Nuclear plant spent fuel cooling pools: Dissapating the heat

Factual Questions
1

Recently on the National Geographic Channel I saw the beginning of the After Humanity series, where they describe what would happen if all of us suddenly disappeared. If I understood correctly, the normal procedure for nuclear power plants in dealing with spent fuel rods is that they are first allowed to cool in tanks, in which cold water circulates around them and gradually carries off the excess heat. I was also under the impression that nuclear plants work by heating water into steam used to drive turbines.

I can understand why there would be a problem without people to monitor the cooling system, but instead of simply dissipating the heat, why could one not try to use it, for example, by feeding it back into the system, say right before where the water is transformed into steam? Wouldn’t this reduce the amount of nuclear fuel needed per megawatt of power?

Or is this already part of the process?

2

(former navy nuke here)
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 these spent cores still need babysitting.

As far as spent fuel is concerned, you can’t use it anymore in the reactor because it cannot generate enough neutrons to sustain a reaction, regardless of how much decay heat is there. I suppose you could milk it for that remaining megawatt for a year, but you probably want to slap in a new core and get up to full power.

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.

3

Thanks for the quick response, I appreciate it. I guessed what you said about the decay heat, but it’s nice to know I’m not way off base.

4

May I ask a few semi-related questions? Probably related more to land-based generating plants. Disclaimer: thermo was not my best subject in college. Anyhow:

At some point in running the steam through the generators, it’s still steam, but it doesn’t have enough oomph left to be able to push a turbine blade decently - right? I’m using non-technical language here, sure, but the idea is generally correct, right?

The steam used to run the generators is shielded enough from any nuclear reactions that it’s not radioactive, right?

It’s not super-heated steam, I understand, but what the heck, it is at least fairly hot water. Are there any industrial uses to which that water could be put? I suppose the answer is, maybe, but no one’s come up with any practical, cost-effective way to use that water a little more - right?

5

One thing that really stood out when working in steam plants was this: if there is a way to milk one more teensy bit of energy out of the power plant, the engineers have already done so. They aren’t throwing away energy unless it is absolutely cost prohibitive or impossible to reuse it.

In a typical steam plant, there are numerous places where small amounts of residual heat are being redirected back into the system. For example, in a traditional coal or oil-fired plant, they use an economizer to capture some of the heat from stack gases that come out of the boilers. Here’s a vendor link for economizers.

So, when the steam is done pushing turbine blades, it is put to the best use that saves the most energy: it is condensed and pumped back to the boilers as feedwater. This is important because the residual heat in the feedwater is that much heat that doesn’t need to be added to fresh feedwater to boil it, and a steam plant’s feed water is quite pure, with special chemicals added to prevent corrosion — you don’t want to have to keep adding new chemicals all the time.

In other words, a modern steam plant runs in a closed loop, forming a Rankine Cycle. The boiler turns feedwater to steam, the steam turns the turbines, the steam is condensed to water (forming a vacuum, helping create better differential pressure across the turbines), and the water is pumped back into the boiler.
All heat engines function by heat flowing from a heat source to a heat sink. In this case, the heat source is a reactor or boiler, and the heat sink is a cooling tower or seawater.

Note: The stuff coming out of cooling towers is not the steam from the plant.

As far as the steam and radioactivity goes…

It depends on the kind of reactor. In a Pressurized Water Reactor uses two loops, with radioactive coolant kept in a closed loop and nonradioactive feedwater/steam in a secondary loop. See this diagram.
In a Boiling Water Reactor design, the feedwater is the reactor coolant. See this diagram.

6

There are two different types or reactor, pressurized water reactors(PWR) and boiling water reactors(BWR).

In PWRs, the steam used to turn turbines is kept separate from the primary coolant(the water inside the reactor) by use of steam generators. These are big bundles of tubes with the primary coolant on one side, and the secondary water on the other. The primary coolant is really hot, but kept under such high pressure it doesnt boil, so it will readily boil the water on the other side. This makes the steam to spin the turbines. The water in this secondary loop is pretty much radiation free, but tube leaks in the steam generators are not impossible, so its not something you really want to use for anything else outside of containment.

In a BWR, the steam generator is taken out of the equation, and the water is boiled in the reactor itself, and sent directly to the turbines. This has the positive benefit of eliminating the very costly steam generators, but it also means that the steam in the turbines and all the secondary systems is radioactive to a certain degree(i’ve no experience with these reactors, so i cannot say for certain how bad it is). Most commercial reactors are of this type, if i recall correctly, while naval reactors are all PWR.
I’ve heard some rumblings of using the excess heat to power Sterling engines, for even greater efficiency, but for the most part, the excess heat in all power plants is just dumped. By the time the steam exits from the turbines, its had the great majority of its useful energy pulled from it(power plants are big on efficiency, as minor7flat5 said), and isn’t much more energetic than the steam rising from a boiling pot of water, and frankly, thats not really of any great use. Sure its hot, but its a lot easier to just buy fuel to heat up whatever it is you need, than to transport the excess heat from the power plant in well insulated pipes to where its needed, before it cools down.

7

The exhaust steam from most power plant turbines has less energy than steam comming from a pot of boiling water. The pot steam is at 14.7 psia and 212 degrees F. The exhaust steam is close to 0.1 psia and around 100 degrees F. The only recoverable energy would be the latent heat of condensation.

8

Bolded pronoun refers to boiling water reactors (BWRs)

I know nothing of naval power plants, but commercial PWRs are quite common. I am very familiar with the Ringhals, Sweden installation, which operates 3 PWRs and one BWR. Initially they installed one of each, and were happier with the PWR performance and operational considerations, so two additional units of that type were installed.

A note on nuclear plant efficiency: Ringhals is fairly typical of nuke plants in that the fuel accounts for well under 10% of the operating budget. It is almost free. The plant is operated not for maximum efficiency, but for minimum wear, as capital cost is the major budget item. Happily this also is a bit safer operating point, due to slower reactions and lower temperature. The main wear item is the steam generator in the PWRs. (as mentioned upthread) Ringhals pioneered the replacement process for this part, greatly extending the operational life of PWR plants of similar design.