Do NERVAs have radioactive propellant?

[quasarrgames]

I know in NERVAs the propellant is passed over a nuclear reactor, which heats it immensely and forces it out the back. However, does the reactor also transfer radiation to the propellant? I'd like to know because for realism's sake, i want to know whether it's okay to light up a LV-N inside the atmosphere in KSP, or whether it would just radiate the planet.

THANKS!

[stupid_chris]

I know in NERVAs the propellant is passed over a nuclear reactor, which heats it immensely and forces it out the back. However, does the reactor also transfer radiation to the propellant? I'd like to know because for realism's sake, i want to know whether it's okay to light up a LV-N inside the atmosphere in KSP, or whether it would just radiate the planet.

THANKS!

The exhaust from an LV-N would not be very dangerous, apart from the fact it's going fast and that it's pretty hot. The really big danger would be if the reactor inside was to break, aka a crash.

[shynung]

With proper shielding, even an actual NERVA prototype engine would probably spew no radiation-emitting particles out of the exhaust. The real danger comes from, short of an actual crash, the reactor itself going critical beyond specs. In the worst case scenario, you're looking at a Chernobyl-style explosion (radiation shields are quite heavy). Even then, there's probably a big red EJECT REACTOR button somewhere on the control panel, set to decouple the reactor and send it away with Sepratrons.

[Ralathon]

It kind of depends on the reaction gas you use and how the reactor works on the inside. If the nuclear fuel is directly exposed to the propellant then overheating it could cause some bits of fuel to escape into the exhaust. This is why there is usually some ceramic layer between the fuel and the exhaust gasses.

Theoretically your reaction gas can also become radioactive thanks to the thermal neutrons. Stray deuterium atoms in hydrogen may absorb neutrons to become radioactive tritium. Carbon atoms in methane may capture neutrons to form carbon 14. Nitrogen and oxygen in amines can also transmute into carbon 14 via neutron capture etc.

In practice however these radiation sources are negligible. Their effects are so small that you can just ignore it.

[shynung]

I'm quite sure no rocket engineers are stupid enough to expose nuclear fuel rods directly into an expansion chamber that has a direct access to everything outside the rocket. Therefore, I don't think we should expect any significant radioactive materials coming out of an NTR's exhaust gas.

[Seret]

I'd like to know because for realism's sake, i want to know whether it's okay to light up a LV-N inside the atmosphere in KSP, or whether it would just radiate the planet.

It's not really the planet that would be at risk, it's your spacecraft. The engines would generate a high neutron flux around them, even if you assume they'd be shielded well enough to stop them cooking the crew, it would embrittle and induce radiation in the structure.

[Sirrobert]

Since the fuel is only in indirect, very short contact with the radioactive material, it can not make the fuel radio active.

[Kryten]

I'm quite sure no rocket engineers are stupid enough to expose nuclear fuel rods directly into an expansion chamber that has a direct access to everything outside the rocket. Therefore, I don't think we should expect any significant radioactive materials coming out of an NTR's exhaust gas.

Then I suppose that makes the engineers of both the US and USSR NERVA programs 'stupid'. Remember we're talking about a rocket engine, weight and performance are far higher priorities than a few particles in the exhaust.

[shynung]

Then I suppose that makes the engineers of both the US and USSR NERVA programs 'stupid'. Remember we're talking about a rocket engine, weight and performance are far higher priorities than a few particles in the exhaust.

They actually did?

So they went back to the Soviets' early-rockets-design-protocol. Thank god they didn't fly that one.

In that case, their designs are probably supposed to work solely in deep space. Neither upper stages nor LEO tugs.

[lajoswinkler]

If NERVA uses hydrogen, it's a pretty safe exhaust. Hydrogen has a tiny cross section, meaning it doesn't capture thermal neutrons easily, and it takes two captures to reach tritium, so the amount of tritium expelled would be pretty small, probably negligible.

Ignoring the danger from the reactor meltdown and fission product release, the only danger from firing such engine in atmosphere would the the flammability of hydrogen. You'd need to make sure there is a safety flame at the nozzle, so that a great cloud of hydrogen-air mixture doesn't appear and detonate.

[shynung]

Aren't high-temperature gaseous hydrogen quite reactive already? I thought they would ignite in contact with the air. So high-temp GH₂ is not pyrophoric?

[wasmic]

They actually did?

So they went back to the Soviets' early-rockets-design-protocol. Thank god they didn't fly that one.

In that case, their designs are probably supposed to work solely in deep space. Neither upper stages nor LEO tugs.

Have you heard of the Saturn V-N? Yes, the N stands for nuclear. It was supposed to have a nuclear third stage. The Soviets also had plans for nuclear upper stages on the N1. However, I'm not quite sure how exposed the fuel rods were in those designs.

[Sirrobert]

Aren't high-temperature gaseous hydrogen quite reactive already? I thought they would ignite in contact with the air. So high-temp GH₂ is not pyrophoric?

We're talking about RADIOACTIVE, not reactive

[SargeRho]

It should be said that while solid core (NERVA type) and closed-cycle gas core NTRs don't have radioactive exhausts, things like open cycle gas core engines for example would have very radioactive exhausts.

[Moar Boosters]

Don't forget this was designed in the same era America nuked themselves about 900 times just to **** off the soviets. As long as the astronauts were protected, I doubt they had much concern what kind of radiation they'd be pumping into the upper atmosphere.

[lajoswinkler]

Aren't high-temperature gaseous hydrogen quite reactive already? I thought they would ignite in contact with the air. So high-temp GH₂ is not pyrophoric?

Wolfram Alpha says autoignition temperature of hydrogen is 536°C, so yes, active engine would release readily flammable hydrogen and it would look like this.

Safety flame, or perhaps sparklers, would simply have to be used because liquid hydrogen readily boils off. It is lighter than air and would go up, but explosive mixtures might form nevertheless.

[shynung]

Don't forget this was designed in the same era America nuked themselves about 900 times just to **** off the soviets. As long as the astronauts were protected, I doubt they had much concern what kind of radiation they'd be pumping into the upper atmosphere.

I see. Though, I can't help but feel somewhat insecure. I don't know what would happen to people's health if Saturn V-Ns were to start flying.

I wonder if they ever heard of the N1 accident. Have they ever thought of what would have happened should a Saturn V-N were to suffer the same fate?

At least, we don't have to worry about it for now. Even a single NTR rocket test is probably enough to cause significant public uproar. They're getting nucleophobic for some reason.

Edited May 12, 2014 by shynung

[Nibb31]

I see. Though, I can't help but feel somewhat insecure. I don't know what would happen to people's health if Saturn V-Ns were to start flying.

I wonder if they ever heard of the N1 accident. Have they ever thought of what would have happened should a Saturn V-N were to suffer the same fate?

At least, we don't have to worry about it for now. Even a single NTR rocket test is probably enough to cause significant public uproar. They're getting nucleophobic for some reason.

The S-N would only have been used once the stack was in LEO. It was lighter than the S-IVB that it would have replaced, so the S-II could have put it into orbit in one go without requiring it to do a circularization burn like the S-IVB did.

If the launch had failed, it would have sank to the bottom of the Pacific. The only case in which there might have been a release of radioactive particles is if the Saturn V had exploded on the pad. However, that is no different that RTGs that have been commonly launched by both the US and Russia.

[shynung]

Well, at least NASA cares enough about the environment to not fire it in the atmosphere (other than a few tests in a desolate area).

Regarding launch failures, I don't remember the last time NASA have gotten themselves a pad explosion for the last 30 years or so. If they are to fire the NTR only when in LEO, then I see nothing too dangerous with the exhaust; it would have scattered too widely to have any visible effects.

Not really sure about NK, though. But I won't delve further into that.

[wasmic]

Well, at least NASA cares enough about the environment to not fire it in the atmosphere (other than a few tests in a desolate area).

Regarding launch failures, I don't remember the last time NASA have gotten themselves a pad explosion for the last 30 years or so. If they are to fire the NTR only when in LEO, then I see nothing too dangerous with the exhaust; it would have scattered too widely to have any visible effects.

Not really sure about NK, though. But I won't delve further into that.

I'm not sure whether NASA has had any pad explosions recently, but they have had some explosions mere seconds into flight. There was a video of a Titan IV blowing up very soon after liftoff, and one of a Delta II.

[Idobox]

The S-N would only have been used once the stack was in LEO. It was lighter than the S-IVB that it would have replaced, so the S-II could have put it into orbit in one go without requiring it to do a circularization burn like the S-IVB did.

If the launch had failed, it would have sank to the bottom of the Pacific. The only case in which there might have been a release of radioactive particles is if the Saturn V had exploded on the pad. However, that is no different that RTGs that have been commonly launched by both the US and Russia.

The NERVA is a solid core reactor, ie rods of metal inside a ceramic casing. These things are pretty sturdy and don't catch fire easily, and even if they do, the ashes don't go far. And even if they do, it's not even very radioactive stuff.

The problem with nuclear accidents arise when the fuel is old and full of nasty fission products like iodine, which would not be the case for a Saturn V-N

[lajoswinkler]

The NERVA is a solid core reactor, ie rods of metal inside a ceramic casing. These things are pretty sturdy and don't catch fire easily, and even if they do, the ashes don't go far. And even if they do, it's not even very radioactive stuff.

The problem with nuclear accidents arise when the fuel is old and full of nasty fission products like iodine, which would not be the case for a Saturn V-N

NERVA reactor can't catch fire. Nuclear fuel in all reactors is in the form of oxide, which is a ceramic material with 2865°C melting point, inert to atmosphere. Only nuclear bombs have metallic uranium/plutonium inside, for reasons of critical mass achievement.

True, such engine would be a radiological hazard after it has been fired for some time.

[ScallopPotato]

I see. Though, I can't help but feel somewhat insecure. I don't know what would happen to people's health if Saturn V-Ns were to start flying.

I wonder if they ever heard of the N1 accident. Have they ever thought of what would have happened should a Saturn V-N were to suffer the same fate?

At least, we don't have to worry about it for now. Even a single NTR rocket test is probably enough to cause significant public uproar. They're getting nucleophobic for some reason.

Go to page 22 of this powerpoint. An unfired nuclear reactor on the pad or during ascent is much less radioactive than an RTG. Those poison rods are going to be deep in there until the reactor reaches a safe orbit. The presentation also talks about testing NTRs, and open-air tests aren't needed anymore.

[Nibb31]

The NERVA is a solid core reactor, ie rods of metal inside a ceramic casing. These things are pretty sturdy and don't catch fire easily, and even if they do, the ashes don't go far. And even if they do, it's not even very radioactive stuff.

The problem with nuclear accidents arise when the fuel is old and full of nasty fission products like iodine, which would not be the case for a Saturn V-N

I never said it would catch fire. I said that it might release radioactive particles into the atmosphere.

The explosion of the fuel of a Saturn V on the pad would be devastating. 500kT of TNT equivalent, a 2600°C fireball. A nuclear reactor located on top, even if it was protected by a casing, would be obliterated and any fuel rods inside would end up blown to pieces all around the pad. It would be the equivalent of a dirty bomb that would shut down pad operations for years.

[Kryten]

The explosion of the fuel of a Saturn V on the pad would be devastating. 500kT of TNT equivalent, a 2600°C fireball. A nuclear reactor located on top, even if it was protected by a casing, would be obliterated and any fuel rods inside would end up blown to pieces all around the pad. It would be the equivalent of a dirty bomb that would shut down pad operations for years.

How exactly would a small reactor full of fresh fuel be 'equivalent to a dirty bomb'? There'd be no fission products, all you'd have would be widely-scattered alpha-emitters. The best actual equivalent would be the K-431 accident-that resulted in a dockyard being unusable for only a few months, with much higher concentrations than would be present in your scenario.