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third person
08-31-2010, 06:06 AM
Hi, I've a hypothetical situation to present to the more sciencey types here. When something is described as nuclear-powered, it is fission that generates that power, correct? What then, in a hypothetical situation, could cause that fission to cease immediately? Barring the removal of the source of radiation (assume it's impossible to remove it) what kind of substance or material could be introduced that would diffuse the power generated?

If my question still makes no sense, I'll try to further break down what I'm looking for. In the meantime I'll be trying to figure it out myself through strenuous internet research. Thanks in advance for any help or advice you can give.

hammerklavier
08-31-2010, 06:18 AM
1. There are various materials which soak up the neutrons which are causing the nuclear fission chain reaction. Control rods are the usual terminology. I think the soviets used graphite, but other materials can work also.

2. If the nuclear fissile material is embedded into an expanding medium, such as metal rods, then heating the metal rods will move the atoms apart. Provided the concentration was just right, this would shut down the reaction.

third person
08-31-2010, 06:30 AM
1. There are various materials which soak up the neutrons which are causing the nuclear fission chain reaction. Control rods are the usual terminology. I think the soviets used graphite, but other materials can work also.

2. If the nuclear fissile material is embedded into an expanding medium, such as metal rods, then heating the metal rods will move the atoms apart. Provided the concentration was just right, this would shut down the reaction.

Thank you for your hasty reply! Let's go with the neutron-soaking theory using graphite as an example. Are you saying if something made of this material were introduced into the environment in which fission occurred that the process would be significantly disturbed? Say, a graphite artillery shell penetrating a (hypothetically) small nuclear reactor?

EDIT: As I thought, graphite is somewhat brittle with a Mohs hardness of say 1.5. My hypothetical "graphite shell" would therefore be implausible. Dangit!

hammerklavier
08-31-2010, 08:09 AM
Just blowing up the reactor would end the reaction, if you want to go that route. It would spread radiation too, though.

third person
08-31-2010, 11:27 AM
Just blowing up the reactor would end the reaction, if you want to go that route. It would spread radiation too, though.
Although you make a good point, the reactor was hypothetical and unrelated to my WIP. I have a creature that synthesizes energy via nuclear fission. I was trying to brainstorm a feasible weapon to use against such a creature. Perhaps graphite-based ammunition would do the trick.

zerospark
08-31-2010, 11:50 AM
Is halting the nuclear reaction essential to stopping the creature, versus just hitting it with exploding things?

Cyia
08-31-2010, 02:20 PM
Is this a biological creature you're talking about? If it is, then you need to consider the amount of heat its going to put off if it routinely initiates fission. It's going to have to have some sort of internal cooling system (like was needed for the rods mentioned, IIRC). It might be more practical to find a way of interrupting the cooling system than to interrupt the reaction itself, then it could succumb to its own biology.

Kenn
08-31-2010, 03:53 PM
A reactor needs a moderator to convert fast neutrons to thermal ones and maintain the fission process. This can be graphite but it is very commonly water (dualling as the coolant), which would fit in well with the creature. The reaction is controlled by rods which absorb the neutrons. There are various substances you could use and boron is the one most people have heard about. Silver and cobalt would work as well. So if you shot the beastie with silver arrows, you could theoretically kill it! In practice, there is a minimum size for a reactor, although you can get them in the 10s of kW range. A human produces only 0.1 kW of heat though, so it would need to be quite a big creature.

third person
08-31-2010, 06:11 PM
Is halting the nuclear reaction essential to stopping the creature, versus just hitting it with exploding things?
Yes, halting the reaction will render it unable to heal, thus clearing the way for the "exploding things" to finish it off.


Is this a biological creature you're talking about? If it is, then you need to consider the amount of heat its going to put off if it routinely initiates fission. It's going to have to have some sort of internal cooling system (like was needed for the rods mentioned, IIRC). It might be more practical to find a way of interrupting the cooling system than to interrupt the reaction itself, then it could succumb to its own biology.
No, it is synthetic with no living tissue whatsoever. I have to admit I never thought of the heat problem, however. thanks for giving me something to think about.


A reactor needs a moderator to convert fast neutrons to thermal ones and maintain the fission process. This can be graphite but it is very commonly water (dualling as the coolant), which would fit in well with the creature. The reaction is controlled by rods which absorb the neutrons. There are various substances you could use and boron is the one most people have heard about. Silver and cobalt would work as well. So if you shot the beastie with silver arrows, you could theoretically kill it! In practice, there is a minimum size for a reactor, although you can get them in the 10s of kW range. A human produces only 0.1 kW of heat though, so it would need to be quite a big creature.
I'd like to be "different" and not have a "silver bullet" solution. However, when I saw Boron in your reply I went to trusty Wikipedia (I know I know) and looked the element up...and discovered boron carbide. Now that is some very hard stuff that also absorbs neutrons. Interesting ;)

I am now pondering the believability of ammunition made of boron carbide with a graphite core.

Thanks for the help, people. I really appreciate it.

Lhun
08-31-2010, 06:30 PM
A reactor needs a moderator to convert fast neutrons to thermal ones and maintain the fission process.Not necessarily, that all depends on the specific fission chain that's being used. A cooling system is also not necessarily required. The energy output of fission is pretty much only in the form heat energy, which has to be converted into something useful. If that conversion has a high enough efficiency, there is not a lot of waste heat.(steam turbines don't)


Thank you for your hasty reply! Let's go with the neutron-soaking theory using graphite as an example. Are you saying if something made of this material were introduced into the environment in which fission occurred that the process would be significantly disturbed? Say, a graphite artillery shell penetrating a (hypothetically) small nuclear reactor?Unfortunately, there is nothing that works in the quantities you describe here. I.e. "soaking up neutrons" isn't so much soaking as just blocking them, so a small amount of graphite or water will not do much. You can't use it like a poison, you have to use it like water against a fire, and the absorbing material needs to be mixed with the fissile material.
Maybe you could just used sufficiently powerful explosives to hit the part of the creature where the fission takes place. If the fissile material gets scattered, the reaction also stops.

third person
08-31-2010, 06:45 PM
Unfortunately, there is nothing that works in the quantities you describe here. I.e. "soaking up neutrons" isn't so much soaking as just blocking them, so a small amount of graphite or water will not do much. You can't use it like a poison, you have to use it like water against a fire, and the absorbing material needs to be mixed with the fissile material.
Maybe you could just used sufficiently powerful explosives to hit the part of the creature where the fission takes place. If the fissile material gets scattered, the reaction also stops.

Thank you for your reply. A couple of responses to your post:



A small amount of whatever neutron soaking/blocking material would not work, I agree. But in my idea to weaponize it an overwhelming assault of said material would certainly be effective.
In saying the absorbing material needs to be mixed with the fissile material do you mean my "ammunition" would also have to be radioactive?

Lhun
08-31-2010, 07:25 PM
A small amount of whatever neutron soaking/blocking material would not work, I agree. But in my idea to weaponize it an overwhelming assault of said material would certainly be effective.
In saying the absorbing material needs to be mixed with the fissile material do you mean my "ammunition" would also have to be radioactive?
I think you need a basic idea of how fission works for this.
Basically, one atom of the fissile material spontaneously decays and emits a neutron. That neutron hit another atom of fissile material and causes it to decay, emitting more than neutron in the process. These neutron each hit more atoms etc. You don't get a runaway reaction since many neutrons are lost to the environment (i.e. don't hit anything before leaving the reactor, or a hit an atom which does not emit any more). To get a stable reactor you need to pick the specific fissile material, and have a required amount density etc. since it doesn't just work on its own. But that's not really relevant here.
What is relevant however is that neutron absorbing materials simply work by getting hit by a neutron, and instead of decaying and emitting more neutrons, they just absorb the incoming neutron.
So, to prevent nuclear fission from happening, the neutron absorbing material has to be placed between the atom that emits the neutron, and the atom that was supposed to get hit by the neutron. You have to mix the radioactive material and absorptive material.
Maybe visualize it with dominoes. The neutron absorbing material is like a tile that doesn't fall over. Just putting it next to the other tiles doesn't work. And unfortunately, just putting one the middle doesn't work either, since a reactor block isn't a single line of tiles, but a big three dimensional block where one tile can kick over other tiles in all directions. To stop the reaction, you have to thoroughly mix stopping tiles with normal tiles.
Likewise in a reactor, a neutron absorbing material works just like putting a lot of space between clumps of fissile material, i.e. the fissile material on the right side of the graphite can no longer react with the fissile material on the left side of the graphite. (in practice, the common design is a hollow ring of fissile material so the control rod can be put in the middle) But if there's enough fissile material on either side, the control rod won't stop that.
So, you can probably see why one, or even several bullets sticking in a clump of fissile material aren't going to work. You'd have to put about as much absorptive material in the bullets as there's fissile material in the reactor. It would be far more practical to try and (violently) scatter the fissile material around. Distance does the same job as neutron absorbing material, i.e. neutrons get lost since they don't hit atoms of the right material to keep the continuous reaction going.

Kenn
08-31-2010, 08:26 PM
Lhun, it is true that fast breeder reactors are unmoderated and that it is possible that these can be built on a small scale. These use coolants such as liquid sodium. However, I am not sure what would you think is going to happen if you have a reactor without any coolant at all. It would just get hot. If you meant that it doesn't generate electricity then that is different. If you meant that it can lose heat without a pumped cooling system then that is different also. As for controlling the reactor, what you say depends on the size of the core and whether the controlling rod has a significant effect on the overall neutron flux. For someting the size of a PWR or an AGR then obviously it wouldn't make much difference. For much smaller systems then I think it would all depend on the design. It's all a bit hypothetical anyway mind!

Third person, Lhun means that it has to penetrate the fuel in the core blocking the neutron flux. Fire it into there, in other words, but it doesn't need to be radioactive. The graphite bit is a red herring and it is the boron (or whatever) that is the important thing. If you Google "control rods" then you will find something that sounds a lot more exotic than boron.

GeorgeK
08-31-2010, 09:05 PM
"Wait a minute ma'am," the policeman glanced at his watch and wondered why he'd been called to this address and whether the custard filled doghnuts would still be there by 9AM. "You are telling me that a giant snake crawled into your pool last night and made it too hot?"

"Well, if you put it that way?" she answered, gradually pulling her nightgown into a more closed position.

"There's nothing missing. Nobody threatened you...Ok ma'am we'll put out an APB for a giant snake that super heats swimming pools and get right back to you..."

Lhun
08-31-2010, 11:37 PM
Lhun, it is true that fast breeder reactors are unmoderated and that it is possible that these can be built on a small scale.I was actually thinking of liquid metal reactor concepts, but yeah, simply based on the fissile material used you can also have a breeder type without moderator.

However, I am not sure what would you think is going to happen if you have a reactor without any coolant at all. It would just get hot. If you meant that it doesn't generate electricity then that is different. If you meant that it can lose heat without a pumped cooling system then that is different also.The "coolant" in a nuclear reactor is somewhat misnamed. It is not so much there to cool the reactor as it is necessary to transport the heat which is generated in the reactor core to the place where your heat-electricity changer (usually a steam turbine) is located. Since a nuclear reaction only produces heat as a product (nuclear batteries that produce electricity work different than a fission reactor) i'm assuming you have some other way to generate whatever type of energy the creature needs from the heat. A steam turbine seems rather unlikely inside a creature, but if that's the case, don't worry about the reactor, just blow up the turbine.

As for controlling the reactor, what you say depends on the size of the core and whether the controlling rod has a significant effect on the overall neutron flux. For someting the size of a PWR or an AGR then obviously it wouldn't make much difference. For much smaller systems then I think it would all depend on the design. It's all a bit hypothetical anyway mind!Well, i don't think the design is so important, since firing a moderator inside in the form of bullets is going to mess it up anyway. I'm just saying that using a moderator to stop a nuclear reaction is, in general, only possible with quite a lot of moderator, definitly enough that you could just use explosives instead and scatter the fissile material to make the reaction stop. More reliable than hoping for bullets that will get stuck in the right area of the target.

Kenn
09-01-2010, 12:22 AM
Lhun, it's not my creature so don't ask me :)

The coolant is necessary to cool the reactor core though, otherwise it would just get hotter and hotter until it melted. There are two cooling systems in most power reactors (three if you count the cooling towers) and the primary one is most certainly for cooling. The secondary one drives the turbines and you could argue that this is not really a cooling system (although the primary would over-heat if it failed). A boiling water reactor is different since the primary drives the turbines and the secondary cools the condenser.

I think you mean absorber rather than moderator and I think it depends on the scale of the reactor. Reactor vessels need to be very tough so blowing one up is not as easy as it sounds. But then again, it's just going to be a story!

Lhun
09-01-2010, 01:40 AM
The coolant is necessary to cool the reactor core though, otherwise it would just get hotter and hotter until it melted.Yes, that's the general idea of the liquid metal (not light-metal cooled!) design. The core is always running at "full speed" and kept liquid, which has several advantages. For example, it can't get damaged by overheating, and the choice of reaction chain would be so that, unlike in a breeder, the final result is inert, eliminating the problem of waste. There are also other designs that are inherently immune to overheating.

I think you mean absorber rather than moderator and I think it depends on the scale of the reactor. Reactor vessels need to be very tough so blowing one up is not as easy as it sounds. But then again, it's just going to be a story!No, i mean moderator. Not all nuclear decay produces neutrons that are so fast they need to be slowed down to work for fission.
The blowing up part is a given anyway, because any vessel to difficult to blow up would also be immune to shooting bits of a moderator into them. ;)

Kenn
09-01-2010, 01:55 PM
Yes, that's the general idea of the liquid metal (not light-metal cooled!) design. The core is always running at "full speed" and kept liquid, which has several advantages. For example, it can't get damaged by overheating, and the choice of reaction chain would be so that, unlike in a breeder, the final result is inert, eliminating the problem of waste. There are also other designs that are inherently immune to overheating.
No, i mean moderator. Not all nuclear decay produces neutrons that are so fast they need to be slowed down to work for fission.
The blowing up part is a given anyway, because any vessel to difficult to blow up would also be immune to shooting bits of a moderator into them. ;)

I think you might be getting a little confused because liquid sodium is by definition a light metal. Do you mean light water? If so, any reactor needs a coolant otherwise it would melt. The heat has to go somewhere.

I am not sure why you would want to fire a moderator into the core in preference to an absorber. But whatever, it must be easier to penetrate a hard substance than to smash it. In reality of course, it would have a vent which would be its weak spot. Even the Death Star in Star Wars had one of those. If it was a creature, then it is a fair guess where it might be;)

Lhun
09-01-2010, 03:09 PM
I think you might be getting a little confused because liquid sodium is by definition a light metal.Yes and it is used in liquid metal cooled reactors. Which are different from liquid metal reactors where the fissile metals are present in liquid state.

If so, any reactor needs a coolant otherwise it would melt. The heat has to go somewhere.Sure. The heat goes to whatever you use to turn it into usable energy. (mostly electricity)

I am not sure why you would want to fire a moderator into the core in preference to an absorber.Just a slip, i meant absorber.
But whatever, it must be easier to penetrate a hard substance than to smash it.No that's actually not easier. Not necessarily anyway. It depends pretty much only on whether it's easier to get the required energy into the bullet, or use some energetic material to make the bullet out of. That depends mostly on the gun, not much on the target. But given the quite large amounts of absorber you'd need to stop the reactor, using a similar amount of high explosive rounds would certainly destroy it. That's my point really, shooting an absorber into a reactor doesn't work like using a silver bullet on a werewolf. It works like pouring sand on burning coal, you've got to smother the fire, you can't "poison" it. And if you unload that much firepower on it, you don't actually need the absorber anymore.

In reality of course, it would have a vent which would be its weak spot. Even the Death Star in Star Wars had one of those. If it was a creature, then it is a fair guess where it might be;)And anti-nuclear suppository. The thought boggles the mind.

Kenn
09-01-2010, 08:23 PM
They're called liquid core reactors (not liquid metal ones). That said, they are still very much a concept.

The bullet thing would be a bit more complicated and it would depend on the nature of the containment. As an example, the core of an AGR has several feet of reinforced concrete surrounding it. It would be virtually impossible to smash it, but fuel and control rods are frequently taken in and out through the top. I think third person's original idea was for something to close down rather than suddenly die - so putting sand on the fire is just what would be wanted. Whether or not it would work would depend on the nature of the core and the size of absorber. In other words, it would depend on how many dominoes you started with in your model. But then again, it only has to be feasible rather than be exact.

Lhun
09-01-2010, 10:55 PM
They're called liquid core reactors (not liquid metal ones). That said, they are still very much a concept.Liquid core reactors is a wider classification, i.e. they don't necessarily use molten metal. Molten salt for example is another possibility or an adaption of the nuclear salt-water rocket design for a reactor. But yeah, the essential feature of those is the same.

<snip>I think third person's original idea was for something to close down rather than suddenly die - so putting sand on the fire is just what would be wanted.Sure. I was only pointing out that the required amounts of "sand" make bullets a very impractical delivery vehicle, thus if bullets are necessary something different than "sand" should be used.

Kenn
09-01-2010, 11:38 PM
One more question Lhun. When do you actually sleep? You always seem to be on here!

Lhun
09-02-2010, 01:15 AM
In a word: irregularly. ;)

Nivarion
09-02-2010, 02:46 AM
One moderator for a fission reaction is distance. Simply shell the reactor and the reaction will stop.