How dangerous is a NERVA during its lifetime?

[Elthy]

Since nuclear thermal rockets are quite promissing for manned interplanetary travel im thinking about future usage. What i want to know is:

How dangerous is a nuclear thermal rocket engine

-before first use (e.g. while still on the pad)

-during usage (e.g. earth escape burn)

-shortly after usage

-several months after usage (e.g. while staying in mars orbit)

-hundreds of yeas after usage (e.g. as a spend stage that reenters the atmosphere)

Are there new, better/saver nuclear fuels to be considered for them (e.g. thorium)? Whats the lifetime of the fuel rods, how many earth<->mars trips would be possible (with new hydrogen fuel)?

[Albert VDS]

It should be dangerous at all, if it's only used in space and shielded from the crew.

Ditching a NERVA should always be on a trajectory where it wouldn't impact an other celestial body.

[peadar1987]

Since nuclear thermal rockets are quite promissing for manned interplanetary travel im thinking about future usage. What i want to know is:

How dangerous is a nuclear thermal rocket engine

-before first use (e.g. while still on the pad)

Not dangerous at all, radiologically speaking. Chemically quite toxic, but less so than, say, nitrogen tetroxide or hydrazine, both of which are commonly used today.

-during usage (e.g. earth escape burn)

-shortly after usage

Screamingly radioactive inside. You don't want to get too close. Docking with space stations shortly after use wouldn't be a good idea! A shadow shield could easily completely protect the crew though. More radioactive when being fired than after shutdown, but that doesn't really matter, because the difference between "lethal dose in 1/10th of a second" and "lethal dose in 1/100th of a second" is only of academic importance!)

-several months after usage (e.g. while staying in mars orbit)

Again, will kill you extremely quickly. Do not lick!

-hundreds of yeas after usage (e.g. as a spend stage that reenters the atmosphere)

Will give radiation sickness after prolonged exposure, but the primary risk would be that of cancer. It's probably safest to crash spent stages into the moon or put them in an orbit that will take several thousand years to decay.

Are there new, better/saver nuclear fuels to be considered for them (e.g. thorium)? Whats the lifetime of the fuel rods, how many earth<->mars trips would be possible (with new hydrogen fuel)?

[Elthy]

If they are that dangerous i cant understand how NASA planned to use them. The shadowshield would work while the crew is on board, but what would happen if they tried to dock after mars ascent? Is it possible tom somehow mitigate the radiation problem?

[peadar1987]

If they are that dangerous i cant understand how NASA planned to use them. The shadowshield would work while the crew is on board, but what would happen if they tried to dock after mars ascent? Is it possible tom somehow mitigate the radiation problem?

Yep, don't approach it from the rear! As far as I can tell, the NERVA was actually planned to be shielded to the sides as well, so as long as you didn't point the nozzle at anything squishy, you'd probably be okay

[Elthy]

Maybe if you want to use them e.g. as a earth/mars shuttle with space stations as destinations on both ends it would make sense to place a heavy shield over the engine (e.g. with a roboter arm) before someone does e.g. EVA maintainace on other parts of the vehicle.

[peadar1987]

Maybe if you want to use them e.g. as a earth/mars shuttle with space stations as destinations on both ends it would make sense to place a heavy shield over the engine (e.g. with a roboter arm) before someone does e.g. EVA maintainace on other parts of the vehicle.

Hey, what do you know? Somebody already did the maths:

Yep, if you were using one as a shuttle, you would probably have a shield to place over it at one end of the journey, and just keep it there. However, as I said before, making a nuclear thermal rocket that's both efficient and long-lived is beyond our current capabilities, we have to choose one or the other.

[Wesreidau]

This is the same NASA that put a nuclear reactor on a B-36 Peacemaker. Men were men then. When you consider all of this is taking place in a pressurized tin can being thrown from one spinning rock to another on a mission that will last a good quarter or half of an orbit around a thermonuclear fireball, you realize monkeys weren't supposed to come down from trees so you may as well throw your tin can with style.

The shadow shield and propellant tanks are enough radioactive shielding to permit a docking with a station, if they approach and leave nose-first.

[lajoswinkler]

Yes, you need to approach the target carefully. This reflected stuff is commonly called skyshine when it happens on Earth.

[Kerbart]

Yep, don't approach it from the rear! As far as I can tell, the NERVA was actually planned to be shielded to the sides as well, so as long as you didn't point the nozzle at anything squishy, you'd probably be okay

To the best of my knowledge the propellant used by the real world NERVA would be hydrogen. I'm not a nuclear physicist (and not a lawyer eitherâ€â€it never hurts to state that) but how would hydrogen pick up any radioactivity? What would make the exhaust plume dangerous?

[peadar1987]

To the best of my knowledge the propellant used by the real world NERVA would be hydrogen. I'm not a nuclear physicist (and not a lawyer eitherâ€â€it never hurts to state that) but how would hydrogen pick up any radioactivity? What would make the exhaust plume dangerous?

Not the exhaust plume, the hole in the nozzle that goes into the reactor core. You'd have a shine path pretty much straight from the fuel elements.

[Elthy]

@peadar1987:

Is there something missing in your post?

@lajoswinkler:

Im not sure what you want to show with that picture...

@Kerbart:

Afaik Hydrogen could pick up a neutron, but that would make it harmless deuterium. As long as the fuel containment remains unbreached the only thing emited should be gamma rays, right?

[lajoswinkler]

To the best of my knowledge the propellant used by the real world NERVA would be hydrogen. I'm not a nuclear physicist (and not a lawyer eitherâ€â€it never hurts to state that) but how would hydrogen pick up any radioactivity? What would make the exhaust plume dangerous?

Nah, just some tiny amount of tritium. Skyshine would be a worse issue.

Im not sure what you want to show with that picture...

Docking with a target. You approach head on.

Not the exhaust plume, the hole in the nozzle that goes into the reactor core. You'd have a shine path pretty much straight from the fuel elements.

That can be avoided by coiling the hydrogen delivery conduit.

[peadar1987]

That can be avoided by coiling the hydrogen delivery conduit.

As far as I can see, none of the plans do that. Maybe fluid dynamics issues, maybe just not included on the prototypes, I don't know

[Hcube]

Afaik Hydrogen could pick up a neutron, but that would make it harmless deuterium. As long as the fuel containment remains unbreached the only thing emited should be gamma rays, right?

I'm pretty sure that's not possible since the only (or ultra majority of) interaction between hydrogen and neutrons (fast *and* thermal) is elastic diffusion... (That's why you want your nuclear-war shelter to be under a body of water btw !)

About emissions from the spent reactor, depending on the nature of the products of fission, there could be other stuff than gamma rays like Beta particles. Although those are quite easily and completely/reliably stopped by thin and light shields, i guess the problem would indeed be the gamma.

EDIT : on a (very much) more useless and pedantic note, the spent reactor would also emit (anti)neutrinos. Though those are very unlikely to hurt you ^^

Edited November 10, 2015 by Hcube

[davidy12]

I just read Voyage by Steven Baxter. (FANTASTIC book BTW, highly recommend it. Not the best Alt History/Mars mission book, but still amazing, and I feel as if it's under-rated.) And there are two things that kind of confuse me.

1.) When testing the engine in the beginning he said you shouldn't cluster multiple engines. Is that true? Granted most the Saturn Nerva designs I've seen have one engine on the S-IVB. But still.

2.) If the reactor core ruptures, would it be a danger to the crew (Granted I know that anyone who is close to a reactor core would be in danger but how badly and->)? Because how would it get in?

Edited November 10, 2015 by davidy12

[Hcube]

2.) If the reactor core ruptures, would it be a danger to the crew? Because how would it get in?

I guess the extreme heat of the reactor could damage the spacecraft, the radiation shield between the astronauts and the reactor, and maybe some radioactive material could be spilled around the spacecraft and irradiate the astronauts from the sides ? Anyway it would at least be as bad as having a regular engine exploding... Wich is very bad, especially if you only have 1.

[Jovus]

I suspect that, all told, a NERVA wouldn't be any more dangerous than certain rockets that used hypergolic propellant upper stages and were man-rated. The main difference, methinks, is that we're used to the former, and not the latter. (Which isn't just 'radiation scary', but we've done the engineering and testing for man=rated hypergolics, but no-one's ever ridden a nuclear thermal rocket.)

[RainDreamer]

You know, I feel like flying these things around is like pointing a gun at everything in the vicinity from the way you talk about it. Can any of these ships even pass by each other? Docking is going to be a much more nerve wrecking procedure than it already is now that accidently pointing your craft to the wrong place can kill a bunch of people.

Maybe instead of a shadow shield we have to bite the bullet and shield it more?

[Kryten]

I suspect that, all told, a NERVA wouldn't be any more dangerous than certain rockets that used hypergolic propellant upper stages and were man-rated. The main difference, methinks, is that we're used to the former, and not the latter. (Which isn't just 'radiation scary', but we've done the engineering and testing for man=rated hypergolics, but no-one's ever ridden a nuclear thermal rocket.)

The statistics on orbiting reactors don't really back that up... most of them ended up shedding debris, including the only US example, in many cases quite energetically.

[wumpus]

One dangerous thing about NERVA is that while it can take you to distant planets, nobody really knows how to get it to bring you home.

NERVA uses hydrogen. It has to, otherwise you don't get enough exaust velocity to bring your ISP up to sufficient reasons to use NERVA (presumably helium *should* work, although much less efficiently: but I think you would need to crank up the temperature to chemical rocket levels. My guess is that things [i.e. the nuke parts] would get *really* dangerous at that point.) Hydrogen leaks. Hydrogen also typically boils off: the X15 + B52 carried 2 and a half tanks worth of H2 for the X15. One tank to fly with and the other tank and a half to refill the tank as it boiled off. Current NASA plans to Mars include a "zero boiloff" H2 tank, but so far no such thing exists (presumably you "just" need to keep the tank under 33K (H2's critical point)). Even if you do that, expect to lose at least 1% of your H2 each month. That Isp of 800 looks at lot less good when you suddenly have a lot less fuel.

[NuclearNut]

On the launch pad, not dangerous at all, same goes for bringing it to orbit. Once you get the object into orbit, conduct criticality testing (which I assume would be done in orbit), and get the thing running, very radioactive. Now mind, the crew would be shielded, but if you pointed the exhaust part (assuming front and sides) at something squishy or most things electronic you would fry them. Thus it would be advisable to treat the device with a bit of respect, rather than acting like it is any other rocket engine. So a minor disadvantage for a major boost in ISP and possibly an extra power supply.

To the best of my knowledge the propellant used by the real world NERVA would be hydrogen. I'm not a nuclear physicist (and not a lawyer eitherâ€â€it never hurts to state that) but how would hydrogen pick up any radioactivity? What would make the exhaust plume dangerous?

From my understanding, some of it would be converted into tritium, most of it would be non-radioactive, and the tritium itself, as long as you do not ingest it or are in an atmosphere with significant quantities of it in it, is not a problem. This is because it is a beta emitter, and on a spaceship, the hull would provide sufficient shielding.

- - - Updated - - -

One dangerous thing about NERVA is that while it can take you to distant planets, nobody really knows how to get it to bring you home.

NERVA uses hydrogen. It has to, otherwise you don't get enough exaust velocity to bring your ISP up to sufficient reasons to use NERVA (presumably helium *should* work, although much less efficiently: but I think you would need to crank up the temperature to chemical rocket levels. My guess is that things [i.e. the nuke parts] would get *really* dangerous at that point.) Hydrogen leaks. Hydrogen also typically boils off: the X15 + B52 carried 2 and a half tanks worth of H2 for the X15. One tank to fly with and the other tank and a half to refill the tank as it boiled off. Current NASA plans to Mars include a "zero boiloff" H2 tank, but so far no such thing exists (presumably you "just" need to keep the tank under 33K (H2's critical point)). Even if you do that, expect to lose at least 1% of your H2 each month. That Isp of 800 looks at lot less good when you suddenly have a lot less fuel.

Well you can always use ISRU to extract hydrogen from the water that Curiosity found is quite present in martian soil. If you are going other places I am certain you can find sources of the most abundant element in the universe.

- - - Updated - - -

1.) When testing the engine in the beginning he said you shouldn't cluster multiple engines. Is that true? Granted most the Saturn Nerva designs I've seen have one engine on the S-IVB. But still.

You should not cluster unshielded reactors as each unit will emit neutrons, the neutrons will then be absorbed by the other reactor and in essence the multi reactor unit would be one big reactor, and that would mean more calculations and more complex controls, and you already have enough thrust as it is, so there is no point in adding more than one. Or at least that appears to be the thought process.

[Hcube]

snip.

How exactly would you get tritium in the exhaust ? Superheated hydrogen isn't exactly a very good at capturing thermal neutrons...

[radonek]

I would like to point you to http://www.projectrho.com/public_html/rocket/radiation.php#id--Radiation_Shielding. (Article is not specificaly about NERVA, but being only nuclear engine ever developed, it is featured prominently.)

[SomeGuy12]

Current NASA plans to Mars include a "zero boiloff" H2 tank, but so far no such thing exists (presumably you "just" need to keep the tank under 33K (H2's critical point)). Even if you do that, expect to lose at least 1% of your H2 each month. That Isp of 800 looks at lot less good when you suddenly have a lot less fuel.

What are you talking about? People store hydrogen long term all the time. It's really easy. You just have it in a sealed metal tank. It must be completely sealed, with the valves exiting the tank the kind that don't leak. (I'm not an expert on valve composition, you could just solder the valve itself if you had to)

Hydrogen likes to leak but it can't go through a solid metal wall. Then, you mount a cryocooler - NASA has tested some on the ground - on the tank and power it with solar panels, RTGs, or in the case of a NERVA rocket, you have a way to generate power using the same nuclear reactor you can use as an engine. (there are heat exchanger loops that go through the reactor core). The cryocooler recondenses the hydrogen vapor so the pressure inside the tank doesn't rise to the point it explodes.

Your monthly losses will be vanishingly close to zero. Why hasn't NASA flown something like this? Actually, they have, how do you think IR telescopes work?