Freon moderated reactor

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kunkmiester
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Freon moderated reactor

Postby kunkmiester » Thu Jul 07, 2016 4:45 am

https://chiefio.wordpress.com/2016/04/2 ... d-reactor/

Stumbled across this, thought it was a quite fun look at just how simple and easy a nuclear reactor can be if you put your mind to it.

I found it looking for information on silicon as a moderator--I'm toying with a hard sci-fi idea of a privately built nuclear thermal rocket with a quartz core. I'm still looking at the quartz core since it will handle higher temperatures, but I'm also now looking at CO2 as a propellant, at least on earth. I figure it'd be multi fuel in some way--water, hydrogen, CO2, methane, whatever the adventurers can scrounge up.
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krenshala
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Re: Freon moderated reactor

Postby krenshala » Thu Jul 07, 2016 1:23 pm

That is the big advantage of a NERVA style motor: it really doesn't care what type of "fuel" you push through it, as long as it doesn't interfere with the heating process.

While double checking I was remembering things correctly, I found the Isp of the NERVA-2; 380 s at sea level, 825 s vacuum, and a thrust of 867 kN! For comparison, the SSME is 366 s at sea level, and 452.3 s vacuum, though it does have two to three times the thrust (1,860 kN at sea level, up to 2,279 kN vacuum).

hanelyp
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Re: Freon moderated reactor

Postby hanelyp » Thu Jul 07, 2016 4:02 pm

My impression is that a flexible propellant NERVA style engine is a bit tricky, as different propellants moderate and absorb neutron flux differently. A reactor where the primary coolant is also a primary moderator tend to be self regulating to some degree, a hotter reactor pushing coolant out and slowing the reaction. In contrast, a reactor dependent on a solid moderator, with a coolant tending to damp the reaction by absorbing neutrons, can go the way of Chernobyl on loss of coolant. From the figures in the linked essay, fluorocarbon moderated and cooled reactors would tend to exhibit the former more stable behavior. A hydrogen or light water reactor, on the other hand, may exhibit the less stable behavior depending on other factors.

You'd lose some performance, but having a separate coolant loop and heat exchanger to deliver heat to your flex propellant would make a lot of engineering easier. Or if your design is for a fluid core reactor (fluorocarbon with dissolved fuel?) have the heat exchanger outside the fission core.

As a structural moderator, have you looked at silicon carbide? Though given the reactor stability discussed above, I'd be more comfortable with a reactor design where the coolant constitutes a sizable fraction of the total moderator.

Brain storm: fluid moderator control "rods" in a reactor that needs a moderator other than the coolant.
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kunkmiester
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Re: Freon moderated reactor

Postby kunkmiester » Thu Jul 07, 2016 7:35 pm

Someone on another forum pointed out a reactor in Ohio that used an organic coolant. Apparently neutrons tend to make them polymerize, and this happens to fluorinated ones too, so freon is probably not the best choice. Don't know about his idea for a solid Teflon moderator though.

How hard is silicon carbide going to be to get as an SLS material versus quartz(and how hard is it to laser sinter)? The thought was to print the core. Using a high temp material would mean higher temp exhaust meaning higher isp etc., and it was partly safety--it needs less cooling when shut down or broken.

Fuel would be natural uranium--engineering is available, infrastructure for enriching is a vulnerability. I'm looking at co2 as coolant and primary moderator--you'd also be turning a supercritical turbine for power. Just having carbon dioxide in the reactor makes things simpler, though I'm still debating fuel salts in water as a liquid fuel, or just do solid fuel.

I'm also looking at a colony ship several times the size of a Falcon 9 so you'd actually have several reactors, all but one would be shut off after launch and kept as spares.

The scenario is a billionaire heading out with friends and family, possibly with two or three such ship colonies. Once the engineers and fabricators are on board the project, building stuff is fairly easy as long as you avoid exotic materials. Operational security takes over though and means you want to minimize your footprint.
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kunkmiester
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Re: Freon moderated reactor

Postby kunkmiester » Sat Aug 06, 2016 4:24 am

*poke*

When comparing ISP and payload of different systems, all else being equal, which is more accurate, "increasing ISP increases payload fraction" or "increasing ISP reduces fuel fraction?"

Not sure if a billionaire would try for a smaller rocket or just say screw it and build an Orion drive launcher for the big stuff.
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krenshala
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Re: Freon moderated reactor

Postby krenshala » Sat Aug 06, 2016 12:21 pm

When it comes to the rocket equation, higher Isp with all else being equal means higher delta-v. You then have a choice with the rocket - leave the mass (fuel and payload) as it is, and be able to go further, or increase payload mass to bring the delta-v back down to just what the mission requires. Due to the nature of rockets, the mission determines the delta-v requirements (which are based on what you are doing/were you are going, not the rocket), which when combined with the desired payload determines what amount of rocket you need to get the payload to have that amount of delta-v.

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Re: Freon moderated reactor

Postby kunkmiester » Sat Aug 06, 2016 3:30 pm

Makes sense, I was getting that, let me clarify the question a bit. I doubt the relationship between payload and fuel fractions is linear, so if I double my ISP, say by going from a chemical RP-1 rocket to a Proteus NTR (not quite even, but close enough), I was guessing I have two choices for the extra d/v: half the fuel fraction, or double the payload fraction. I'm pretty sure one of those is wrong though.

Going back to the story idea, the ship being launched--and the earth exit orbit it's going to--would be well into the territory of a Nova class launcher. If the increased ISP is substantially reducing the fuel load, allowing for a much smaller rocket, it's perfect. If it's simply doubling the payload fraction from say 4% to 8% the issues with a nuclear first/single stage aren't worth it. You're not saving enough mass and volume to make the rocket much easier to build.
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hanelyp
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Re: Freon moderated reactor

Postby hanelyp » Sat Aug 06, 2016 6:54 pm

kunkmiester wrote:How hard is silicon carbide going to be to get as an SLS material versus quartz(and how hard is it to laser sinter)?

SiC is a common industrial abrasive. I think I have a couple pounds of the stuff around here for use in a rock tumbler. For SLS of SiC I expect you'd want an inert atmosphere. Heated in an oxygen atmosphere it tends to react on the surface to produce SiO2 and CO2.
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krenshala
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Re: Freon moderated reactor

Postby krenshala » Sat Aug 06, 2016 8:04 pm

kunkmiester wrote:Makes sense, I was getting that, let me clarify the question a bit. I doubt the relationship between payload and fuel fractions is linear, so if I double my ISP, say by going from a chemical RP-1 rocket to a Proteus NTR (not quite even, but close enough), I was guessing I have two choices for the extra d/v: half the fuel fraction, or double the payload fraction. I'm pretty sure one of those is wrong though.

And this is when the fun part of the Tsiolkovsky Equation kicks in. :D When you change engines, you are changing the ships mass. NTRs typically have a much higher mass than a kero-lox/hydro-lox engine, but they make up for it through their improved Isp. It depends on what the mass of the ship is whether that helps or hinders your delta-v budget by switching to an NTR. Keep in mind the actual math, even if you fudge things a bit.

Δv = 9.81m/s^2 * engine I_sp * ln( wetmass / drymass )

In the above drymass is the wetmass (the mass of the fully fueled vessel) minus the mass of the fuel, and the function ln() is the natural logarithm (spreadsheets and calculators using the scientific setting should have an option for this). If the mass of the NTR is enough greater than the 'normal' engine, switching to it could actually hurt your final Δv number due to the additional drymass it adds.

Now, this assumes everything not fuel is payload. Basically, everything the engine is pushing, including itself. Each stage in a multi-stage vessel counts all non-active stages it is lifting (fuel and all) as payload (drymass). This is why staging helps; you are dropping deadweight that used to be holding fuel and/or an engine that was more efficient where you were than it is were you are now (e.g., the engine that was optimal at launch may suck in vacuum, and vice versa). A possible NTR advantage is using ambient atmosphere as fuel, thus increasing its effective Δv by not having to push the mass of that "fuel". The number get trickier at that point, and to be honest starts to get a bit beyond me. ;)

Since it is a story, of course, you can pick whatever Isp that does what you need. :D

kunkmiester wrote:Going back to the story idea, the ship being launched--and the earth exit orbit it's going to--would be well into the territory of a Nova class launcher. If the increased ISP is substantially reducing the fuel load, allowing for a much smaller rocket, it's perfect. If it's simply doubling the payload fraction from say 4% to 8% the issues with a nuclear first/single stage aren't worth it. You're not saving enough mass and volume to make the rocket much easier to build.

From what I understand there is a point at which adding more fuel simply leads to the same amount of aditional fuel being required, thus adding zero change to your Δv total. That gets into the portions of the math I have trouble with, however.

If you think it might help, try planning out the mission/flight path and getting the actual required Δv budget needed using a Δv map for our star system (just seach on 'delta v map', and make sure you aren't pulling one for Kerbal Space Program :roll: ). The budget needed is simply the sum of the Δv values of the maneuvers you are performing (e.g., ~9km/s for launch from Earth to LEO, plus however much it is for a trans-lunar-injection burn, plus however much it is to establish low-lunar orbit, etc). The Δv budget is completely separate from the mass of the ship, while the payload/ship mass and rocket equation show how far you can get in that Δv budget with the Isp and fuel-mass you are using/bringing. Once you know what Δv budget you are aiming for, start throwing numbers at the rocket equation above to see what kind of (fuel) mass you need to reach or exceed that Δv total.

Hopefully this wall-o'-text is helpful. :)

kunkmiester
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Re: Freon moderated reactor

Postby kunkmiester » Sat Aug 06, 2016 9:18 pm

It's been a while since I've done math with logs. I'll start looking at that, though I'm not even sure how much the habitat they'll be hauling will weigh. I'm thinking the payload will need to be well over 100 tons though. At it's basic, this would be a 3-5 core rocket using an analogue of the Phoebus 2.

I'm having trouble finding weights/masses for the Phoebus and other NTRs. Closest I could find was a pdf that assumed 2 metric tons for it and some other structure*. The pictures I've seen of the engine though make me assume at least 5,000lbs, probably 10,000 once everything is put together. At at least 200,000 lbs of thrust, that's a pretty good ratio, and fuel likely makes up a more important part of the math than the engine.

*http://www.aben.com.br/Arquivos/212/212.pdf page 15. Worth looking at, didn't see a mention of fuel fractions for the given throw weights, but also didn't see anything about fission products contaminating the fuel, so not sure how much to take from it.
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93143
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Re: Freon moderated reactor

Postby 93143 » Sat Aug 06, 2016 9:43 pm

krenshala wrote:That is the big advantage of a NERVA style motor: it really doesn't care what type of "fuel" you push through it, as long as it doesn't interfere with the heating process.

Since no one else has mentioned it, I'll just note that this is entirely untrue, for two separate reasons, neither of which has anything to do with the nuclear side of the design (which has already been covered).

1) chemical compatibility is a significant issue for both reducing propellants like hydrogen and oxidizing propellants like CO2 (it dissociates), and it is not trivial to come up with a core design that can withstand both.

2) the only propellants I'm aware of that seem to produce Isp values better than a hydrolox rocket are hydrogen, ammonia (which is only better than hydrolox at fairly extreme core conditions), and methane (which isn't a whole lot better than ammonia, plus it dissociates to produce huge quantities of solid carbon, about 3/4 of the exhaust by mass). A LOX/CO chemical rocket has a higher Isp than an NTR using CO2. A hydrolox chemical rocket has a higher Isp than an NTR using H2O. (Solid-core, of course. Temperature is key in a nuclear thermal rocket.)

krenshala
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Re: Freon moderated reactor

Postby krenshala » Sat Aug 06, 2016 10:32 pm

I was being pretty general, and would consider corrosion and other problems (e.g., carbon buildup from dissociation of CO/CO2) to be "interfering with heating". You are, however, completely correct in your corrections to my post. I have, however, seen some papers on the feasibility of using "air" as the fuel for NERVA/NTR engines, and while they list numerous problems and issues with doing so, they all pointed to being at least marginally workable (as it, they work, but aren't good for long term use that way).

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Re: Freon moderated reactor

Postby kunkmiester » Sat Aug 06, 2016 11:25 pm

The main concern I'd have with air is I understand it'd be full of stuff that gets radioactive while in the reactor. It may not stay that way for long, but it's an issue for a voluntarist. I'm looking at ammonia as an alternative, and wondering where to find how the reactor materials will react to the various environments I'm looking at here.

I was originally looking at CO2 on the thrust curve--mass in rocket propellant gives raw thrust, at the cost of ISP. A CO2 engine running at the same temps might have more thrust--good for getting off the ground--than a hydrogen or ammonia engine, but apparently isn't efficient enough to get you to orbit. Rather than saddle tanks, the hydrogen or dissassociated ammonia with a ramjet afterburner might be more appropriate. Don't know yet. Big issue I see with hydrogen is the same reason Musk used kerosene--the volume needed for the tanks, that impacts weight.

Did some math on 200,000 Kg dry mass on the rocket equation, using 20,000 m/s target d/v and ISP of 800 (using Pheobus 2 numbers). Got 1.2 million Kg of hydrogen. Still in Nova size range I'd bet, but that's also without ramjet air added after the reactor for more power. Looks like this would be pure fiction, fun as it would be to do.

Right now it'd be a silicon carbide core reactor. Two sets of fuel channels exist--using thorium or natural uranium, one set is to optimize a breeding region. Moderator channels run CO2 as the moderator, which also goes through a closed cycle Rankine turbine. Propellant goes through another set of channels. During liftoff, ramjet air is used to "afterburn" for extra thrust. While the reactor is charged with solid fuel, the design intent is to let it melt, running a liquid fuel system which is managed just above the reactors. This puts the melting point of the core material (silicon carbide) as the temperature limit. Obviously this would never fly because of this too--while the design shouldn't easily fail, if it does it'd be a disaster.

Ideally, the CO2 flow can be adjusted so that when the reactor is shut down, decay heat can be used to run the generator. A low propellant flow is an alternative for cooling the reactors. The third shutdown mode would be to just leave the reactor, and the design will simply take the heat generated by decay and just be hot. Don't know how practical that would be. Multiple cores wouldn't just provide more thrust, once in space they're backup reactors, since only one core at partial power would be needed to cruise in space.

Make a nice paper for some undergrad in nuclear power.
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paperburn1
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Re: Freon moderated reactor

Postby paperburn1 » Tue Aug 09, 2016 12:40 am

I am not a nuclear physicist, but play one on the internet.

D Tibbets
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Re: Freon moderated reactor

Postby D Tibbets » Tue Aug 09, 2016 3:53 am

As mentioned, radioactive heating even once the fission has stopped has to be handled. From my reading after the Japanese nuclear accidents, and from earlier reading, the post shutdown radiactive heating is about 10% of the heating with fission turned on. As such the heating of a propellant can only be about 10 times the ability to cool the reactor with radiators, perhaps helped some by a sterling engine or thermoelectric conversion of some of the heat to useful electricity. Maintaining ~ 10% of the propellent flow post shutdown would also suffice,but you are perhaps throwing away the mass at lower
ISP.

Perhaps regulating the propellent/ coolent flow at gradually decreasing flow rates would maintain the temperature and thus ISP efficiency. The problem is that the radioactive decay is perhaps too slow for timely thrust- at least for some situations. The radioactive decay heating from fission products has a drop off rate of ~ 10 fold for every 7 fold increase in time. At one hour, if the radioactive heating was a thousand units, at 7 hrs it would be 100 units, and at 49 hours it would be ~ 10 units, and at two weeks after shutdown it would be ~ 1 unit.. It is this residual heating that cannot be controlled with rods, boron dumping, etc. that has led to reactor meldowns after active coolent capacity was lost.

Fusion- especially aneutronic fusion greatly to tremendously mitigates this problem as there is relatively little residual radioactive heating once the fusion is stopped or idled.

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