Author Topic: Role of NTR/BNTR/NEP in future architectures  (Read 182696 times)

Offline Firestarter

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #40 on: 01/02/2006 06:58 PM »
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Orbiter Obvious - 2/1/2006  1:24 PM

Are those new forms available to be used in space transportation? Or are they like the Nuclear fission possibilities, which are meant to be about 20-30 years away?

And if not, that is too much a risk. Space travel should not be a test bed for Earth energy potential. It should be the other way around.

Offline kfsorensen

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #41 on: 01/02/2006 07:01 PM »
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Orbiter Obvious - 2/1/2006  1:24 PM

Are those new forms available to be used in space transportation? Or are they like the Nuclear fission possibilities, which are meant to be about 20-30 years away?

I believe this reactor could be used very effectively in space transportation...not as a nuclear thermal system, but as an heat source to generate electrical power to drive electric thrusters.  For systems like this, nuclear-electric systems, one of the most important overall parameters is the specific power, also called the "alpha".  It is a measurement of how many kilograms of reactor, power conversion, radiators, etc. it takes to generate a kilowatt of electricity.  The key to getting a good (low) alpha is to get the masses of the individual system components down--reactor, shield, power conversion, radiators, and so forth.  Surprisingly, the reactor is a rather small mass in the overall system, but it drives all the others, and the reactor really needs to have a high temperature capability.  This ripples through the power conversion system and allows waste heat to be rejected at higher temperatures, which leads to smaller radiators, since the effectiveness of the radiators is proportional to the fourth power of temperature.

The need for a high-temperature reactor is why typical terrestrial reactors, which are water-cooled in big pressure vessels, would not do well in space at all.  It is one of the reasons why I think the specific needs of the space reactor drive us in directions that will later lead to attractive terrestrial reactors.

Offline truebeliever

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #42 on: 01/02/2006 09:59 PM »
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Orbiter Obvious - 2/1/2006  2:24 PM

Are those new forms available to be used in space transportation? Or are they like the Nuclear fission possibilities, which are meant to be about 20-30 years away?

I have been following the threads of vanilla ice ( or is it just vanilla :) ). I have read Alvin Wienberg's  book, "The Life and Times of a Technological Fixer". He used to be the director of the DOE's Oak Ridge National Laboratory.  Great book. He talks about the development of the molten salt reactor (MSR). They built the first molten-salt reactor back in the 1950's just to see if it could be done. It ran for about a month and attained a then unheard of fuel temperature of 1133 K. They then built a second test reactor that ran for 5 years. Remarkable machine. They would run the reactor from Monday-Friday, and shut it down on Friday afternoon by dumping the fluid fuel into a storage tank. It would quickly cool down and solidify. On Monday morning, they would show up for work, turn on the electric heaters, remelt the salts, and pump the fuel back up into the sytem and restart the reactor. Pretty cool.

Weinberg called it " a pot, a pump and a pipe", in reference to the simplicity of the reactor itself.  

This is what NASA needs for space nuclear power. Something that is simple, cost effective and reliable.

Offline Colby

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #43 on: 01/02/2006 10:12 PM »

Vanilla, would you mind posting a little more background information on yourself? You already stated that you are a nuclear engineer and you gave us some other hints, but it would be very nice to see it all in one post. ;) Your knowledge is definately going to be an asset to this website!

Colby

Offline kfsorensen

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #44 on: 01/02/2006 10:43 PM »
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Colby - 2/1/2006  5:12 PM

Vanilla, would you mind posting a little more background information on yourself? You already stated that you are a nuclear engineer and you gave us some other hints, but it would be very nice to see it all in one post. ;) Your knowledge is definately going to be an asset to this website!


Um, that's probably not the best idea.  I'm not exactly working at a place that would smile on these kinds of comments.  Hence, my ability to post is rather connected to my anonymity.  I realize that that makes any information I post suspect, so I will try to document this information as much as possible.  If I can get some FTP storage on this site, I will upload a number of documents that will shed more light on many of these topics.

Sorry I can't say more, but I'm just a pretty vanilla-type person...

Offline kfsorensen

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #45 on: 01/02/2006 11:03 PM »
The Weinberg book is a very good one---its full title is "The First Nuclear Era:  The Life and Times of a Technological Fixer".  I ordered it on Amazon and really enjoyed it.

A good website about molten-fluoride reactors (more commonly called molten-salt reactors--I prefer the term molten-fluoride because there are also concepts for molten-chloride reactors) is Bruce Hoglund's:

http://home.earthlink.net/~bhoglund/

Offline Flightstar

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #46 on: 01/02/2006 11:04 PM »
Colby, some of us are unable to indentify who we are, especially when working for NASA or a NASA contractor.

You'll see more NASA, USA etc. people on this forum than anywhere else on the open net as Chris owns the site and has a history of extreme confidentiality, so we're safe here. There's one USA worker on SDC who doesn't hide his indentity and is actually risking his job in the process, but appears to add false information in with true information to cover his own back, maybe.

I'd love to add a picture of myself with Atlantis, but that'll have to wait until my retirement, which isn't all that far away!

Offline David AF

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #47 on: 01/02/2006 11:07 PM »
A similar case with myself. I'm not NASA, but a Lt. Col in the USAF.  Websites are public access anywhere in the world, but as above, Chris is very highly briefed (he's military himself) on security, so this is a rare site to be free to talk on.
F-22 Raptor instructor

Offline Colby

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #48 on: 01/02/2006 11:50 PM »

I understand perfectly vanilla. I was mostly hoping for some more information that wouldn't identify you, although I'm not in your situation, so I'm not quite sure what that could be without risking your job. I am just very interested in aerospace engineering, but I also have a fascination with nuclear engineering, so your posts obviously caught my attention!

People like you, Flightstar, and David AF (and so many others) make this site truly unique, so do what you must so you can continue making this site one of the best!

Colby

Offline kfsorensen

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #49 on: 01/03/2006 01:43 AM »
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Chris Bergin - 2/1/2006  12:12 PM
Thorium-fueled thermal breeder reactor, and more so the molten fluoride reactor - do we have any available web-based resources on these concepts for further learning?
http://www.nasaspaceflight.com/_docs/

I have uploaded the complete text of "Fluid Fuel Reactors" to this location.  The files of interest are titled "FFR_chap01.pdf" for the first chapter, and so forth.

Chapters 1-10 of the text deal with the aqueous homogeneous reactor, chapters 11-17 deal with the molten-fluoride (molten-salt) reactor, and chapters 18-25 deal with the liquid metal reactor, which was another fluid-fuel reactor concept where uranium would be dissolved in a mixture of lead and bismuth.  There are also three files that are indices for each section.  Altogether the book is about 47 MB in size.

Offline Jamie Young

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #50 on: 01/03/2006 03:27 AM »
And there was me thinking the ESAS report was long! :) I'm going to have to book a week off school! ;)

Offline James Lowe1

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #51 on: 01/03/2006 03:33 AM »
Thanks Vanilla. I just used Chris' login to see if I could place all the files you've uploaded into one seperate folder, to keep them in one place away from the videos etc. It won't let me, but if you know a way, then that might help as I'm sure we'd like to highlight this documentation in an area that is specific.

However, if not, I'm sure it's not a problem. It is hugely appreciated by all of us here that you've made such information available.

Offline kfsorensen

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #52 on: 01/03/2006 04:10 AM »
I tried the same thing during the upload, to make a separate folder, but it wouldn't let me....sorry!

Online Chris Bergin

RE: Role of NTR/BNTR/NEP in future architectures
« Reply #53 on: 01/03/2006 01:58 PM »
Excellent, thanks for uploading this vast resource of information.

I'll be setting up a more direct way for people to download specific elements of the information at some point, inter-linking the information on this thread.

Offline Mark Max Q

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #54 on: 01/03/2006 04:33 PM »
Thanks Vanilla, I'm very interested in this subject and will read through the documents.

Offline kfsorensen

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #55 on: 01/03/2006 06:03 PM »
Now that some documents on molten-fluoride reactors are available, I would like to further explain why this reactor could have some very attractive advantages as a space reactor, either for surface power or in a nuclear-electric vehicle.

During Prometheus, there were three basic reactor concepts studied.  They all used solid uranium fuel that was highly enriched in uranium-235.  In order to minimize the size of the reactor, they didn't slow down (moderate) the neutrons before fission....such types of reactors are called "fast" reactors, and there are only a handful of such reactors in operation today.  By running on fast neutrons and using highly-enriched fuel, they lacked two of the most important aspects in a typical reactor that give you a negative temperature coefficient:  neutron moderation and Doppler absorption.  When you have a reactor that uses low or moderate uranium-235 enrichments (2-20% roughly) you get a nice safety feature in the form of Doppler absorption.  This comes about because the other 98-80% of the uranium is uranium-238, which tends to absorb neutrons most of the time.  When the fuel gets hotter, the 238 gets more absorptive, tending to "drink" up neutrons and shut down the reaction, hence it is a big contributor to a negative temperature coefficient.  235 also gets more absorptive when it gets hotter, but its absorptions typically lead to fission, so using very-highly enriched fuel with little absorbing material present (238) makes your Doppler absorption turn from an effect that gives a negative temperature coefficient to an effect that gives a positive temperature coefficient.

The difference between the negative and positive temperature coefficients is extraordinarily important.  A negative coefficient is like having a marble in the bottom of a bowl.  If you displace the marble, it wants to roll back where it started.  It is "dynamically stable".  Now flip the bowl over and put the marble on the back of the bowl.  If you displace it, it will roll further and further away and off the bowl--it is "dynamically unstable."  There are experts in control theory who spend their whole careers figuring out how to stabilize dynamically unstable systems, like fighter aircraft, through active control.  You do the same sort of thing when you balance a ruler on your finger.  But try to hold still and the ruler falls over.

In a dynamically unstable reactor, with a positive temperature coefficient, you may only have a very, very brief amount of time to stabilize the reactor before it gets away on you and melts down or disassembles.  Unfortunately, there have been a few reactors built on Earth that were dynamically unstable, and they have needed very good control systems and very careful operation to control them.  No Western commercial reactor has ever been built that was dynamically unstable--it's just not allowed, and for good reason.

A solid-core, highly-enriched, fast reactor is almost certainly going to be dynamically unstable.  Now you better have an incredible control system onboard to keep it from melting down or disassembling, and if the reactor is out at the Moon or Jupiter, you will have to rely on that control system to be incredibly redundant, always work, and never fail, even in a terrible radiation environment.  I think that's a bit too much to ask for a space reactor.  Thus I think you must pursue a reactor that is dynamically stable.

I should note that this criticism does not apply to the NTR since the hydrogen in the core will lead to a moderated reactor, and the hydrogen will lead to a fairly strong negative coefficient.  An NTR should be dynamically stable in its neutronic operation, but the fast-spectrum, highly-enriched reactors that were investigated for Prometheus, I don't think so.

Offline Justin Space

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #56 on: 01/03/2006 07:35 PM »
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vanilla - 3/1/2006  1:03 PM

The difference between the negative and positive temperature coefficients is extraordinarily important.  A negative coefficient is like having a marble in the bottom of a bowl.  If you displace the marble, it wants to roll back where it started.  It is "dynamically stable".  Now flip the bowl over and put the marble on the back of the bowl.  If you displace it, it will roll further and further away and off the bowl--it is "dynamically unstable."  There are experts in control theory who spend their whole careers figuring out how to stabilize dynamically unstable systems, like fighter aircraft, through active control.  You do the same sort of thing when you balance a ruler on your finger.  But try to hold still and the ruler falls over.

So the Doppler absorption soaks up neutrons to keep everything at a managable pace, keeping the 'bowl' the right way up, so that the "marble" stays where you want it, because it's aiding a negative coefficient? And thus the Doppler absorption is a safety barrier?

Too many neutrons, or neutrons speeding up out of control, your heading to positive temperature coefficients, and that's going to flip your bowl over and then you've got a battle on your hands in keeping the "marble" where you want it?

I hope I've got close to this. I've never touched on this subject before, but it's bloody facinating! :)

Offline kfsorensen

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #57 on: 01/03/2006 07:57 PM »
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Justin Space - 3/1/2006  2:35 PM
So the Doppler absorption soaks up neutrons to keep everything at a managable pace, keeping the 'bowl' the right way up, so that the "marble" stays where you want it, because it's aiding a negative coefficient? And thus the Doppler absorption is a safety barrier?

Too many neutrons, or neutrons speeding up out of control, your heading to positive temperature coefficients, and that's going to flip your bowl over and then you've got a battle on your hands in keeping the "marble" where you want it?

I hope I've got close to this. I've never touched on this subject before, but it's bloody facinating! :)
What Doppler absorption does is it makes things that absorb neutrons absorb them better as they get hotter.  Uranium-238 (which is the abundant component of uranium) tends to absorb neutrons and turn into uranium-239, then decay to neptunium-239 and then to plutonium-239.  But these decays take a few days, so on the time scale of the fission process, all the U-238 does is drink up neutrons.  So when you have a fuel that has a lot of "resonance absorber" in it (which is nuclear-talk for a material that tends to absorb neutrons in resonances, which is another way to call Doppler [are you confused yet!]) then the Doppler really helps create a negative temperature coefficient, which leads to reactor stability.

The reason I point out U-238 is that it is the most common resonance absorber in a typical reactor core.  Your average terrestrial reactor has fuel that is probably 97% U-238, so this is a strong contributor to the negative coefficient.  Other resonance absorbers are thorium-232 or tungsten...most heavy elements are resonance absorbers.  Your average terrestrial reactor also has moderated neutrons, which is another big way to get a negative temperature coefficient.

U-235, on the other hand, tends to absorb a neutron and then fission, which sprays out a bunch more neutrons.  So if you have a core that is mostly U-235, like these space reactors I was telling you about, then the Doppler absorption (resonance absorption) tends to make the temperature coefficient positive, because higher temperature leads to more absorption leads to more fission, which leads to higher temperature....and so on....boom!

So the use of highly-enriched fuel, and the absence of resonance absorbers in the core, and no neutron moderation (fast reactors) lead to reactor designs with positive temperature coefficients....trouble, big big trouble.  Chernobyl happened because a reactor that was generally designed to have a negative coefficient got into an operating regime where it had a positive coefficient, and there was an explosion.  Western reactors are REQUIRED to have negative coefficients in ALL possible regimes.  The newer Russian reactors (VVERs) are like Western reactors in that regard.  The Chernobyl-type reactor (the RBMKs) I believe have all been decommisioned.

There are a number of things to make sure you get right in a reactor design, but I don't think there is any one more fundamental than the temperature coefficient of reactivity.

Offline Avron

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #58 on: 01/04/2006 03:34 AM »
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vanilla - 3/1/2006  3:57 PM


U-235, on the other hand, tends to absorb a neutron and then fission, which sprays out a bunch more neutrons.  So if you have a core that is mostly U-235, like these space reactors I was telling you about, then the Doppler absorption (resonance absorption) tends to make the temperature coefficient positive, because higher temperature leads to more absorption leads to more fission, which leads to higher temperature....and so on....boom!


Is that one of the key reasons for using U-235 in bombs? The Positive coefficient drives the yield? more positive, the  bigger the bang? and does that also mean that U-238 cannot go bang?

Offline kfsorensen

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #59 on: 01/04/2006 04:25 AM »
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Avron - 3/1/2006  10:34 PM
Is that one of the key reasons for using U-235 in bombs? The Positive coefficient drives the yield? more positive, the  bigger the bang? and does that also mean that U-238 cannot go bang?
No, in a bomb your only goal is to make the reaction go supercritical, basically as fast as possible.  Designing a reactor is MUCH more difficult than designing a bomb, which is probably one of the reasons that the US built a bomb in 1945 but didn't get a civilian power reactor until 1957.  U-235 is fissile, at all neutron energies.  U-238 will actually fission, but only at very high neutron energies, which ironically, you typically find in a bomb!  Some bombs use U-235 or Pu-239 as the first stage, a fusion second stage where deuterium and tritium fuse to helium and neutrons, and then those high energy neutrons hit a third stage of U-238, which will fission under the intense energies of those fusion neutrons.

The negative temperature coefficient keeps your reactor exactly critical--you go subcritical, the reactor cools down, the reactor gets more reactive, and heats up again.  You go supercritical, even a little bit, the reactor gets hotter and less critical, cools down, and you're back where you want to be.  It's a beautiful feature, and it's what allows well-designed reactors to "follow the load"--they will adjust the power they produce to the power demand that they "feel" from the grid.

One of the ways German scientists messed up Hitler's attempts to develop the bomb was to tell him that you needed to moderate (slow down) the neutrons to make the bomb work.  It doesn't work--the bomb will disassemble before it blows up.  They knew it--but they didn't want Hitler to have to the bomb.  Aren't we glad they didn't?

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