ShittyBeatlesFCPres t1_j5taye2 wrote on January 25, 2023 at 12:34 PM

Does anyone know how large these would be compared to a civilian energy reactor? When these come up, there’s always a debate about the risk of a catastrophic failure spewing radioactive material in unpredictable ways. But I’ve never been clear on what the scope of the disaster would be. Is it way more radioactive material or far less (and even that spread more thinly)? How uninhabitable is how much area for how long from a nuclear disaster on the way to space?

A worse case disaster, I mean. It sounds like for a Mars trip, we wouldn’t be using these engines until far enough in space. But let’s say this tech becomes routine. Maybe the Congressman for wherever these engines get made has a meeting with the contractor’s lobbyist and decides NASA needs to buy more engines. And then 💥kaboom💥.

zypofaeser t1_j5unajn wrote on January 25, 2023 at 6:09 PM

Relatively short burn times. Thus only a small radioactive inventory of fission products.

[deleted] t1_j5tgvhs wrote on January 25, 2023 at 1:28 PM

[removed]

ttkciar t1_j5vy5oz wrote on January 25, 2023 at 10:58 PM

> Does anyone know how large these would be compared to a civilian energy reactor?

Tiny. A civilian energy reactor has to implement two heat exchange systems -- one for transferring heat from the core, and one for heating water to steam to turn turbines and then condense it again.

For NTP there are no circular heat exchanges, and no turbines. It's just a hot core in your reaction chamber, which heats the hydrogen you squirt on it, and the hot hydrogen gas escapes out the rocket nozzle.

The smallest critical mass of plutonium is about four inches across. In theory that's all you need in the reaction chamber, but in practice you will also want cladding so that your hydrogen reaction mass erodes the cladding and not the plutonium (else you'll be squirting plutonium out the rocket nozzle along with your hydrogen), and a bisecting neutron reflector shutter or something so you can turn the core on and off.

So, maybe something about twelve inches across? Still much smaller than a civilian power reactor.