Do Nervs pollute the atmosphere?

[A_name]

I was wondering how harmful to the environment it is to fire them Nervs inside the atmosphere. What do you guys think? Should a sustainable space program avoid doing that?

[KerikBalm]

No, solid core nuclear thermal rockets do not have radioactive exhaust.
 No Uranium or fission products escape the fuel rods, its just hydrogen passing by something really really hot.

That said, after you fire up a reactor, it will emit a lot of radiation, and you wouldn't want to get close to it, a spacecraft would have a shadow shield, but approach it from any other direction and its bad.

As soon as its recovered and stored in a place enclosed in lead/concrete/etc, everything is fine, and there is no lingering radioactivity from emitted exhaust.

The Nerv is radioactive, but nothing else is, its all contained in one place... unless your rocket explodes... then it could be bad.

[ModZero]

Putting anything at very high temperature in the atmosphere and you'll end up with interesting nitrogen compounds (atmosphere provides its own nitrogen and oxygen for some basic pollutants, you just need to heat up and stir). Whether the amount is high enough to be a bother is another matter, though with the kerbalistic amount of launches anything is possible. 

I'd hope that ksp nukes are closed cores, so should be fine, but seeing the general mood of the game, I wouldn't be surprised by anything. Ah, and apparently the cores should be able to survive the rocket not going to space today. The bigger wonkier cousins (pulse engines) are fun, as they produce part of the "pollution" as electromagnetic pulses (see: Starfish Prime, please no Orion drives in my magnetosphere). 

[radonek]

Well there is a story about how during first Nerva run engineers saw bright flashes in exhaust plume. These turned out to be white-hot specks of core plumbing which could not stand thermal stress and cracked&melted. Whole thing had to be scramed and decontaminated…  Plus there is this small thing with neutron flux activating everything, including ship itself. So - yes, exhaust itself is fine under normal conditions. But "common" failure modes would involve spewing hot radioactive stuff Very Fast. let's say there is no pollution in normal operation but don't expect FAA to let it break free from remote desert testing stations. 

[lajoswinkler]

It's not like anyone would use such engine inside atmosphere. Its thrust is pathetic. That's a vacuum engine.

In any case, it would release only a tiny amount of tritium generated by hydrogen being bombarded by an intense neutron flux. Radiation, however, would be severe. It's basically a naked fission core. Doses near it would be absolutely insane.

And we also know what does it look like when such reactor goes supercritical and blows itself apart.

[PB666]

2 hours ago, lajoswinkler said:

It's not like anyone would use such engine inside atmosphere. Its thrust is pathetic. That's a vacuum engine.

In any case, it would release only a tiny amount of tritium generated by hydrogen being bombarded by an intense neutron flux. Radiation, however, would be severe. It's basically a naked fission core. Doses near it would be absolutely insane.

And we also know what does it look like when such reactor goes supercritical and blows itself apart.

The heat from the engine itself would fry you, why are you worried about gamma, tritium is considered an environmental threat, most of the neutrons would end up in dueterium anyway before injection, they would not spend enough time close enough to the engine to become tritium.

The problem is that the best Nerva have the uraniums and hydrogen mixing in close quarters some radioacitive uranium would be lost.

 

[lajoswinkler]

No, most of the heat is carried away by hydrogen. That's why NERVA doesn't need radiators in its most basic setup.

The worst threat it offers is insane neutron and gamma ray flux. I mean it's absolutely insane, it would probably kill in seconds few metres away.

As I've said, only a tiny amount of tritium would be produced, nothing to worry about. No uranium is lost and no fission products are lost as they're isolated. The core components aren't naked and leaking out.

Edited December 21, 2015 by lajoswinkler

[KerikBalm]

The radiation would fry you at a much greater distance than the heat of a naked core would fry you. Tritium production would be miniscule... and tritium is already present in our atmosphere, the concentration wouldn't be changed by running these engines at any reasonable scale of operation.

And tritium's half life is only 12 years, so its not that bad in the longer term.

[A_name]

Hey, thank you all for your comments! I guess I'll keep firing away those nukes!

@lajoswinkler The reason I would want to use nukes in the atmo is for SSTO's that use no oxidizer. Go fast and high enough on jets and you can get into orbit with just the nukes. They actually become useful (i.e. producing almost their peak thrust at almost their peak ISP) lower than I expected, at about 10km. That's an awesome pic, by the way!

 

[K^2]

It's important to distinguish between "shouldn't" and "doesn't" in this case. A well-built and well-shielded closed-cycle nuclear thermal rocket running on pure hydrogen as reaction mass, will not produce any radioactive contamination. Any impurities in your hydrogen supply, and this claim is void. Any cracks, leaks, flakes, etc on your reactor or plumbing, and this claim is void. Any dust that managed to accumulate on pretty much any internal part of the rocket, including just under the shielding, and this claim is void.

There are a lot of little things that can go wrong to result in radioactive isotopes in the plume. And this can vary from insignificant to catastrophic.

[PB666]

1 hour ago, K^2 said:

It's important to distinguish between "shouldn't" and "doesn't" in this case. A well-built and well-shielded closed-cycle nuclear thermal rocket running on pure hydrogen as reaction mass, will not produce any radioactive contamination. Any impurities in your hydrogen supply, and this claim is void. Any cracks, leaks, flakes, etc on your reactor or plumbing, and this claim is void. Any dust that managed to accumulate on pretty much any internal part of the rocket, including just under the shielding, and this claim is void.

There are a lot of little things that can go wrong to result in radioactive isotopes in the plume. And this can vary from insignificant to catastrophic.

I dont think there can be enough shielding to attenuate neutronic radiation once it gets started, iron doesnt have enough scatterring potential in a 2 cm thick containment to stop the nuetron, but by the time you are 100 meteres away it would hardly matter. 

The are some high isp designs that churn the heater and hydrogen together, i can almost guarantee theses designs have radioactive elements in the plume. Higher operating temp means higher isp means more plume contaminants. 

[fredinno]

Generally, the public won't allow for NERVA use in the atmosphere anyways. Even in LEO is a big problem for the public, as they probably would think it's spewing radiation across the world (it's not).

[RainDreamer]

Nope if operating normally. It is only when things are not operating normally that carries the risk of pollution.

[lajoswinkler]

14 hours ago, A_name said:

Hey, thank you all for your comments! I guess I'll keep firing away those nukes!

@lajoswinkler The reason I would want to use nukes in the atmo is for SSTO's that use no oxidizer. Go fast and high enough on jets and you can get into orbit with just the nukes. They actually become useful (i.e. producing almost their peak thrust at almost their peak ISP) lower than I expected, at about 10km. That's an awesome pic, by the way!

 

There is one problem, though. When you release gamma rays and neutrons in air, they are deflected and in the process they lose some of their energy. X-rays are emitted as secondary radiation. So even if you had a beam of neutrons and gammas, you'd end up with an "umbrella" radiating some of it back at you, and you don't have a beam but approximately spherical source, making things even worse.

[wibou7]

(Wow, interesting thread!)

Soooo...  Since the NERV existed (in a lab setup lost in the desert, but still), what were they trying to accomplish with it if you can't basically use it without frying out the astronauts in the ship attached to it?

Since it was back in the early '60, they couldn't have realistically planned to use it for unmanned rocket?

[Nibb31]

They planned to use it as an upper stage for a variant of the Saturn V. The Nerva engine would be used in the S-N stage, which would replace the good old S-IVB for missions to Mars. However, when you look at the numbers, the trade studies weren't that favorable. The S-N would have had a higher Isp, but the Nerva with LH was much heavier than the J-2 with LOX and LH. It also had lower thrust, was much more expensive, and required the construction of a whole new nuclear logistics infrastructure at KSC with all the specialized handling, safety, and security.

In the end, it wasn't worth it. The plans for Mars were scrapped, S-N was cancelled, and Nerva was abandoned.

[SomeGuy123]

Could it be worth it if you used a NERV the way we do in KSP, as a workhorse engine for an interplanetary vessel?  If you get to use a single NERV that is relatively low mass compared to the total propellant it processes, and it essentially increases the efficiency of every kilogram of propellant by about 2.5 times (1000-1200 ISP instead of 400), it works out, right?

Basically you'd build the interplanetary vessel by launching a heavy lift rocket like the S5 dozens of times, gradually adding and bolting together the crew module, science payload, lander, many many propellant tanks that keep themselves cold and the H2 in them in a slush state, and then you'd launch the NERV as a series of separate subcritical pieces.

I guess the unfueled reactor would be a piece, then the fuel would be packed into separate modules that you'd somehow install in orbit.  Be tricky to come up with a way to pack the fuel so you don't have to haul a bunch of dead weight in packaging but can prevent it from hitting critical mass.  

As a side note : does anyone have any specific information on if H2, kept in a slush state (where there is solid H2 frozen around the cooling tubes and it is in equilibrium with a liquid phase inside the same tank) will leak through the metal walls of a storage tank?  I know gaseous H2 will, but the liquid phase has drastically lower velocities, so...

[MarvinKitFox]

On 2015/12/22 at 1:15 AM, wibou7 said:

(Wow, interesting thread!)

Soooo...  Since the NERV existed (in a lab setup lost in the desert, but still), what were they trying to accomplish with it if you can't basically use it without frying out the astronauts in the ship attached to it?

Since it was back in the early '60, they couldn't have realistically planned to use it for unmanned rocket?

Using a nuclear thermal rocket in the earth's atmosphere is quite insane, as:

1) if anything, but *anything* goes wrong, it can spew its guts all over the landscape. This is the very definition of Bad.

2) Backscatter from the surrounding atmosphere would require shielding in all direction for any crew or electronics on board.

 

Using a nuclear thermal rocket in space has great potential, although still a bit risky.

1) The radiation flux around your motor is intense* . You need to be several hundred meters away, or you need to impose many tons of radiation barrier between the engine core and anything delicate. (like pilots, electronics, plastics, steel girders, etc) Yes, i said steel girders!! Just what do you think happens to a steel beam exposed to a longduration intense neutron flux?

2) If it blows up, and *any* of the core debris could hit earth at any time in the next several millennia, you will have a bad day. This kinda means you need to operate the thing only outside of Earth's gravitational influence, **including** any earth-orbit-crossing orbit track!

 

Note that an unfired Nerva-class motor is reasonably benign, you have neatly packaged fuel that can even be in a reentry-safe housing. The danger is that after your motor has operated for several minutes, the whole motor + casing + superstructure becomes intensely radioactive due to the ludicrous unshielded neutron flux that is a basic part of the operation of this sort of motor.

 

Definition of "Intense" as used here:

At full operating thrust, the engine emits a fast neutron flux of about 2 * 10^14 neutrons per square cm per second, at a distance of 3 meters outside the unshielded motor casing.

Even ignoring the gamma flux, this is a 100% lethal dose of neutrons in 1/8600 of a second!!!!!!!!!

Or a lethal dose in ONE SECOND at a range of 300m

 

 

Edited December 25, 2015 by MarvinKitFox
aHem. table used is per hour, not per second. figures fixed

[SomeGuy123]

7 hours ago, MarvinKitFox said:

Definition of "Intense" as used here:

At full operating thrust, the engine emits a fast neutron flux of about 2 * 10^14 neutrons per square cm per second, at a distance of 3 meters outside the unshielded motor casing.

Even ignoring the gamma flux, this is a 100% lethal dose of neutrons in 1/8600 of a second!!!!!!!!!

Or a lethal dose in ONE SECOND at a range of 300m

Source?  I believe you, I'd like to see the numbers because I wonder

     1.  What does power level do to this?  Would a futuristic fusion drive with a huge ISP and enormous energy level be many times worse?

     2. Just how many tons of shielding do you need.  Is it enough to reduce the neutron dose to "fatal dose takes 1 year"?  That would mean 24 halvings of the neutron flux.  Simple google searches won't tell me how much shielding you need to do that.  

    3.  Geometry wise, your shadow shield must be at least the diameter of the engine, right?  It cannot be smaller than that, because if you do a ray diagram, you end up with a small cone that gets smaller with distance from engine.  If the shadow shield is slightly larger than the engine diameter (if the engine is a cylinder, this is a disk bolted on top), your ray diagram creates a large shadow, growing bigger with distance.  

    4.  I read that hydrogen is one of the best forms of radiation shielding.  Conveniently, it also is what you use for propellant.  Why not skip the heavy shielding materials and have hydrogen tanks.  This also lets you use the remaining hydrogen as propellant if you get desperate or would rather get the ship back home even if it kills the crew.

[MarvinKitFox]

58 minutes ago, SomeGuy123 said:

Source?  I believe you, I'd like to see the numbers because I wonder

     1.  What does power level do to this?  Would a futuristic fusion drive with a huge ISP and enormous energy level be many times worse?

     2. Just how many tons of shielding do you need.  Is it enough to reduce the neutron dose to "fatal dose takes 1 year"?  That would mean 24 halvings of the neutron flux.  Simple google searches won't tell me how much shielding you need to do that.  

    3.  Geometry wise, your shadow shield must be at least the diameter of the engine, right?  It cannot be smaller than that, because if you do a ray diagram, you end up with a small cone that gets smaller with distance from engine.  If the shadow shield is slightly larger than the engine diameter (if the engine is a cylinder, this is a disk bolted on top), your ray diagram creates a large shadow, growing bigger with distance.  

    4.  I read that hydrogen is one of the best forms of radiation shielding.  Conveniently, it also is what you use for propellant.  Why not skip the heavy shielding materials and have hydrogen tanks.  This also lets you use the remaining hydrogen as propellant if you get desperate or would rather get the ship back home even if it kills the crew.

Nerva technical docs: http://fas.org/nuke/space/nerva-spec.pdf

        the more relevant bits are around page 64

 

Some info derived from:

http://www.calculator.org/property.aspx?name=radioactive%20dose%20equivalent

https://www.osha.gov/SLTC/radiationionizing/introtoionizing/ionizinghandout.html

http://www.kayelaby.npl.co.uk/atomic_and_nuclear_physics/4_7/4_7_3.html

 

Power levels:

The radiation output scales almost linear with reactor power output.

Nerva was designed around a 4Gw reactor, final plans were for a 7.5-8Gw design.

 

 

Yes, Hydrogen is an **excellent** shield against neutron radiation, especially as only a miniscule amount of it will get changed into anything radioactive. H->Deuterium->Tritium is needed, very unlikely.

Tank heating is a factor you will have to work around, but it is by no means unsolvable.

In space, if there are no friendlies within several kilometers to the side or rear, a nuclear thermal can be made quite safe to operate without needing any explicit shielding.

Just put your H2 fuel tank in front of the reactor core, then your water+foodstuffs+inert supplies, *then* your delicate crew and electronics.

The Aerojet Nerva spec allows for shielding sufficient to limit crew exposure to under 10 rem per hour of thrust.

 

Where a Nerva-style nuclear thermal is dangerous is when operating under full thrust, and for anyone/anything that is near it in a direction where you can not afford to slap in several tons of shielding. Such as to the rear!

Edited December 25, 2015 by MarvinKitFox

[MarvinKitFox]

On 2015/12/23 at 3:09 AM, SomeGuy123 said:

As a side note : does anyone have any specific information on if H2, kept in a slush state (where there is solid H2 frozen around the cooling tubes and it is in equilibrium with a liquid phase inside the same tank) will leak through the metal walls of a storage tank?  I know gaseous H2 will, but the liquid phase has drastically lower velocities, so...

Hydrogen does permeate through most materials, but for more-or-less unpressurised LH2 the amount is utterly negligible as to loss of fuel.

You do need to worry about just what all the hydrogen is doing to your fittings (embrittlement, etc..) but simple thermal boiloff is a cause of fuel loss several magnitudes bigger than permeability losses.

On 2015/12/23 at 3:09 AM, SomeGuy123 said:

I guess the unfueled reactor would be a piece, then the fuel would be packed into separate modules that you'd somehow install in orbit.  Be tricky to come up with a way to pack the fuel so you don't have to haul a bunch of dead weight in packaging but can prevent it from hitting critical mass.  

Just keep your fuel rods well separated, and keep them away from the delicates.

Silicon and carbon-based lifeforms (computers and Kerbals) seem to be allergic to the effects of too much radiation.

[Rune]

On 23/12/2015 at 2:09 AM, SomeGuy123 said:

Could it be worth it if you used a NERV the way we do in KSP, as a workhorse engine for an interplanetary vessel?  If you get to use a single NERV that is relatively low mass compared to the total propellant it processes, and it essentially increases the efficiency of every kilogram of propellant by about 2.5 times (1000-1200 ISP instead of 400), it works out, right?

Basically you'd build the interplanetary vessel by launching a heavy lift rocket like the S5 dozens of times, gradually adding and bolting together the crew module, science payload, lander, many many propellant tanks that keep themselves cold and the H2 in them in a slush state, and then you'd launch the NERV as a series of separate subcritical pieces.

I guess the unfueled reactor would be a piece, then the fuel would be packed into separate modules that you'd somehow install in orbit.  Be tricky to come up with a way to pack the fuel so you don't have to haul a bunch of dead weight in packaging but can prevent it from hitting critical mass.  

As a side note : does anyone have any specific information on if H2, kept in a slush state (where there is solid H2 frozen around the cooling tubes and it is in equilibrium with a liquid phase inside the same tank) will leak through the metal walls of a storage tank?  I know gaseous H2 will, but the liquid phase has drastically lower velocities, so...

Actually, a NERVA core is sub-critical by itself, so no need to worry about keeping the fuel in separated pieces. The thing that takes the core past criticality are neutron reflectors (to increase the neutron flux over the fuel rods past criticality), that are kept misaligned whenever the reactor is not in operation. That is how they control the power output, and how they make sure that even is the reactor suffers a catastrophic hydrogen explosion that mangles it, you don't get a dirty nuke out of it, just a shower of nasty stuff that is activated only if the reactor has already been turned on. So actually carrying NERVAs to space should be much safer than, say, launching RTGs already full of radioactive material.

And yes, a NERVA such as the one studied on the sixties would be a wonderful in-space engine, coupled with a secondary thruster system for operations in proximity to other manned vessels (i.e. space stations). But NERVA wasn't the "thrustiest" NTR you can build, some other studies have looked at making TWR>>1 engines, for SSTO applications, and apparently you can actually lift off vertically with enough fuel on top to make orbit, plus a very significant payload. Good luck getting permission to activate one of these reactors inside the atmosphere, though. Neutron scattering alone would be a very serious thing, never mind a launch failure spewing nasty stuff all over the place.

 

Rune. Hope that clears the misconception.

[Bill Phil]

As much as a stream of hot hydrogen harms the environment. And the radiation, if you have a shadow shield and not a complete shield.

But you wouldn't use one in atmo. That thrust level is much too low.

[AeroGav]

On 22/12/2015 at 0:23 AM, A_name said:

Hey, thank you all for your comments! I guess I'll keep firing away those nukes!

@lajoswinkler The reason I would want to use nukes in the atmo is for SSTO's that use no oxidizer. Go fast and high enough on jets and you can get into orbit with just the nukes. They actually become useful (i.e. producing almost their peak thrust at almost their peak ISP) lower than I expected, at about 10km. That's an awesome pic, by the way!

 

If you're going to use a nuclear reactor in the lower atmosphere you may as well have it do some work lower down too.

Interstellar mod allows you to mount a pebble bed reactor and channel it's heat to either a nuclear turbojet or nuclear thermal rocket.   

In turbojet mode, reactor operating temperatures can be lower than in thermal rocket mode, since we are no longer trying to heat the propellant as hot as possible to maximise the delta-v gains from a finite quantity of stored liquid.  Instead we have limitless quantities of atmospheric air , but are constrained by the max working temperature of the turbine blades of the jet engine, which are some way below the operating temperatures of a NERV. 

This actually suits pebble bed reactors, whose heat output declines with increasing core temperature -so there should be more power in turbojet mode.

[lajoswinkler]

On 25.12.2015. at 8:50 AM, MarvinKitFox said:

Just what do you think happens to a steel beam exposed to a longduration intense neutron flux?

Nothing that doesn't happen to reactor vessels in same or worse conditions. Granted, those vessels are much thicker, but then again, NERVA works for minutes/hours compared to power plants.