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.

Wednesday, May 16, 2018

Falling fertility rates shouldn't be a problem forever

There are a lot of countries in the world with declining populations.  Historically this isn't unusual if there's famine or disease or war causing it but in the developed world this is mostly just people choosing to have fewer children.  In Japan it's all the way down to 1.46 children per woman which, since half of kids are male, means that every generation will tend to be three quarters as large as the one before it.

There are a lot of reasons for this.  People having better options than they did in the past seems like the biggest one.  There are differences within the industrialized world in many ways involving religionattitudes towards child care, and other thing.

But those are societal factors and there are also individual factors at play in a couple's decision to have kids.  People have different personalities and personality seems to have an effect on child bearing just like you'd expect.  Here's a study I recently saw linked on twitter.

So, for instance, people who are more Agreeable based on the Big Five personality traits social scientists normally use seem to be more likely to have children.  And these personality traits seem to be substantially heritable.  So if nothing were to change we should probably expect evolution to have its say and the population decline to eventually halt and reverse itself through natural means.

I don't actually expect things to turn out that way.  Rather, I expect technological improvements in the ease of having and raising children to make a bigger difference sooner.  And societal improvements would be nice too but I'm not counting on those.  How we're going to take care of large numbers of older people as they live longer without a large working population is going to be a concern for the next few generations but there's no reason to worry about humanity going extinct or anything.

Of course technological progress cuts both ways and maybe virtual reality will reduce childbearing rates further.  But again I expect the next generation being more like the sort of people who had kids in the last generation to eventually rescue things.

But the sad moral of the story is that we might have we might have gotten out of the Malthusalian trap.  Maybe even for quite a while.  But I'm pretty sure that in the very long run the human population will increase again until it's reduced to a subsistence level. 

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