Tuesday, May 22, 2018

Nuclear power in space

I'm a little behind on my blogging but a couple of months ago NASA released some information on their new Kilopower small reactor for use in space.  So I thought this would be a good opportunity to write something about power generation in space. 

Most satellites and probes in the inner solar system use solar power.  This was one of the first practical uses of solar power back when solar panels were very expensive and now that solar panels have gotten much, much cheaper it's even more of a good idea.  The solar panels on your roof are built tough and will weigh about 10 kg for every square meter or, on the Earth.  At 1300 watts of sunlight coming in times an optimistic 20% efficiency for typical solar cells you've got 26 watts per kilogram.  According to Wikipedia more expensive spacecraft-grade solar panels will give you 77 watts per kilogram, but even then the majority of the expense is going to be lifting the weight of the panel into orbit.

Solar panels are great if you're as near to the Sun as Earth is but they start to work less well as you get further from the sun.  The incoming sunlight per meter goes down as the square of the distance from the sun so when you're out at Jupiter, 5 times as far as Earth from the Sun, you're only receiving 50 watts per square meter and your spacecraft grade panels are only generating 3 watts per kilogram.

For trips to the outer solar system NASA has traditionally relied on radioisotope thermoelectric generators or RTGs.  The idea is that you take some particularly radioactive isotope, such as Plutonium-238, and just let it generate heat by decaying.  These will actually generate about the same electric power per kg of weight as solar panels will at Jupiter's orbit but without the worry of always having to point your panels at the sun.  And as you travel beyond Jupiter to Saturn they become much more mass efficient.

They do have their problems, though.  In order to be intrinsically radioactive enough to generate such heat the fuel has to have a pretty short half life.  Plutonium-238 will half decay away in just 90 years.  That means all the natural P-238 is gone and any we use has to be synthesized at great expense.  It also means that half the power will be gone after 90 years.  Missions don't last 90 years but over the long distances of the outer solar system they can last more than a decade and the power output from these generators will go down noticeably on those time frames.

And then there's the problem of radiation.  To generate heat the material must be very radioactive by its nature and that means dangerous.  If there were to be an explosion during launch the plutonium might be lost.  The RTG is designed to stay as one intact unit in the event of catastrophe and sink to the bottom of the ocean but it still causes worry.

That's one area where a nuclear reactor rather than an RTG has benefits.  Kilopower and most other reactors mostly run of Uranium-235.  In small quantities U-235 is pretty safe.  Well, it's pretty toxic but toxic in the manageable sort of way that lead or mercury or bismuth are toxic.  It has a half life of 700 million years, quite a bit more than P-238.  Long enough for large amounts of naturally occurring U-235 to remain since the creation of the solar system.  And while it gives off a little bit of radiation from decay that decay occurs so slowly that you can mostly just ignore it.

Why do nuclear accidents like Fukushima release so much radiation then, if the uranium fuel that goes into the reactor is fairly safe?  Because reactors work by transforming uranium into other elements to release the energy stored in it.  And the elements that are created are frequently even more naturally radioactive than P-238 is.  So a reactor that has been running is just as dangerous as an RTG is.  But a reactor that has not yet been turned on is actually fairly safe.

So that's a clear reason to prefer a reactor over an RTG in the outer solar system where solar doesn't work very well.  What about closer in?  Mostly you would want to consider nuclear in cases where you need reliable power for things such as life support through the night.  On Mars the night is going to be 12 hours long and adding enough lithium-ion batteries to last that long would effectively be a 20 Watt/kg system, and since you need twice as much solar panel during the day to both charge your batteries and run your life support you're down to a net of 13 continuous watts of power per kg of solar panel and batteries.  Which is still better than nuclear.

But what about the Moon which rotates once a month, with nights over 300 hours long?  With that the heaps of batteries you have to pile under your solar cells only give you .8 watts per kg of equipment making it clearly less mass efficient than nuclear.  Is it worth the problem of dealing with radiation?  That's for NASA to decide.  And it would only be NASA that gets to make that decision.  Such lightweight reactors have to use highly refined uranium that could be made into nuclear weapons in the wrong hands.  So there isn't much chance of SpaceX or other private ventures getting their hands on these.

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