Liquid uranium

RobertDyck · 2014-01-26 17:26:59

Just saw a re-run of Battlestar Galactica: Blood and Chrome. They use liquid fuel for their space fighters, able to land on a planet and take off to return to a carrier in orbit. The only way you'll get that kind of specific impulse is with some sort of nuclear propellant. So what is that liquid fuel? The series made up a name, but is there something in reality?

I did a little search today, found one possibility. Uranium hexachloride (UCl6) is solid at room temperature, dark green in colour, usually prepared as a powder, and melts at 177.5°C. It's soluble in carbon tetrachloride (CCl4). That's dry cleaning fluid, so safe to handle and fairly easy to get. CCl4 is liquid at one atmosphere pressure between -22.92°C and +76.72°C. Could this be used as fuel for a gas core nuclear thermal rocket? Fill the fuel tank with a mesh of neutron absorber to prevent the fuel from becoming a reactor.

Of course this would still require liquid hydrogen as propellant. A little thing not mentioned in the TV show. This eliminates a problem with solid core reactors: contamination of fuel with fission fragments. Of course that means fission fragments are expelled with exhaust, so not something you want to do in an atmosphere. And naturally the reaction chamber would be lined with neutron reflector to minimize critical mass. Liquid fuel would be easier to handle than pressurized UF6 gas.

Wikipedia says the usual coating for the inside of a GCNR is beryllium oxide. This acts as a moderator. Not mentioned is how beryllium reacts. When struck with an energetic alpha particle, it emits a neutron and becomes carbon 12. When struck by a neutron, it emits 2 neutron and splits into 2 alpha particles. So it's a neutron multiplier. This also means a little spurt of an alpha emitter would trigger release of neutrons, helping get a reactor started. Perhaps enclose an alpha emitter in an alpha absorber, with a door that can open to expose the reaction chamber. This would act as the "ignition plug". Is that what you would call it? A gasoline engine has spark plugs; a diesel engine has glow plugs. Considering alpha emitters have a short half-life, engines for a reusable vehicle would require their "ignition plugs" replaced regularly. Screw them in like glow plugs?

Wikipedia says 5000-7000s should be achieved, this corresponds to temperature of 50,000-100,000 Kelvin of the exhaust gas. That's way beyond the temperature to decompose the liquid fuel. So the reactor would become uranium gas, with carbon and chlorine expelled with exhaust.

Last edited by RobertDyck (2014-01-26 19:04:06)

SpaceNut · 2014-01-26 17:41:30

This sounds like it would also work with other nuclear fuels such as thorium...this is definately an ISS experiment if you ever saw one....

RobertDyck · 2014-01-26 18:16:20

Interesting. Thorium tetrachloride is water soluble.

RobertDyck · 2014-01-26 19:27:21

100% of thorium in nature is Th-232. When thorium absorbs a moderated neutron, it becomes Th-233. That has a half-life of 22.3 minutes, beta decay to protactinium-233. That has a half-life of 27.0 days, beta decay to uranium-233. U-233 absorbs a neutron more readily than thorium, so in a reactor it's consumed as quickly as it's produced. It's far more common in Earth's crust than uranium, so a plentiful fuel. The slow reaction means it can't be used for a bomb, and practically impossible to melt down. All this makes it a great fuel for a power reactor. But is it too slow for a rocket engine?

Ps. Yea, I know, atomic mass should be written as a superscript before the element symbol. But BBCode doesn't support superscripts, so I do it this way.

Last edited by RobertDyck (2014-01-26 19:28:59)

SpaceNut · 2014-01-26 21:52:00

Thank you for the reply, the way you wrote the element was fine and was understandable.

Radioactive Elements List on this page

Just searching for a refueling possibily for mars for a return ship to make use of.

RobertDyck · 2014-01-26 22:28:21

I think uranium has such high fuel density that a Mars mission could bring enough from Earth. It's propellant you have to worry about, not nuclear fuel. A permanent Mars settlement would have to worry about fuel, but they could use thorium for a power reactor. Mars Global Surveyor already made a map of thorium from orbit. Some people consider thorium an indicator mineral for uranium, but I think it could be used directly.

I consider a nuclear engine "would be nice to have", but not necessary for Mars. Would hate to think some Congressman would use this as an excuse to delay a Mars mission even further. Especially considering any form of ISPP for NTR would require a reliable source of hydrogen. Again, that "pack ice" or "frozen sea" at Elysium Planetia looks promising, but the European team for MARSIS was not able to confirm ice exists there. It could simply be that the ice is not thick enough for their instrument. After all, their instrument is designed to penetrate up to 20km deep, so a layer just 45 metres deep is rather thin. We would have to send a rover with a drill to check it out.

But yea, testing the engine in space sounds like a good idea. Would the guys managing ISS get squeamish about nuclear technology? Or a thruster?

By the way, I look up elements at this web site. There's a link for Isotopes on the right side of the page for each element.
www.webelements.com

One idea is to use uranium-233. It has a greater neutron absorption cross section than U-235, which means it absorbs neutrons more readily. Not as good as plutonium, but better than U-235. And releases more energy per unit mass than U-235, but again not as much as Pu-239. So it's an intermediary. Natural abundance of U-233 is so ridiculously low that the web site doesn't even list it, so the only way to get it is a breeder reaction from thorium. Since it's uranium, it's not chemically poisonous like plutonium. So one option is GCNR fed by the liquid I started with, but using U-233. That allows a smaller critical mass.

GW Johnson · 2014-01-27 07:44:19

Some of the original GCNTR stuff I saw was in a report written by a Maxwell Hunter at United Aircraft, ca. 1960. I ran across it about 1970. The concept then was solid U-233 feed as a metal rod fed through some sort of gland nut.

This liquid feed idea is quite intriguing, as it would solve a number of practical problems with ideas like those Hunter wrote about.

Ultimately, some sort of GCNTR that could use water directly as the working fluid would be a very practical type of propulsion, able to "refuel" almost anywhere, as long as the nuclear fuel held out. Water as ice seems to be in a lot of places. All that is really required is gravity-settling to get the solids out, once the ice has been melted.

Although a GCNTR sheds radioactive reaction products in its exhaust, the amount contaminating a local atmosphere is actually quite small for any given launch, even here on Earth. For complete nuclear burn-up, the flow ratio of uranium to hydrogen in the old UA designs was about 1/1000. And at Isp's above 2000 s, the total plume mass is actually fairly small compared to the thrust you get. Even at Earth launch mass ratios, 1/1000 of the tonnage expelled is a pretty small number. Coal plant exhaust radioactivity is actually the larger problem, and by far (that's 1/3 of the "natural" background in the US).

GW

RobertDyck · 2014-01-27 17:33:03

Ok. Here's an interesting engineering project. How would you build an open cycle gas core NTR that doesn't melt itself? Let's push Isp to the max. Wikipedia claims exhaust temperatures of 50,000°K - 100,000°K are requires for 5,000s to 7,000s Isp respectively. But this paper claims 10,000°K to 20,000°K results in >5200s Isp. Most papers show a spherical reaction chamber with an ellipsoid uranium plasma in the centre, however this web site shows a toroid. The paper is on NASA's website while the image is on a website of speculative technology, but text does talk about a toroidal vortex to contain uranium.

Last edited by RobertDyck (2014-01-27 18:25:52)

SpaceNut · 2014-01-27 17:46:36

Have been reading about fast neutron and slow neutron reaction. Thorium would be a fast but only if you use He3 rather than water from what I have found

Facebook community
http://www.quora.com/Thorium-Fueled-Nuc … ace-travel

But I am still reading

RobertDyck · 2014-01-27 18:42:38

I don't use websites that require access to my Facebook account. How do they claim it gets around the things I described above? I don't see how He3 would have any effect.

Last edited by RobertDyck (2014-01-29 18:53:20)

GW Johnson · 2014-01-29 18:24:03

If you put a thorium fuel plant "somewhere", you will need lots of power for it. Why not use a thorium reactor for heat and power, but modified to breed U-233? Use the U-233 directly in your "liquid uranium" fuel. That way you can use water or hydrogen in your GCNTR, and need not worry about He-3.

I dunno about ellipsoidal versus toroidal fission fireball geometries in a GCNTR. The one they were aiming at in the late 1960's had a nearly-spherical fireball inside a spherically-symmetric flow field. It was a liquid hydrogen propellant design, with LH2 injection all around the spherical chamber, angle-biased aft. Accordingly, there was an extreme temperature gradient from the cold wall to the edge of the fissioning fireball.

What I remember reading was that up to about 2000-2500 sec Isp, regenerative cooling was thought to be adequate, and no waste heat rejection scheme was needed. Above that level, they were looking at a high-temperature radiator scheme, something that was likely to be very large and heavy. This had a limit, too: thermal radiation-induced propellant transparency to the extreme thermal radiation from the fissioning fireball. This was believed to occur somewhere between 6000 and 10,000 sec Isp power levels. But nobody really knew for sure back then.

This device never got beyond benchtop concept demonstrations of a couple of the most critical individual technology items, at academic-institution grant funding levels, which are very low, actually. One outfit demonstrated successful controlled fission in a uranium gas. Another outfit demonstrated 1000:1 flow ratio of the hydrogen vs uranium, with some sort of plasma flow device, in that spherical geometry.

That's as far as it got. Everything gas core nuclear died in 1973-ish, same time as ready-to-test-fly solid-core NERVA died, by presidential executive order killing Apollo and restricting men to LEO only. NASA's thinking was "who needs the rockets if we're not going to go?" The money got spent instead on shuttle, then a series of X-planes that never flew, then ISS. And here we are.

GW

RobertDyck · 2014-01-29 19:25:32

When I say "elipsoid", that's from images on the internet. The combustion chamber is spherical, but shows the uranium plasma as almost, but not quite spherical. I think we're talking about the same thing. The "torus" is from more recent documents, attempts to use technology from uranium enrichment. The enrichment guys have well documented how uranium gas flows, so this is an attempt to use that knowledge to reduce uranium loss. Of course there's no details; it says is "they know". They aren't going to post details how to enrich uranium on the internet. And that's all I have access to.

The spherical thing appears to have more than one regenerative cooling system. Is it just using heavy water as a moderator, and primary coolant to keep the ractor lining from melting? Is deuteriom oxide just recycled? With a heat exchanger from LH2?

Last edited by RobertDyck (2014-01-30 10:17:20)

GW Johnson · 2014-01-30 09:50:41

Hi RobertDyck:

I honestly don't know about moderators or heavy water stuff. None of that was in the ancient report I read long ago. I don't remember regenerative cooling system details. Hard to say what you saw, because the photo didn't come through in your post just above.

Way back then, the design target was a 6000 s Isp orbit-orbit engine. The radiator for 2000-6000 sec Isp was supposed to be be a 4000 R (2000 K) device, crudely. At that time, no such thing existed, and I have yet to hear of one, so their weight estimates from the late 60's would be bogus.

If the GCNTR could be made to run on water instead of LH2, there might be better regenerative cooling capability, plus the water could be stored as ice. That melt-off process would be a good use of the excess waste heat for operating above 2000 s Isp, instead of the radiator. Melt more than you need for the burn at hand, then let the cold of space re-freeze it before the next burn.

GW

RobertDyck · 2014-01-30 10:19:50

Found another website with the same image.

Tom Kalbfus · 2014-01-30 11:09:41

Too bad these diagrams have remained just that for the last 40 years or so.

RobertDyck · 2014-01-30 12:06:26

Image of engine with toroidal vortex uranium core. From the website already linked. The hope is this one would lose less unreacted uranium. Not as detailed, but notice the radiator fins.

Tom Kalbfus · 2014-01-31 15:47:18

Wouldn't a fusion rocket based on the Tokomak reactor also looks sort of like that? Imagine if we could put the ITER in space.
http://en.wikipedia.org/wiki/ITER

Quaoar · 2014-02-17 15:44:37

With thorium derived U233 it is possible to build a 50% lighter solid core rocket. Another interesting possibility is the solid core matrix rocket MITEE that reach an exaust velocity of 10 km/s in the basic version, 13 km/s in low pressure version and 17 km/s in the hybrid electro-thermal version.

http://web.archive.org/web/200503071638 … mitee.html

http://web.archive.org/web/200503171455 … /PUR-1.PDF

http://web.archive.org/web/200503171455 … /PUR-8.PDF

http://web.archive.org/web/200503171454 … PUR-12.PDF

The same MITEE project team have also studied a liquid core rocket LARS (Liquid Annular Reactor System) with an exaust velocity of 20 km/s, but they have not completely solved the problems of core containement douring rocket acceleration.

http://web.archive.org/web/200503131626 … /PUR-6.PDF

GW Johnson · 2014-02-18 10:54:15

Interesting stuff, this MITEE thing. I doubt that team exists anymore. These small business set-asides rarely (if ever) lead to anything mainstream significant. Unfortunately.

The PUR 12.pdf file has a nuclear ramjet variant. The Marquardt engine they used as a baseline shape is old indeed, there are far better ramjet components and designs now, and have been since the 1970's. And, in addition to the old Project Rover nuclear rocket engines that got tested in Nevada long ago, there was also a Project Pluto nuclear ramjet that also got tested in Nevada long ago.

The nuclear rocket got shut down by 1974, at the ready-to-flight test stage. Little has been done since then except paper studies and a couple of brief "resurrections" done by a couple of spy agencies. If that work had been continued, the remaining problems NERVA had would have been fixed by now, and we might even have gas core technology by now. After all, NERVA took place approximately half a century ago. I remember it in the news back then.

The nuclear ramjet in old "Rover" was far less successful. It had serious problems with a lethally-radioactive exhaust, and a lethally-strong trailing shock wave flying Mach 3 at a few hundred feet altitude. It was abandoned when they figured out these two effects would have killed more people than the thermonuclear bomb it was to push to target (the thing was to be a strategic cruise missile, before there ever were any ICBM's).

We first had working LOX-LH2 engines in the 1960's. The only new thing added since then is electric propulsion, but without any solutions to its idiotically-low thrust weight problems. Kerolox we have had since the 50's. There just hasn't been that much fruitful propulsion technology in recent decades, other than incremental improvements on what we have had for a long time.

Breakthroughs in propulsion are now long overdue. Undeveloped concepts would be fertile ground for those breakthroughs. Nuclear thermal offers one of those. So would nuclear pulse (explosion) propulsion. So would a lightweight energy source for electric propulsion.

GW

Quaoar · 2014-02-18 17:37:04

GW Johnson wrote:

Interesting stuff, this MITEE thing. I doubt that team exists anymore. These small business set-asides rarely (if ever) lead to anything mainstream significant. Unfortunately.

GW

MITEE higher exaust velocity of 10 km/s is due to reduct core-chamber radius, that consent higher temperature (almost 3000°K) with less mechanical stress due to Laplace's Law (wall tension = pressure*radius/2 )

The low pressure variant works at 3400°K so hydrogen is dissociated and exaust velocity can reach 13 km/s. This version is particulary interesting because it can be pressure feeded, resulting a very simple and reliable rocket.
Is it possible to build another variant of this low pressure MITEE that use water instead of hydrogen, reaching more than 5 km/s of exaust velocity?

Last edited by Quaoar (2014-02-18 17:38:56)

RobertDyck · 2014-02-18 18:16:00

Timberwind 250 was an engine developed by the US air force for a new Titan missile, to launch a nuclear warhead at the Soviet Union. It was part of Regan's SDI (Star Wars). It was a pebblebed reactor, radial flow. Isp 1,000 seconds in vacuum, 780 seconds at sea level. Thrust in vacuum 250,000 kgf (hence the name "250"). Engine mass 8,300 kg.

Compare that to MITEE. ~1000 sec Isp, Engine mass of ~140 kg
It says reactor mass is 70kg using U-235, or 40kg using U-233, and says total engine mass is "2x Reactor Mass - Includes Turbo Pump, Controls, and Contingency". So yea, they're aware of U-233.

It's interesting they also list Am-242m: reactor mass 25kg. So has even lower engine mass. However, Am-242m is not normal Am-242. It has to be made in a specific way so the nucleus of each atom has additional energy. It isn't a normal breader cycle, it's tricky and expensive. Worth it? Depends just how expensive.

Thrust is difficult to find. Reading the report "MITEE-3" from your link, it says baseline thrust is 14,000 Newtons (1,427.6 kgf), but that is hand written while everything else is typed, and that same table claims engine mass is 100kg. Timberwind had the problem that overheating caused pebbles to agglomerate, making it non-restartable. But it's hard to compare without numbers.

Quaoar · 2014-02-19 08:05:11

What's about ammonia propellant?
With a low pressure MITEE, it can reach an exaust velocity of 6.3 km/s: almost half than LH2, but ammonia is very easy to store. We can imagine a Mars mission with a MITEE spaceship that use LH2 for Earth-Mars trip and liquid ammonia for the return trip.

Last edited by Quaoar (2014-02-19 08:05:56)

GW Johnson · 2014-02-19 09:26:01

Hi Robert:

When you say Timberwind 250 was "developed" by USAF, do you mean something actually built and tested in some way? If so, where did they test this thing? The old Jackass Flats facility in Nevada was already in serious disrepair by then. I remember talk of NTR being revived ca. 1980-ish, to counter an effort thought to be an NTR in Russia. Back then, the talk said this was CIA, but USAF would be the more experienced bunch. That Russian effort was thought by various groups to be any of several things, but did turn out to be some NTR work, similar (I think) to the old Project Rover NERVA stuff.

Myself, if we do bring back solid core NTR, I wish we would upgrade it to a water working fluid, as in the original 1950-vintage concept. There's an Isp penalty, to be sure, but the working fluid is available as ice in many locations throughout the solar system, including Mars. Melt it, gravitationally settle-out the solids, and run the cleaned water through the engine. Very easy to process with minimal equipment. Your "fuel tank" needs a door through which to load the ice, and a tap from which to draw off the separated solids.

If this can be made to work solid core, then it can likely be made to work in gas core, when that technology is ready to fly.

GW

Quaoar · 2014-02-19 10:43:31

GW Johnson wrote:

Hi Robert:
Myself, if we do bring back solid core NTR, I wish we would upgrade it to a water working fluid, as in the original 1950-vintage concept. There's an Isp penalty, to be sure, but the working fluid is available as ice in many locations throughout the solar system, including Mars. Melt it, gravitationally settle-out the solids, and run the cleaned water through the engine. Very easy to process with minimal equipment. Your "fuel tank" needs a door through which to load the ice, and a tap from which to draw off the separated solids.
If this can be made to work solid core, then it can likely be made to work in gas core, when that technology is ready to fly.
GW

A water version of the low pressure MITEE can reach an exaust velocity of almost 5-5.5 km/s. Thinking that water is very easy to handle and we can refuel everywhere, a water NTR can be a very good chioce: is it possible to use LH2 for departure trip and water for the return trip with the same rocket?

RobertDyck · 2014-02-19 10:44:00

Yea, I'm simplifying. Serious development was started. When nuclear activists realised this was for a ground launched ICBM, they didn't want it. They campaigned hard, got it shut down. How far did development go? Not sure. But total engine mass is so much lower than NERVA that I keep using Timberwind as a baseline. These figures make MITEE appear to have even smaller engine mass, but that may be simply that it's a smaller engine with smaller thrust.

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Liquid uranium

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