the reaction; it enables it. The word seems to be chosen poorly, since a layman might think that moderators moderate the reaction—the reality is that they moderate the speed of the neutrons, thereby enhancing the desirable conditions for fission to occur.

Two more, just for fun:
The rods don’t throttle power; steam demand does. Nobody needs to touch a thing for the reaction to automatically increase and decrease following the throttles in the steam plant.
And “critical” isn’t a bad word. It just means that the fission rate is self sufficient and stable: each subsequent generation of neutrons is sufficient to maintain the same reaction.

It’s my understanding that this is only true for relatively small nuclear reactors with high power densities, like the naval plants we’re most familiar with, minor7flat5.

I have a couple of friends (former Navy nuclear techs) who now work at commercial nuclear plants, and I’m pretty sure they indicated that reactor power is, in fact, at least partially controlled with the control rods (maybe even wholly controlled, for all I know), even in the power range.

This may have something to do with both the scale of the plant, and the much lower power density of the core.

It’s also my understanding that control of commercial reactors is considerably more complicated that we we’re used to, and usually involves computer assistance.

Agreed…I’m pretty confident that as plant size goes up, complexity increases exponentially, the rods might be dancing up and down in time with everyone’s air conditioners in the summer, and computers and such would be needed to manage things.

The first time I saw video of the control room of TMI, I was amazed at the complexity and surprised at how simple our naval control rooms appeared in comparison.

I have no clue about commercial power plants. But I’m getting tired of qualifying every darned thing said on this subject.

They’re pumping and dumping tons of water in. I’ve asked before, "where is all the waste/sea water going? What are they doing with it?

I got a response from a nukie asking, “ever hear of evaporation?”

Nice try. Tons and tons of water is not evaporating. If it’s disappearing, it’s going somewhere. Is it boiling off as fast as they add it? Is it soaking into the ground under the reactors? Is it pooling under the reactors?

There’s your moderator, speeding up whatever nuclear chain reaction that *might *be going on. Yes I know they add boron, a neutron poison, that sucks up the neutrons that are naturally flying all over the place.

There’s already been partial meltdown, right? That means the fuel rods are no longer fuel rods. They’ve melting into a very hot, very radioactive slag that’s mixed with all sorts of other melted materials and become what’s called Corium. At that point, all sorts of things can be happening and the experts can’t know exactly. If Corium melts its way through containment, spreads out a bit, mixes with water - you can get a temperature, pressure and hydrogen gas spike and go boom.

If the water has boron in its not a moderator. Its an absorber. Your second sentence flatly contradicts the first.

If you use heavy water (Deuterium oxide) you can also get away without enriching the uranium. The CANDU reactor design used in Canada are the only examples I’m aware of that use this method for power generation.

Again I ask, where is the water? Is it there, keeping everything nice and cool, and all the boron is preventing a re-criticality?

Or is the water gone? If it’s gone, no boron, no cooling, and normal radioactive decay is driving temps way up. New fissile materials, or “fuels” if you will, are or could very well be, being formed.

Those fuel rods aren’t fuel rods anymore (I guess, I’ve got the same internet everyone else has) and the molten slag they are now is a mix of various uraniums, plutonium, and more.

We don’t know exactly what’s going on, because those fuel rods weren’t all the same age. Some are “fresh” some are old, most are somewhere in between, and they all have different compositions and proportions of different, fissile materials, interacting in all sorts of interesting ways with all the melted core material.

It seems like a big waste - the spent fuel continues producing heat that not only is wasted, it actually needs cooling? Can’t they reprocess it to make new fuel?

Spent fuel rods can be reprocessed to make new fuel - most of the material in the fuel rods is still perfectly usable. The primary reason fuel rods need to be removed from the reactor after a while is not due to the uranium and plutonium fuel being used up, but due to the levels of various fission byproducts building up enough to interfere with the normal fission reaction. During reprocessing those byproduct elements are removed. Typically this is done after the fuel’s activity has decayed enough that it no longer needs active cooling, although there are some experimental designs such as the LFTR (liquid fluoride thorium reactor) which internally reprocess the fuel constantly during operation.

The decay heat is mostly coming from the fission byproducts, new fuel elements that haven’t been in use don’t generate much heat at all. So in that sense, the heat being given off by spent fuel is wasted. It’s not really worth capturing when the reactor core itself normally gives off so much more higher-quality, easier to capture power.