Do NERVAs have radioactive propellant?

[Red Iron Crown]

Nice point, but remember that the leidenfrost effect will form an isolating barrier of steam

I wasn't aware of the proper name of that effect, thanks for mentioning it. Wouldn't the effect be more pronouced for liquid hydrogen, with its much lower boiling point?

[lajoswinkler]

The N1 explosion was huge, reckoned to be among the largest non-nuclear explosions in history, and it was RP-1/LOX. True, it may not be a detonation strictly speaking, but with enough of the stuff that doesn't really matter, so long as it mixes reasonably well. Fuel-air bombs are plenty explosive enough to see military use - and indeed, raise the point that in a rocket failure the fuel can still react with the surrounding air even if it doesn't mix well with the oxidizer.

It does matter. You are mixing energy and power here. A huge deflagration (fireball) can be so immense that it sets on fire some stuff around it by heat rays, but it is not a detonation. Detonations have much greater powers than deflagrations, though can, and often have, lower energies.

The Soviet N-1 explosion was estimated to be equivalent to 0.5 kT of TNT. The W54 nuclear warhead (used in the Davy Crocket nuclear bazooka or the AIM-26 air-to-air missile) ranged from 0.01 to 0.25kT of TNT. So yes, a Saturn V pad explosion would have been akin to a small yield nuclear device.

An earthquake can release as much energy as as several hundred megaton bombs, and the consequence can be few shaken buildings and spilled tea. That's why it's futile to put such info in the media. Again, power and energy aren't the same thing.

There are some definite high-energy pad explosions in that video. Detonation or explosion, the payloads of those rockets get a heavy beating. If an NTR had been on top of a Saturn V in the same situation, I would be hard pressed to guarantee that the reactor would survive in one piece.

In the worst case scenario, you'd get fairly large chunks of it, scattered over the launch area, with most of it crumpled somewhere. That is not a significant problem. Hazmat team would pick the mildly radioactive pieces and use special foams to imobilize any dust. It's enriched uranium, and that isn't very radioactive. It's fissile, but not very radioactive.

NERVA which has been working, and is about to reentry, now that would be a disaster. I-131, Cs-137, Sr-90, wow... A disaster.

Hazmat suits protect from alpha and beta radiation, not gamma rays which are the worst. The main reason they use hazmat suits is to avoid contamination of skin and clothes. They throw them away after use and they still have to limit exposure to gamma radiation.

The worst is hard beta, because we're opaque to it. Gamma poorly interacts with organisms, so even those photons have higher energy, much less of them actually manage to ionize the water in our bodies, which is the main reason why ionizing radiation is dangerous.

Hard beta is like bullets which explode inside you.

So those suits actually help a lot, and in addition, pose an obstacle to radiological contamination.

[Idobox]

Nice point, but remember that the leidenfrost effect will form an isolating barrier of steam

This is one of the main challenges in conventional nuclear reactor design, the hotter the reactor is, the better the bubbles insulate the core.

In most reactors, the cooling liquid (ie water, heavy water or here hydrogen) also serves as moderator. As a result, bubble formation is not too much of a problem, since it drastically reduce moderator density, and thus reaction power. This type of strong negative feedback is very good for safety. There is even a type of reactor called boiling water reactor that uses it.

The reason we try not to have bubbles is because they reduce the turbine efficiency and cause mechanical damage.

[Wahgineer]

Yes, the exhaust would be "radioactive": heat is a form of radiation (hence the "").

[Brotoro]

Radioactivity refers to the emission of ionizing radiation, so radiating infrared energy is in no way related to this.

[Wahgineer]

Radioactivity refers to the emission of ionizing radiation, so radiating infrared energy is in no way related to this.

I was kinda being sarcastic (and yes I understand that heat and ionizing radiation are not the same.)

[lajoswinkler]

In most reactors, the cooling liquid (ie water, heavy water or here hydrogen) also serves as moderator. As a result, bubble formation is not too much of a problem, since it drastically reduce moderator density, and thus reaction power. This type of strong negative feedback is very good for safety. There is even a type of reactor called boiling water reactor that uses it.

The reason we try not to have bubbles is because they reduce the turbine efficiency and cause mechanical damage.

What do you mean by reducing turbine efficiency? Bubbles in boiling water reactor are in the reactor vessel, where the water is. Turbine handles dry steam because there is a separator between the reactor and the turbine. If the separator should fail, droplets are the stuff that errodes the turbine.

If the bubbles are large enough, they displace enough water so that the rods can overheat even if the reactor is in a shutdown mode because of the fission products decay. In any way, you don't want them to clog up the fuel bundles.

In pressurized water reactor, you don't want them at all, so a pressurizer, using the feedback loop systems, holds the pressure high enough so that no boiling occurs.

The negative feedback you're mentioning serves the purpose of removing the threat of sudden power excursion, but can not prevent a meltdown.

[Idobox]

What do you mean by reducing turbine efficiency? Bubbles in boiling water reactor are in the reactor vessel, where the water is. Turbine handles dry steam because there is a separator between the reactor and the turbine. If the separator should fail, droplets are the stuff that errodes the turbine.

If the bubbles are large enough, they displace enough water so that the rods can overheat even if the reactor is in a shutdown mode because of the fission products decay. In any way, you don't want them to clog up the fuel bundles.

In pressurized water reactor, you don't want them at all, so a pressurizer, using the feedback loop systems, holds the pressure high enough so that no boiling occurs.

The negative feedback you're mentioning serves the purpose of removing the threat of sudden power excursion, but can not prevent a meltdown.

The water from the primary loop doesn't go through the turbine, but through a heat exchanger. If the water in primary boils, the heat transfer is much less efficient, which result is lower power ouput from the turbine.

The mechanical problem with bubbles is in the reactor: bubbles appearing and popping out inside the reactor apparently cause wear. Nothing terrible, but the reactor has to be designed for this.

It's true the feedback loop doesn't do anything for metltdown, since they are caused by natural decay, but having your core lying in a tank of coolant with a low boiling temperature can help keep it cool, simply by letting the coolant boil and escape. In this regard, BWR are safer than PWR because the coolant will keep circulating as long as the pipes are there, thus you will get a meltdown only if the water leaks out, while a PWR will melt down if the pumps stop working.

[lajoswinkler]

The water from the primary loop doesn't go through the turbine, but through a heat exchanger. If the water in primary boils, the heat transfer is much less efficient, which result is lower power ouput from the turbine.

The mechanical problem with bubbles is in the reactor: bubbles appearing and popping out inside the reactor apparently cause wear. Nothing terrible, but the reactor has to be designed for this.

You're describing PWR, but were referring to BWR. No steam bubbles are allowed in PWR, of course. The temperature is too high and it would be stressful for its zirconium cladding to be constantly heated and cooled by the frothing water.

It's true the feedback loop doesn't do anything for metltdown, since they are caused by natural decay, but having your core lying in a tank of coolant with a low boiling temperature can help keep it cool, simply by letting the coolant boil and escape. In this regard, BWR are safer than PWR because the coolant will keep circulating as long as the pipes are there, thus you will get a meltdown only if the water leaks out, while a PWR will melt down if the pumps stop working.

PWR has emergency systems which drop the pressure, allowing the boiling, and the excess tritium laden steam is dumped through the exhaust stack as one of the last resorts while waiting for the pumps to go back online.

I think the operators did that at TMI, resulting in very weak release of radionuclides into the environment.

[cantab]

One of the fun things regarding reactor safety is that the NERVA is practically a nuclear reactor cooled by liquid hydrogen.

Can you imagine a more effective way to cool something than pumping cryogenic liquid hydrogen through it with a rocket engine's turbopump?

I can think of many more reliable ways.

It does matter. You are mixing energy and power here. A huge deflagration (fireball) can be so immense that it sets on fire some stuff around it by heat rays, but it is not a detonation. Detonations have much greater powers than deflagrations, though can, and often have, lower energies.

It's true that most rocket explosions do look like fireballs, but more generally some deflagrations can be fast enough to to create a strong pressure wave, and a big deflagration will create more power than a small detonation by simple scale. I've not seen a clear pressure wave in any footage of rocket failures, but then I've not seen much high-quality footage, a lot of it's old. You can see one quite nicely in this explosion of a ton of gunpowder, showing a deflagration in general can be more than just a fireball: http://youtu.be/eFytcsA9mU8?t=1m21s. For fuel/air mixtures, or presumably fuel/oxidizer ones, I believe getting such a wave requires a quite precise mixture ratio of fuel vs air/oxidizer, making it unlikely in a rocket failure - but I wouldn't want to risk assuming it's impossible.

[lajoswinkler]

It's true that most rocket explosions do look like fireballs, but more generally some deflagrations can be fast enough to to create a strong pressure wave, and a big deflagration will create more power than a small detonation by simple scale. I've not seen a clear pressure wave in any footage of rocket failures, but then I've not seen much high-quality footage, a lot of it's old. You can see one quite nicely in this explosion of a ton of gunpowder, showing a deflagration in general can be more than just a fireball: http://youtu.be/eFytcsA9mU8?t=1m21s. For fuel/air mixtures, or presumably fuel/oxidizer ones, I believe getting such a wave requires a quite precise mixture ratio of fuel vs air/oxidizer, making it unlikely in a rocket failure - but I wouldn't want to risk assuming it's impossible.

Deflagrations do not cause pressure waves. That's what detonations do and it's the key thing which distinguishes them from deflagrations which by all means can be also devastating.

The process on the clip you've linked is detonation. Confined gunpowder, or a large pile of unconfined one, will detonate upon ignition. It is an intimate mixture of reagents.

Two tanks sitting one on top of another, containing LOX and kerosene, breaking and spilling their contents, can not cause such immense detonation. At best, it can be a strong deflagration. The reagents simply aren't properly mixed, and the flame front doesn't do the proper job, as it increases the volume.

Mind that a deflagration can turn into a detonation in some cases (MOAB, for instance).

The destructive nature of exploding liquid fuel rockets is due to the huge amount of fuel they store inside. But if that amount was premixed... oh god. You wouldn't want to be near that.

Those explosions in movies are all weak deflagrations. I cringe when I see a war movie where grenades look like Molotov cocktails. In real life, bombs cause pressure waves, and in most cases there's only a flash, but your soft tissues get destroyed if you're near them.

[Brotoro]

I believe that you mean that deflagrations don't cause shock waves, while detonations do cause shock waves.

My butt can produce pressure waves.

[Sirrobert]

I believe that you mean that deflagrations don't cause shock waves, while detonations do cause shock waves.

My butt can produce pressure waves.

Technicly, your but can also produce shockwaves. Though I don't think those are strong enough to count in this discussion

[Seret]

Mind that a deflagration can turn into a detonation in some cases (MOAB, for instance).

They can certainly be strong enough to induce sympathetic detonation in explosives. That's typically what happens when explosives are in a fire, one of them will deflag and cuase adjacent ones to detonate. That's why the rules about how much stuff you can stack in one pile and distances between them are so strict.

The principle difference is that deflagration is just a rapid oxidation, while detonation is a fundamental molecular breakdown. Different reaction, much more powerful.

Confined gunpowder, or a large pile of unconfined one, will detonate upon ignition.

Nope, still just deflagration, it's just that you can get enough smash out of it that containing it is a bad idea. That stuff is designed to generate lots of gas in a confined space.

[Seret]

I've not seen a clear pressure wave in any footage of rocket failures

This close enough for you?

www.youtube.com/watch?v=_KuGizBjDXo

Not strictly a rocket, but a factory making ammonium perchlorate for rocket motors, including SRBs.

[lajoswinkler]

I believe that you mean that deflagrations don't cause shock waves, while detonations do cause shock waves.

My butt can produce pressure waves.

Yes, that is a proper term, thanks.

They can certainly be strong enough to induce sympathetic detonation in explosives. That's typically what happens when explosives are in a fire, one of them will deflag and cuase adjacent ones to detonate. That's why the rules about how much stuff you can stack in one pile and distances between them are so strict.

The principle difference is that deflagration is just a rapid oxidation, while detonation is a fundamental molecular breakdown. Different reaction, much more powerful.

They indeed can, but a baby can flip the switch and cause a detonation, too, though we don't call that baby a detonator.

Nope, still just deflagration, it's just that you can get enough smash out of it that containing it is a bad idea. That stuff is designed to generate lots of gas in a confined space.

You mean a 10 metre pile of gunpowder, ignited in the bottom, won't detonate? Oh, it will. These things are exactly dependant on the bulk amount.

This close enough for you?

www.youtube.com/watch?v=_KuGizBjDXo

Not strictly a rocket, but a factory making ammonium perchlorate for rocket motors, including SRBs.

That is not the topic here. We're discussing liquid fuel rockets. PEPCON had ammonium perchlorate which is a compound containing all it needs for an intramolecular reaction, so that means it's in the class of the most thoroughly "mixed" explosives.

Two tanks of LOX and kerosene, ruptured by a falling rocket on a launch pad will cause a huge disaster, but won't produce these detonations.

If, however, the tanks are intact and in fire,

. That's a different scenarion.

[Sirrobert]

This close enough for you?

www.youtube.com/watch?v=_KuGizBjDXo

Not strictly a rocket, but a factory making ammonium perchlorate for rocket motors, including SRBs.

I don't think rockets have as much fuel anywhere near them as there was in that plant...

So no, not comparable

[Seret]

You mean a 10 metre pile of gunpowder, ignited in the bottom, won't detonate? Oh, it will. These things are exactly dependant on the bulk amount.

Correct, it won't detonate. Gunpowder (whether you mean black powder or modern nitrocellulose) won't detonate, it can't. Whether something can detonate is determined by it's molecular structure, not how much of it there is.

[Seret]

I don't think rockets have as much fuel anywhere near them as there was in that plant...

So no, not comparable

I wasn't really trying to prove any point, except that any large explosion will generate an overpressure, which can be visible. Just look at Vietnam footage of the Daisy Cutter bombs and you'll see that HE doesn't hold the monopoly on pretty blast waves.

[PakledHostage]

Then it's a good thing there wouldn't be any gamma, isn't it? Fresh reactor=no fission products=only alpha emitters, this has been pointed out twice already.

But the point Nibb made that you may have missed is:

I would be hard pressed to guarantee that the reactor would survive in one piece.

Perhaps the experts here can explain to the rest of us how it would be ensured that the reaction would not START as a result of a catastrophic failure of the rocket? How would it be ensured that the parts of the engine that control the rate of reaction in the core would not be damaged in some way that would affect their ability to function following a launch failure?

As has been mentioned several times in this thread, there is a difference between energy and power. But any reactor capable of powering a NERVA engine would have to have a very high reaction rate, presumably generating dangerous reaction products at an equally high rate. Can someone please explain how that reaction rate would be safely controlled following a catastrophic launch failure or pad explosion, given that the damage the engine sustains is difficult or impossible to quantify?

[Brotoro]

Perhaps the experts here can explain to the rest of us how it would be ensured that the reaction would not START as a result of a catastrophic failure of the rocket? How would it be ensured that the parts of the engine that control the rate of reaction in the core would not be damaged in some way that would affect their ability to function following a launch failure?

As has been mentioned several times in this thread, there is a difference between energy and power. But any reactor capable of powering a NERVA engine would have to have a very high reaction rate, presumably generating dangerous reaction products at an equally high rate. Can someone please explain how that reaction rate would be safely controlled following a catastrophic launch failure or pad explosion, given that the damage the engine sustains is difficult or impossible to quantify?

The reactor in the NERVA engine is a solid core (graphite moderator, ceramic fuel elements, support structure) surrounded by a beryllium neutron reflector. There are "control drums" located inside the beryllium reflector around the circumference of the reactor core. These drums are rods made of beryllium that are coated for one third of their circumference with boron. When the drums are rotated so that the boron coated sides are facing the core, the boron absorbs neutrons, and the core is sub-critical (the reactor is off). When the control drums are rotated so that their uncoated beryllium sides face the reactor core, there is enough reflector surface around the core to bounce back enough neutrons that nuclear chain reactions can happen (the reactor is on).

If you energetically dissassemble the NERVA with an explosion, you will not lose the control drums unless you also lose the beryllium reflector in which they are embedded. And without that reflector, the rector will not operate (the solid core itself is a sub-critical assembly).

[PakledHostage]

Thank you.

[Brotoro]

Oh…I was doing further reading: the NERVA would also have wires made of neutron-absorbing material inside the reactor core during prelaunch handling and during launch. These wires would not be removed until the engine was in space and was ready to be used.

[lajoswinkler]

Correct, it won't detonate. Gunpowder (whether you mean black powder or modern nitrocellulose) won't detonate, it can't. Whether something can detonate is determined by it's molecular structure, not how much of it there is.

Detonation is an explosion with a supersonic reaction front. That's how it is defined, and not by the molecular structure. If you have a barell of gunpowder and you seal it tightly, it will detonate upon ignition. A shockwave will be visible.

A three story pile of gunpowder will certainly detonate if it's ignited at the bottom, because the released gases can't escape fast enough, which causes an increase in pressure, temperature and, finally, reaction rate. It all goes very fast, but after few miliseconds, the bottom of the pile is subjected to an outward propagating supersonic shockwave. The pile will not deflagrate as a handful of gunpowder would, if ignited on its top.

Perhaps the experts here can explain to the rest of us how it would be ensured that the reaction would not START as a result of a catastrophic failure of the rocket? How would it be ensured that the parts of the engine that control the rate of reaction in the core would not be damaged in some way that would affect their ability to function following a launch failure?

As has been mentioned several times in this thread, there is a difference between energy and power. But any reactor capable of powering a NERVA engine would have to have a very high reaction rate, presumably generating dangerous reaction products at an equally high rate. Can someone please explain how that reaction rate would be safely controlled following a catastrophic launch failure or pad explosion, given that the damage the engine sustains is difficult or impossible to quantify?

I hardly doubt there is any expert on NERVA on the entire forums (they're mostly old people now), but you don't need to be for basic analysis.

NERVA engine's reactor doesn't have typical "pull out" rods. They have rotating reflector rods which makes the design very compact. I honestly don't see how could a locked, cold reactor be turned on in such event. You can't just turn on a reactor as you turn on a lamp. Every reactor has a procedure which ensures the spatial geometry of the neutron flux is such that criticality can occur. This is not an atomic bomb which can fizzle if you mess with its fissile material.

The procedure lasts for days for power plants and I doubt it's less than several hours for NERVA.

Brotoro, you ninja!

[Seret]

Detonation is an explosion with a supersonic reaction front. That's how it is defined

It's not, but I can see where you'd get that idea. Detonation has a supersonic wave, but not all supersonic waves are detonation.

One of my more interesting previous jobs was as an armourer in the military. You're right that people who work with explosives every day do talk as if velocity of detonation is the be all and end all, but that's because it is important when you're considering two substances that are detonable. You can smack a non-detonable substance as fast as you like, it won't detonate. If I wrap a lump of gunpowder (are we talking nitrocellulose?) in all the det cord in the world it won't detonate. Same goes for petrol, cheese sandwiches, hamsters and other combustible but non-detonable materials.

Let's back the truck up and take another look at your three-storey pile of gunpowder. What purpose is the large bulk of the pile serving? It's to provide a reaction force which counters the force of the expanding gas generated by combustion, yes? So what if we provide that reaction force by any other means, such as a hard steel case. So we have enough a propellant to generate very high pressures and velocities combusting inside a metal chamber. What device have we just described? A firearm, or an artillery piece. You're right that containing it does massively increase pressures, safety class explosives like small arms ammo are completely underwhelming when they combust in their packaging, but within the chamber things are different. IIRC chamber pressures in a 5.56mm rifle spike at about 40,000psi (275MPa). This doesn't result in detonation. I can't say I've studied the exact dynamics of bulk quantities of propellant in extreme reactions, I think it's quite possible you might be able to initiate multiple site of combustion via a supersonic shock wave if you set things up right, but the point is that the combustion reaction created would propagate by heat transfer, and is still a different reaction to detonation. It doesn't matter what the source of your high-VoD pulse is, it just won't create the effect you're looking for.

I'll leave to to a chemist to explain the exact nitty gritty of why some molecules are detonable and some aren't, because that's getting a bit outside my field. When I look at the molecules for things like nitrocellulose (non-detonable) and nitroglycerine (detonable) I see similar nitro groups, so I won't try to pretend I could tell you the difference. I'm pretty sure this forum can cough up someone who's got the chemisty-fu and can vouch for me on this though.