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Name: Scott
Status: Educator
Grade: 9-12
Location: MA
Country: United States
Date: January 2007


Question:
What makes suitable fuel for a Radioisotope Thermoelectric Generator? How radioactive does it have to be to be suitable? Would spent fuel pellets be suitable for an radioisotope thermoelectric generator? How about saturated control rods? Tailings? Waste water? Basically, I was thinking that if you could harness the radioactive waste from nuclear power plants with radioisotope thermoelectric generators so >that they would continue to supply the power grid, then they might not even be considered waste any more. I gather the fuel would last a very long time.

As far as I know, radioisotope thermoelectric generators are only really used on space ships that fly past Mars, and they are very expensive, but every thing is expensive when you fly it past Mars. Anyway, I have not heard any other bright ideas for what you do with all that nuclear waste. So is this a feasible use for nuclear waste?



Replies:
Hi Scott,

The key to answering your question is that the spent fuel pellets are.... "spent". That means that the highly fissionable nuclear fuel has been mostly "used up" and converted to radioactive byproducts that have very long half- lives, and which themselves cannot be made to undergo chain reaction fission at rates sufficient to generate significant heat. As a result, these radioactive byproducts simply "sit there" and very slowly decay, but because the rate of decay is so slow, little significant heat results.

The isotopes you referred to that power some space probes, typically use very fissionable Plutonium, which can undergo a fission chain reaction sufficiently aggressively that moderators are required to slow the reaction sufficiently to keep it producing a useable, controllable amount of heat for long periods. These power generators do not use any spent fuel.

But you are right about there being few "bright ideas" for nuclear waste disposal!

Regards,

Bob Wilson


I have not thought too much about these things, but there are some easy trains of thought you can peruse. For a spacecraft's "RTG", you have a sandwich-shaped Peltier element, heated on one face by a container of radioactive material, cooled on the other by black fins radiating thermal IR into space. The heat flowing through the sandwich from hot to cold generates a voltage difference roughly proportional to the thermal flow. The Peltier or thermoelectric element can convert something like 10-20% of this heat flux into useable electric wattage.

For most space missions the designers want to minimize mass, so it can be thrown farther or maneuvered longer or need a smaller rocket. The designers also need a certain amount of electric power defined by what the spacecraft does.

So a well-optimized RTG will have a small radioactive body getting pretty hot, and pretty warm radiators with a high intensity of IR radiating into space. Modern Peltier elements used for this work well up to 200 degree's C on the hot side, and maybe 100C on the cold side. There is no air-cooling, but at 100C the radiator fins can emit something like 1kW/m2 of infra-red glow into the cold dark of space. Times 10%, that is 3 feet x 3 feet just to make ~100W of electricity. I think you can presume that the radioactive source and Peltier plates are somewhat smaller, and the heat is somehow spread out to the radiator fins. But think creatively, by all means. There are many vendor web-pages for thermoelectric elements. They should have hints about the optimal heat-flux concentration for these elements. I think it is around 1w/cm2, some higher.

Most present missions might not want to double or quadruple the size to use less intensely radioactive stuff. On the other hand, if you had a mission which did not care as much about mass, such as a sustained moon base, or large long-term orbital stations served by more-mature spacecraft, high-grade radioactive waste might be adequate stuff to live on.

Wastewater and mine-tailings are much too weak to heat themselves up perceptibly. Only radioactive enough to be toxic to life by occasionally dinging a DNA molecule.

Spent cores have one other inefficiency. It would be nice to have a radioactive power source which was steady forever. But when you want it small and intense (and maybe you want it to die and be safe before eternity too), you use an isotope with a shorter half-life, just long enough to do your job without weakening too much on the way. And maybe you have gotten something that decays directly to a stable isotope, or to isotopes with shorter half-lives so the whole decay-series is done soon after the first decay.. This isotope's heat output will be decreasing according a "decaying exponential" curve. Not very steady for power, nor very prompt at dying, but perhaps a useable compromise. Actually I think spacecraft designers complain about it. Well, spent reactor cores are worse than that. I think they usually have a broad mix of isotopes and decay rates. The fast rates will be putting out most of the heat, but they die off sooner than a single-rate exponential would. The slower ones are not very intensely heating, and they last too long and have unneeded mass.

Generally such waste sits in storage, dying off stage by stage. The fastest rates not yet dead have a half-life roughly the same as the time they have been in storage. You could more-or-less pick how hot you want it by picking how new it is. Provided you actually have running reactors putting out new waste frequently enough. But the slow half-lives mixed in are still a real safety pest, lasting thousands of times longer.

For spacecraft, even if you pick hot-enough waste:

- Its output/mass is less constant than the ideally-fueled generators
- It is chemically less stable. (I.e., it is not a nice, tidy, durable and leak-proof iron brick.)
So our minimal spacecraft are finicky. Need for optimization can kill all kinds of fun.

If you had people on the moon, and an economy based on fissile materials casually used, I wonder how long the neighborhood would last. Longer than on Earth I imagine, but perhaps eventually you could not come inside without tracking in your death in the dust on your space-suit boots. But we could use better hygiene, and on the moon nature would not disturb a tunnel waste-dump for millions or billions of years. (...Man might.)

Jim Swenson



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