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HIMP_Dahak_172291 t1_j5rscw1 wrote

Reply to comment by BabylonDrifter in NASA to test nuclear thermal rocket engine for the first time in 50 years | CNN by dem676

Given our track record with junk in orbit it could be a positively glowing future here in a century as it all falls back down.

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BabylonDrifter t1_j5rspeu wrote

Nuclear rockets are only deployed in trajectories that do not intersect with any planet.

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CollegeStation17155 t1_j5uayw7 wrote

>Nuclear rockets are only deployed in trajectories that do not intersect with any planet.

unless there is a launch failure or Newton to pound thrust conversion error...

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Underhill42 t1_j5vq0w5 wrote

No reason to launch anything significantly radioactive. It's the waste you have to worry about, the fission fuel itself is (comparatively) safe. It has to be, if it were seriously radioactive it wouldn't still exist after almost 5 billion years in the ground.

Still not exactly *safe*, but so long as you stay well away from critical mass the heavy metal poisoning will probably do more damage than the radiation.

And I think I recall hearing that NASA is moving strongly away from using legacy pre-physics units for anything, in large part because it opens the door to stupid conversion errors like that, and someone inevitably walks through.

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dzhastin t1_j5wayym wrote

If it fails to reach orbit for any reason then it will most certainly intersect with at least one planet.

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HIMP_Dahak_172291 t1_j5rsvl5 wrote

Sure. Until they get caught in the Kessler belt we are busy building.

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BabylonDrifter t1_j5rwo0z wrote

Please explain how an object on a trajectory that does not intersect with any planet gets caught in a Kessler belt.

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HIMP_Dahak_172291 t1_j5rwsjo wrote

Well we have to get it all up there first and we dont have teleporters.

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Shrike99 t1_j5sktby wrote

Fresh nuclear fuel isn't very dangerous. It's only after it's been used for a bit that it gets full of nasty isotopes.

So the simple answer is to launch the reactor cold and only activate it once safely in orbit.

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FrostyAcanthocephala t1_j5sm9xt wrote

Right. Fresh fuel isn't dangerous. /s

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Adeldor t1_j5tjf6q wrote

He's more or less right. In its pure form it's as much dangerous from its heavy metal qualities as it is from radiation, hence the person holding this 90% pure plutonium puck wearing gloves.

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FrostyAcanthocephala t1_j5tkjqp wrote

Yeah, that photo doesn't tell me much about the relative dangers. I get that it's fairly safe in a puck as an alpha emitter, but that's not really the danger we are discussing, is it?

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zypofaeser t1_j5un2ng wrote

Just uranium. Like, what is found naturally all over the place.

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Adeldor t1_j5tm2oh wrote

Referring to the earlier comments, I think it is what we're discussing - the pure "unburned" form being relatively safe.

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Shrike99 t1_j5wqvjj wrote

I mean nuclear fuel is typically comprised of uranium-235 and uranium-238, both of which occur naturally and can be found in low levels within all rock, soil, and water. Soil for example contains about 12 milligrams of uranium per kilogram on average.

A typical space-based fission reactor contains abut 30 kg of uranium. If a rocket blew up halfway through the launch and scattered that perfectly over a radius of say 50km, and it was all absorbed by just the top 1cm of soil, that would amount to an additional 0.3 milligrams of uranium per kilogram of soil.

Of course, if it spread over a smaller area the concentration would be higher, but it would have to be a pretty small area for there to be enough to matter, so it's not likely to be a major hazard in the grand scheme of things.

 

In practice it's more likely that the fuel rods would remain largely or entirely intact and end up at the bottom of the ocean somewhere. The ocean contains approximately 4 billion tonnes of uranium, so even if the fuel rods were gradually eroded, they'd quickly be diluted into irrelevance.

Now, there are some ways that it might be possible for someone to be exposed to a dangerous quantity - for example, say something like a gram of uranium being chipped off and somehow ingested by someone, my point is more that it's not going to be a widespread ecological disaster.

Whereas in the case of a disaster like Chernobyl, there were a lot of nasty isotopes present in the partially spent fuel rods, most notably iodine-131, caesium-134, caesium-137 and strontium-90. These isotopes are tens of millions of times radioactive than uranium-235 or uranium-238, so even the most miniscule quantities are dangerous.

I'd also point out that we already regularly launch other dangerous substances on rockets. Hydrazine for example has comparable toxicity per milligram to uranium, and large satellites are regularly launched with literal tonnes of that onboard.

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RhubarbImportant5750 t1_j5s4wu2 wrote

One idea would be to use reusable 1st booster rockets like the ones space x uses to reduce the amount of debris left in space.

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