Friday, November 6, 2009

Nuclear reactors for space: safety issues

Propulsion and science in space requires a lot of energy, but we pay dearly for every kilogram that we launch. Nuclear reactors are an obvious power source, since they combine large power outputs and a long supply of fuel into a very small package. NASA is designing a 40 kW reactor to power a moon base, most of the high-efficiency ion drives require more electricity than they're going to get from solar panels, and both the US and the Soviet Union launched several nuclear-powered satellites back during the Cold War. The technology isn't too difficult, and it works like nothing else.

But how do we keep them safe? What if a rocket explodes while carrying a nuclear reactor up into space? This is the most common objection: nobody wants to have chunks of highly radioactive debris raining down on them. Nobody. So let's look at the sequence of events when we're launching, say, a nuclear-powered ion-propelled probe to Jupiter's moons.

  1. The reactor is loaded onto a conventional chemical rocket. It hasn't been turned on yet, so it's not radioactive. The radioactivity in nuclear reactors doesn't come from the uranium fuel; it comes from the reaction and from the radioactive isotopes that the reaction produces. At this point, you can handle the fuel with your bare hands.
  2. The rocket is launched into orbit. The reactor still hasn't been turned on, so it's still not radioactive. If the rocket explodes at this point, there will not be a release of radioactive material because we don't have any yet.
  3. The rocket reaches orbit and stops firing.
  4. The reactor turns on. Now it's radioactive. Good thing it can't just fall out of orbit!
  5. The ion drive has power now, so it turns on and starts thrusting. It leaves Earth's orbit and heads for Jupiter.
If the rocket blows up, the reactor will just be an especially heavy chunk of metal.

The only serious safety issue is if we use reactors to fuel satellites. Satellites may decay in their orbit and eventually fall to Earth. The Soviet Union did this; one of their reactors was even so rude as to break up in re-entry over Canada. I'm not saying that having nuclear-powered satellites is inherently unsafe; just that it is not trivially safe. Using nuclear reactors to power ion drives and moon bases, on the other hand, is trivially safe: there is literally no way for them to rain radioactive debris on us without violating basic physics.

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