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Post by realscifi on Dec 26, 2019 12:51:19 GMT
Using the COADE module design you can make a nuclear reactor using 100g of U235 dioxide at 1% enrichment (the energy it produces is small but it still works) as well as a Nuclear rocket using 140g of U235 at 90% enrichment. This feels wrong as the minimum critical mass for a sphere of U235 is 52kg. If a neutron reflector is used the critical mass is reduced (though one was not used in the two examples above) but it seems very unlikely that it could be reduced as much as the game engine allows.
The KRUSTY1 small nuclear reactor for use in space uses neutron reflectors and still requires a 28kg U235 core and only produces 1kWe, which implies you could perhaps halve the critical mass if a neutron reflector is used but reducing it to 100g seems wrong. Is this an error in the game and if so how could we compensate?
Also the game seems to allows enrichment of all of the nuclear fuels. My understanding is that enrichment is the proportion of U235 present in Uranium (20% enrichment has 20% U235 and 80% U238), how does this apply to Plutonium and the other fuels?
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sammi79
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Post by sammi79 on Jan 14, 2020 18:05:39 GMT
I'm not sure but I think the answer to your question involves the neutron source, sort of a primer in a nuclear reactor. The critical masses you mention are for the plain material without an extarnal neutron source and as such are a fraction away from the supercriticality required in weapons. As well as fuel and control rods real reactors normally contain a few rods with very powerful neutron emitters - in the game this is our neutron flux (the actual neutron flux inside the reactor would rise and fall with operating power.)
So we can make low powered reactors from subcritical masses of fissile material by using strong neutron sources to maintain the flux required, by clever use of neutron reflectors etc. each of which have the practical effect of decreasing the amount of mass required to remain critical. You could also try compressing it, although the fact that I've never heard of any nuclear device besides a bomb using this technique probably means it isn't viable in a power generating reactor where we need to keep the fuel assemblies intact.
I don't think it is viable to create such tiny reactors as it is in the game. Using a higher flux to maintain criticality burns the fuel quicker. A device that lasts for a month might be OK for our game but completely impractical in reality when you need to shutdown, refuel or decommision and replace your power plant every month. There are likely a whole bunch of real physical reasons (forces, limits and so on)that a nuclear engineer could tell you that simply aren't modelled in the game.
When it comes to enrichment, I think the game just treats the remainder % as filler. Obviously in Uranium, the enriched % is U235, and in reality the remainder % would be U238. I seem to generate the same amount of heat from the same amount of enriched fuel, so 10Kg @97% produces the same amount of heat as 100Kg @9.7%
If that doesn't answer your questions I'm sorry I'm just a fan of the game. Perhaps you could have a go at mine;
What would a realistic radiator temperature be for a space ship? I get the feeling that my white hot 2500K panels wouldn't be quite as useful in reality.
Would it really be sensible to build a reactor without any biological shielding at all? because my 5mm Li-6 shield protects the crew just fine. What's that you say? someone might need to work on that bit of the ship at some point in the future, preferrably without wearing a meter thick concrete and lead lined suit? not my problem...
and...
Is it actually possible to design armour that will hold for a while against 20Mj+ railgun projectiles - while being light enough for a spaceship? or should I just not bother and rely on never getting hit?
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Post by AdmiralObvious on Jan 15, 2020 4:18:35 GMT
Just something to note. Reactors in game have a requirement that they must be able to function for 6 whole months of nonstop use at max power.
Reactors as mentioned also aren't just giant lumps of supercritical materials. Control rods specifically are there to prevent the whole thing from exploding.
As for the new questions posed.
1. Reasonable temperatures for radiators will vary depending on who you ask, but in my opinion, it will be based on what the outer armor is, and how that refracts off this armor. Lower temperatures for something like Aluminum (the radiators on the ISS don't glow partially for this reason).
2. Reactors should have shielding, basically anything that is next to our unshielded reactors should be microwaved to death nearly immediately, especially high power ones.
3. The rail gun question depends on how big the projectile is, really. A well done whipple shield can effectively negate most of those pretty effectively. All you need is a reasonably dense metal and you're good to go.
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sammi79
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Post by sammi79 on Jan 15, 2020 10:54:23 GMT
Aha! yes of course plain old high energy photons. I was worried more about the fast neutrons and fission products, but it's the Micro-Gamma freq. photons that would be cooking everything instantly.
Thanks for the replies.
The radiator question bugs me a bit, every so often I get the feeling I'd like to make a fleet of standard realistic warships but find it very hard to accept the extra dimensions of anything less at less than 2000K especially for GW+ designs. And just lately I've had some success with armour development using a very hard outer layer and progressively softer, denser and thicker layers at least 3 over the important stuff. Oh and 1 stuffed whipple. And Selenium film for anti laser.
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Post by AtomHeartDragon on Jan 15, 2020 20:03:03 GMT
I typically build fairly heavily shielded reactors. Boron carbide for low temp ones, titanium diboride for moderate temperatures, pyrolytic carbon or even hafnium carbide for high temp stuff.
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Post by realscifi on Jan 17, 2020 18:47:35 GMT
For the fission plant I have noted that I can make the "Reactor has too short of an operational lifetime" warning go away by reducing the size of the core, which feels wrong. My main concern is that the CODE power plant equations may not be valid for very small fuel masses. Further while a neutron reflector does reduce the critical mass required (as in NASA's KRUSTY 1 power plant) I would like to know how much it reduces it can you get a criticality at 10% normal critical mass or do you need more? In CODE the only function of Neutron Reflectors is to reduce the radiation hazard rather than affecting the criticality of the reator and neutron sources are not modelled at all (at least as far as I can tell).
If the equations do break down does anyone know how to estimate the smallest fuel mass that would actually work and how much Neutron Reflector is actually required?
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Post by realscifi on Jan 18, 2020 10:45:13 GMT
That did not come out as understandable english.
What I was trying to say was:
1. A neutron reflector can reduce the critical mass, does anyone know by how much (52kg to 28kg or can it get down to 1kg)?
2. What thickness/material of neutron reflector would you need to reduce the critical mass (presumably more neutron reflector will reflect more neutrons and therefore give a bigger reduction, but does anyone know how to quantify it)?
3. My concern is that CODE does not use neutron reflectors to do anything other than reduce radiation hazard and enables the construction of 100g U235 reactors at 2% enrichment, this suggests that the equations used to model reactors break down at small fuel masses. I would like to be able to check my CODE reactor designs to avoid producting something that is violates the laws of physics.
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sammi79
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Post by sammi79 on Jan 18, 2020 11:41:35 GMT
For the fission plant I have noted that I can make the "Reactor has too short of an operational lifetime" warning go away by reducing the size of the core, which feels wrong. My main concern is that the CODE power plant equations may not be valid for very small fuel masses. Further while a neutron reflector does reduce the critical mass required (as in NASA's KRUSTY 1 power plant) I would like to know how much it reduces it can you get a criticality at 10% normal critical mass or do you need more? In CODE the only function of Neutron Reflectors is to reduce the radiation hazard rather than affecting the criticality of the reator and neutron sources are not modelled at all (at least as far as I can tell). If the equations do break down does anyone know how to estimate the smallest fuel mass that would actually work and how much Neutron Reflector is actually required? The size of the core in game reflects the general size and shape of the reactor core assembly including fuel/control rods, moderator + coolant channels - ie. volume, not mass.
Like I said I think you are right, that COADE reactor physics simulation is not entirely realistic. I think I read somewhere that for an infinite mass of uranium fuel, the minimum percentage of U235 present would need to be 5%+ for it to be critical. Here's a question, is the reverse also true? that somewhere proud of the 97% is a maximum possible enrichment before even infinitesimal masses become critical? It at least seems plausible to suggest that if the overall density of U235 atoms in a given volume is over a certain threshold then it would be a critical configuration whatever the volume.
That said, it is using real equations that are used for nuclear physics in general. I found this page that might shed some light on it; finite-cylindrical-reactor I believe the game is using equations very similar.
The problems are that the equations are linear, which does appear to assume any possible theoretical mass of fissile material as long as the geometry of the core assembly cylinder is proportionately ideal, and a given solution is correct for any overall level of neutron flux (just hotter or colder). Which seems to be exactly how it works in game where we set our overall neutron flux (which could only be logically done with an additional neutron source)
There are more accurate equations that take more variables into account and are consequently more complex, but I guess these were chosen to balance processing power and accessibility (not having an extra 50 sliders to play with and having reduced numbers of failure modes) to answer your question you probably need to run more complex simulations.
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Post by AtomHeartDragon on Jan 18, 2020 11:54:55 GMT
I think COADE switched to prismatic model at some point. Anyway, my observations: - Other than shielding, neutron reflector does zilch in COADE. It's not modeled in any meaningful manner in regards to the reaction itself.
- Moderator can be used to squeeze extra criticality and lifetime out of even very small amounts of fissile.
- Core geometry matters.
- On a spaceship there does not seem to be any good reason to not go with as highly enriched fuel as you can have because not keeping your core light and compact makes everything else become not light and compact in consequence causing mass and cost to balloon.
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Post by realscifi on Jan 25, 2020 18:55:02 GMT
I found a paper "Criticality Data and Factors Affecting Criticality of Single Homogeneous Units" by the LOS ALAMOS SCIENTIFIC LABORATORY, this is available on the net. This gave some data for the effect of Neutron reflectors.
For a 93.5% U235 sphere (rest U238) the critical mass using a Berylium reflector (the best one they tested) was:
35.1kg for 1.27cm reflector,
26.1kg for 2.54cm reflector,
20.8kg for 5.08cm reflector,
14.1kg for 10.16cm reflector
approx 7kg for an infinite reflector.
From the above I calculated the minimum U + Reflector mass appeared to be 30.2kg for 2.54cm reflector.
They also had data for a water neutron reflector which as a critical mass of 24kg for 5.08cm of water for a total mass of 29.8kg.
The above suggests to me that any reactor / NTR where the U235 mass is less than 7kg is probably not actually viable and anything less than 52kg should have neutron reflector to match.
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Post by Pttg on Jan 26, 2020 9:14:59 GMT
Just something to note. Reactors in game have a requirement that they must be able to function for 6 whole months of nonstop use at max power. Reactors as mentioned also aren't just giant lumps of supercritical materials. Control rods specifically are there to prevent the whole thing from exploding. As for the new questions posed. 1. Reasonable temperatures for radiators will vary depending on who you ask, but in my opinion, it will be based on what the outer armor is, and how that refracts off this armor. Lower temperatures for something like Aluminum (the radiators on the ISS don't glow partially for this reason). 2. Reactors should have shielding, basically anything that is next to our unshielded reactors should be microwaved to death nearly immediately, especially high power ones. 3. The rail gun question depends on how big the projectile is, really. A well done whipple shield can effectively negate most of those pretty effectively. All you need is a reasonably dense metal and you're good to go. The six month limit annoys me when I want to build NTRs for missiles that have an operational duration of like 30 seconds to ten minutes. |
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Post by AdmiralObvious on Jan 28, 2020 3:43:35 GMT
Just something to note. Reactors in game have a requirement that they must be able to function for 6 whole months of nonstop use at max power. Reactors as mentioned also aren't just giant lumps of supercritical materials. Control rods specifically are there to prevent the whole thing from exploding. As for the new questions posed. 1. Reasonable temperatures for radiators will vary depending on who you ask, but in my opinion, it will be based on what the outer armor is, and how that refracts off this armor. Lower temperatures for something like Aluminum (the radiators on the ISS don't glow partially for this reason). 2. Reactors should have shielding, basically anything that is next to our unshielded reactors should be microwaved to death nearly immediately, especially high power ones. 3. The rail gun question depends on how big the projectile is, really. A well done whipple shield can effectively negate most of those pretty effectively. All you need is a reasonably dense metal and you're good to go. The six month limit annoys me when I want to build NTRs for missiles that have an operational duration of like 30 seconds to ten minutes. I mean I get why the limit is there, some missiles will last longer than 6 months depending on the mission. |
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Post by realscifi on Feb 23, 2020 14:06:02 GMT
Going through the paper mentioned in my previous post it seems to show that moderating the reactor has a bigger effect on the amount of U235 required to achieve criticality than the presence of a neutron reflector.
Using a water moderator the amount of U235 required drops to 1.47kg.
Using a moderator and a reflector drops the amount of U235 to 0.82kg.
However the minimum mass of U235 + moderator was 18.3kg, for U235 + moderator + reflector it looks more like 31.4kg.
I found a paper "The Smallest Thermal Nuclear Reactor" by Y. Ronen,* E. Fridman, and E. Shwageraus, in which they seem to claim the smallest possible nuclear reactor weighs 4.95kg and used 0.7kg of Am242 with water as a moderator. reactor radius of 9.6cm.
If your reactor weighs less than the above you may have found a glitch in the maths used in COADE (or a way to revolutionise energy production).
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