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Radioisotope Thermoelectric Generator
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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Update: June 2012
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