Author Topic: Compact nuclear power sources for space exploration  (Read 17030 times)

Offline Kaputnik

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I've been trawling the forum but information on this seems to be spread out thinly, so I was hoping that we could have a discussion of the merits/demerits of different methods of using nuclear power in space.

What I am primarily interested in is nuclear power as a viable, practical method of powering spacecraft on the surface of the moon and Mars, in particular ISRU plants and to power or recharge roving vehicles.

What technologies appear to be the most feasible? How small and 'portable' can we make a reactor? How safe are they?
"I don't care what anything was DESIGNED to do, I care about what it CAN do"- Gene Kranz

Offline Tom Ligon

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Re: Compact nuclear power sources for space exploration
« Reply #1 on: 02/28/2008 04:36 pm »
I'm turning blue holding my breath, waiting for news from Santa Fe on Dr. Bussard's project.  That's the "Interesting Fusion Talk on Google" thread of 2000+ posts below.

If it pans out, and I'm really hopeful it does, it probably what you're looking for.

Offline meiza

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Re: Compact nuclear power sources for space exploration
« Reply #2 on: 02/28/2008 04:53 pm »
Well, as a broad view there are a few ways to convert the power of a reactor core to electricity.
At least:
1) thermocouple (really inefficient but reliable)
2) thermionic (Russians did this, kinda in the middle, I don't really know what it is about)
3) heat engines (moving parts, complex, expensive, error-prone, require big radiators, good efficiency)

Inefficiency here means two things: you need lots of mass for power and you need lots of nuclear material too. Both are bad and expensive things.

Offline lambda0

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Re: Compact nuclear power sources for space exploration
« Reply #3 on: 02/29/2008 02:06 pm »
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meiza - 28/2/2008  11:53 AM
...
2) thermionic (Russians did this, kinda in the middle, I don't really know what it is about)
...

Some doc about the Topaz :
http://www-rsicc.ornl.gov/ANST_site/topaz.pdf



Offline hop

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Re: Compact nuclear power sources for space exploration
« Reply #4 on: 02/29/2008 09:14 pm »
Quote
meiza - 28/2/2008  9:53 AM

Well, as a broad view there are a few ways to convert the power of a reactor core to electricity.
At least:
1) thermocouple (really inefficient but reliable)
2) thermionic (Russians did this, kinda in the middle, I don't really know what it is about)
3) heat engines (moving parts, complex, expensive, error-prone, require big radiators, good efficiency)
Some of the studies involving SRG might be applicable, using a small reactor rather than an RTG as a heat source. If you could get the same efficiency from topaz, you'd have ~25kw out of a few hundred kg mass.


wikipedia also mentions thermophotovoltiac systems, which can in theory be combined with thermocouple, giving roughly 2x the overall efficiency of either system alone, and using no moving parts. On the downside, they degrade faster than thermocouples.

Offline Kaputnik

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Re: Compact nuclear power sources for space exploration
« Reply #5 on: 03/01/2008 05:28 pm »
Thanks for the replies so far.
As a starting point I'm assuming that things are kept fairly 'conventional', i.e. fission and (steam?) turbines for power generation, cooling probably radiative given the environments we're talking about.

What feasible designs are on the table? What sort of size, mass, output are we talking about? What are the shielding/separation requirements?

Sorry for so many questions but I think this technology could be very, very important in space exploration over the next 30+ years and I'd like to have more of a handle on it.
"I don't care what anything was DESIGNED to do, I care about what it CAN do"- Gene Kranz

Offline lambda0

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Re: Compact nuclear power sources for space exploration
« Reply #6 on: 03/02/2008 03:39 pm »
Quote
...
What feasible designs are on the table? What sort of size, mass, output are we talking about? What are the shielding/separation requirements?
...
You can try a search on "SAFE-400" or "HOMER-15"...
But it's a bit difficult to have informations about the development status of these projects, I don't know if they are stopped, just delayed, if they are really funded.
http://www.world-nuclear.org/info/inf82.html


Offline Kaputnik

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Re: Compact nuclear power sources for space exploration
« Reply #7 on: 03/02/2008 05:07 pm »
Thanks for the link.
Starting to get some numbers now- good!
However the information on the link about RTG power sources is very confusing: is the GPHS RTG a 290w, 2kw, or 100w system? All three figures are quoted. Also, what would the unit's complete mass be? The SRG appears to be more defined, at "100-120w", but it would be good to get figures on mass.
"I don't care what anything was DESIGNED to do, I care about what it CAN do"- Gene Kranz

Offline hop

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Re: Compact nuclear power sources for space exploration
« Reply #8 on: 03/03/2008 12:09 am »
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Kaputnik - 2/3/2008  10:07 AM

Thanks for the link.
Starting to get some numbers now- good!
However the information on the link about RTG power sources is very confusing: is the GPHS RTG a 290w, 2kw, or 100w system? All three figures are quoted. Also, what would the unit's complete mass be? The SRG appears to be more defined, at "100-120w", but it would be good to get figures on mass.
Most nuclear sources are specified in both thermal and electrical power. So for example:
Quote
The Multi-Mission RTG (MMRTG) will use 8 GPHS units producing 2 kW which can be used to generate 100 watts of electricity and is a focus of current research.
Means 2KW of thermal output for 100w of electrical output. This means 5% efficiency, which is typical for thermocouple systems. The other figures are for the units that make up one MMRTG.

Offline iamlucky13

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Re: Compact nuclear power sources for space exploration
« Reply #9 on: 03/03/2008 10:33 pm »
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Tom Ligon - 28/2/2008  9:36 AM

I'm turning blue holding my breath, waiting for news from Santa Fe on Dr. Bussard's project.  That's the "Interesting Fusion Talk on Google" thread of 2000+ posts below.

If it pans out, and I'm really hopeful it does, it probably what you're looking for.

Unfortunately, as even Dr. Bussard would I'm sure readily admit, the polywell fusor does not scale down very well at all, which is why at the time of his death, he was talking so much about getting funding to build extremely large prototypes. But the Navy didn't even seem to have a lot of confidence in even his larger versions. And if it doesn't fit in an aircraft carrier, it definitely won't fit on a rocket.

There's two companies working on radioisotope Stirling thermal generators. One is down in California somewhere and I believe is the same company building the big solar-thermal plant in New Mexico. The other one is in Washington and not seeing as much commercial success, but I believe they still have a research contract from NASA. I toured their facility a few years ago, and at the time they thought they had a shot building the power supply for MSL, but I believe JPL chose a thermocouple-type RTG for that. They had propane-heated prototypes as big as 1 kWe, but I don't know what kind of efficiency.

I think somewhere in the ESAS there is discussion of lunar research station power supplies, focusing mosty on solar photovoltaic or nuclear. My impression was the nuclear ideas were thermo-mechanical, ie - Stirling cycle or even microturbines, and fission rather than decay-powered.

Offline Sid454

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Re: Compact nuclear power sources for space exploration
« Reply #10 on: 03/03/2008 11:11 pm »
I feel compact nuclear power sources in the 30KW range and larger are more important then even heavy lift vehicles for a mars mission.
ISRU and even just closed loop life support is going to need some serious KW let alone propulsion etc.
But also remember it's gotta break down into easy to assemble parts that weigh less then 25 to 100tons so for near term fusion is out of the question.

Offline iamlucky13

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Re: Compact nuclear power sources for space exploration
« Reply #11 on: 03/04/2008 06:26 pm »
Something on the order of 30 kWe or even 100 kWe in a 25 ton package is not unfeasible, in fission that is, not fusion. The Russian RORSAT radar spy satellites (like COSMOS-954 that re-entered over Canada) developed 3 kWe (and 100 kW-thermal) from a 400 kg reactor. A thermodynamic cycle might only double the mass, but triple or better the effficiency. In addition, the thermal power density of nuclear reactors can be made to scale versus fuel mass at a rate much better than linear.

Radiation shielding for allowing humans to live and work nearby could make it quite heavy, but it's possible (even advisable) to launch and land the reactor cold (U-235 is nearly harmless from a radioactivity standpoint...it's the fission activity and products and decay products you have to worry about). After landing you bury or cover it in regolith a reasonable distance from your base, and connect power cables and radiators. Voila.

BTW, the Exploration Systems Architecture Study, section 4, briefly discusses power options for a lunar outpost and their extensibility to a Mars mission. Their preliminary conclusions put solar and nuclear systems on similar feasibility levels, but favored a nuclear system for its ability to operate equally effectively over a 14 day-long night and for its extensibility to Mars exploration. In particular, they suggest that a 100 kWe reactor could be developed with a 10-20 ton mass. They don't discuss a buried reactor, but instead suggest placing it approximately 2 km away from the habitat.

http://www.nasa.gov/mission_pages/exploration/news/ESAS_report.html

Offline Gene DiGennaro

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Re: Compact nuclear power sources for space exploration
« Reply #12 on: 03/04/2008 06:43 pm »
SP100?

Offline kttopdad

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Re: Compact nuclear power sources for space exploration
« Reply #13 on: 03/04/2008 08:49 pm »
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Sid454 - 3/3/2008  6:11 PM

I feel compact nuclear power sources in the 30KW range and larger are more important then even heavy lift vehicles for a mars mission.
ISRU and even just closed loop life support is going to need some serious KW let alone propulsion etc.
But also remember it's gotta break down into easy to assemble parts that weigh less then 25 to 100tons so for near term fusion is out of the question.

Having a BFR (Bussard Fusion Reactor) as a power source is probably easier on Mars than it is on Earth.  In this case "easier" equates to lighter and less expensive.  I think it's an ideal solution to the problem of the energy needs of an ISRU Mars mission.

I suspect that a BFR on Mars would benefit from the realities of the situation.  A vacuum containment system would require a much less robust structure in the near-vacuum of the Martian atmosphere.  Also, the unit would require less shielding than would be the case on Earth.  (The inverse square law is the best shielding you can get, and there's no reason not to put a BFR a good distance away from the settlement it's powering.  That's not so easy to pull of here on Earth.  :-) ) Both of those factors would decrease the mass of the system per KW.

In addition to the problem of generating the power needed by the station would be that of power distribution.  If the BFR is in a fairly remote location (for inverse square shielding), then what's a good way to get the power back to the station?  Hauling a couple of miles of big power cables across the solar system isn't a great idea, so we'll obviously want to use something along the lines of microwave transmission for power distribution.  The good thing about that technology is that it lends itself to a mobile/flexible power-use architecture.  Base camps, mining operations, extended exploration parties, etc. would all be able to set up mobile antennas for receiving the power they need.  It would require a careful aim from the BFR station, but the atmospheric losses would be minimal so distance isn't really a problem.  

I would imagine that power generation and power storage/portability will have complimentary solutions.  Obviously, we don't have back-pack-sized nuclear power systems on the horizon, so we're going to be relying on batteries as the best technology for using the BFR-generated electricity for mobile infrastructure.  I can see a case for a ubiquitous battery unit (UBU), about the size of a lap-top battery, that can be ganged together for larger uses like powering vehicles and personal atmosphere suits.  If a UBU goes bad it's removed from the gang and reconditioned.  Each UBU would have enough smarts to monitor its own critical parameters (temperature, state of charge, etc.) and report problems to the gang leader.  The gang leader would be responsible for maintaining the overall charge and conditioning of the system and reporting problems to the user.  This is similar to how Tesla has arranged their energy storage system for the Tesla Roadster.  Imagine everything on the Mars Station, from laptops to the main battery backup for the life-support system using the same modular power storage infrastructure.  Very granular.

The batteries required for an electric vehicle will keep a bank of computers and other equipment running for quite a while.  There's already a lot of discussion around the electric vehicle community about the impact of having thousands/millions of EVs plugged into the grid can act as a source as well as a load, as needed.  When the BFR needs to be taken down for Planned Maintenance, all of the UBUs that are in the vehicles and other large, portable equipment could serve as a source for powering critical subsystems while the main power is out.  Plug the vehicles etc. into their normal charging stations and, viola, they are now power sources!

Using a granular approach like this for power storage would reduce the mass needed overall.  Instead of having an entire backup battery system for the station as well as batteries for all of the vehicles, suits, remote equipment, etc., a modular approach would let you combine the power storage needs of the equipment/station in whatever configuration is needed at the time.

I love dreaming about this stuff.
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Offline khallow

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Re: Compact nuclear power sources for space exploration
« Reply #14 on: 03/06/2008 03:07 pm »
One thing about fusion is that it doesn't seem compact to me. I imagine the first viable, positive net power fusion reactors are going to be big. Even aneutronic reactors (which probably will be the first) will need space to absorb the energy of the charged particles emitted from the reaction. A larger volume means that the mass of the plasma to its surface area (and heat radiation) is larger (for a fixed plasma density). That makes it easier to keep the plasma hot. And if the reaction gives off neutrons, then a large reactor is going to keep a larger fraction of its neutrons in the plasma.

Nuclear fission has already been made very compact. The real problem is the heat sinks.
Karl Hallowell

Offline neviden

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Re: Compact nuclear power sources for space exploration
« Reply #15 on: 03/06/2008 05:24 pm »
Nuclear power sources are good since they are independent from the environment. That means they would be good for Mars, but if we are talking about 10 - 100 KW of power, we could get that from solar power (maybe even from the wind). So that means that they are not absolutely needed if you can store enough power.

If we are talking about space power for NEP, then we need 5 – 20 MW of electric power for manned missions. Unmanned missions need far less, but since they are unmanned they can easily be powered by sun, and even manned missions could be powered by sun. They would need solar wings the size of 200 x 200 m, but that doesn’t seem to be too big of a problem. That is the size of ISS which is far more complicated and yet we still managed to build it.

Fusion would be another thing altogether. GWs of power at high isp especially if we could run Aneutronic fusion reaction. It would be massive of course, but it could move massive ships around. While I would love to see fusion ships I have a felling that the costs would also be massive. Not to mention that we should probably make fusion work on Earth first. Bussard Fusion Reactor would be very good of course, but I am far from optimistic that it would work.

So, it would be nice to have cheap, small reactors, but it will (or should) probably come down to costs. If it were up to me I would use existing fusion reactor that is called the sun and go that way. Space travel is expensive enough as it is.

Offline iamlucky13

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Re: Compact nuclear power sources for space exploration
« Reply #16 on: 03/06/2008 07:11 pm »
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khallow - 6/3/2008 8:07 AM

One thing about fusion is that it doesn't seem compact to me. I imagine the first viable, positive net power fusion reactors are going to be big. Even aneutronic reactors (which probably will be the first) will need space to absorb the energy of the charged particles emitted from the reaction. A larger volume means that the mass of the plasma to its surface area (and heat radiation) is larger (for a fixed plasma density). That makes it easier to keep the plasma hot. And if the reaction gives off neutrons, then a large reactor is going to keep a larger fraction of its neutrons in the plasma.

Nuclear fission has already been made very compact. The real problem is the heat sinks.
Right...Tokamaks (and stellerators) are huge. If Brussard's device actually is workable (far from proven), it might be a little smaller, but not overwhelmingly. Nothing else looks really promising for sustainable, net positive fusion at this point. However, anneutronic fusion is even further down the road than D-T fusion. It takes much more energy to start He-3 fusion.

See below for a size comparison a human being, JET (currently the largest fusion reactor, but not capable of self-sustaining fusion), and ITER, just starting construction:

http://www.jet.efda.org/images/level1/jet-iter/jet-iter-s.jpg

Quote
neviden - 6/3/2008 10:24 AM

Nuclear power sources are good since they are independent from the environment. That means they would be good for Mars, but if we are talking about 10 - 100 KW of power, we could get that from solar power (maybe even from the wind). So that means that they are not absolutely needed if you can store enough power.
NASA's current analysis (referenced in ESAS) seems to indicate that for 100 kWe range on the moon, solar comes out heavier than nuclear due the need to oversize and have storage for overnight power. The balance would be even more heavily in nuclear's favor on Mars, both due to added panel size and higher gravity to support against.

Offline neviden

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Re: Compact nuclear power sources for space exploration
« Reply #17 on: 03/06/2008 07:49 pm »
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iamlucky13 - 6/3/2008  9:11 PM
NASA's current analysis (referenced in ESAS) seems to indicate that for 100 kWe range on the moon, solar comes out heavier than nuclear due the need to oversize and have storage for overnight power. The balance would be even more heavily in nuclear's favor on Mars, both due to added panel size and higher gravity to support against.
As far as I am aware there wouldn't be much of overnight power need if Moon base would be on the Peaks of eternal light as it is currently planned. I am not 100% certain if the sun always shines there, but it shines most of the time (and when it doesn’t, don’t use any power hungry things). ISRU could be used as a power storage if it was designed with that in mind.

You could build tents out of thin film solar panels on Mars. It is far worse energy wise, but it is at least possible to do it without nuclear reactor. Nuclear reactor would be nice, but is not that it is exactly cheap to design and build either.

Offline PurduesUSAFguy

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Re: Compact nuclear power sources for space exploration
« Reply #18 on: 03/07/2008 01:39 am »
Quote
iamlucky13 - 6/3/2008  2:11 PM

Quote
khallow - 6/3/2008 8:07 AM

One thing about fusion is that it doesn't seem compact to me. I imagine the first viable, positive net power fusion reactors are going to be big. Even aneutronic reactors (which probably will be the first) will need space to absorb the energy of the charged particles emitted from the reaction. A larger volume means that the mass of the plasma to its surface area (and heat radiation) is larger (for a fixed plasma density). That makes it easier to keep the plasma hot. And if the reaction gives off neutrons, then a large reactor is going to keep a larger fraction of its neutrons in the plasma.

Nuclear fission has already been made very compact. The real problem is the heat sinks.
Right...Tokamaks (and stellerators) are huge. If Brussard's device actually is workable (far from proven), it might be a little smaller, but not overwhelmingly. Nothing else looks really promising for sustainable, net positive fusion at this point. However, anneutronic fusion is even further down the road than D-T fusion. It takes much more energy to start He-3 fusion.

See below for a size comparison a human being, JET (currently the largest fusion reactor, but not capable of self-sustaining fusion), and ITER, just starting construction:

http://www.jet.efda.org/images/level1/jet-iter/jet-iter-s.jpg

Quote
neviden - 6/3/2008 10:24 AM

Nuclear power sources are good since they are independent from the environment. That means they would be good for Mars, but if we are talking about 10 - 100 KW of power, we could get that from solar power (maybe even from the wind). So that means that they are not absolutely needed if you can store enough power.
NASA's current analysis (referenced in ESAS) seems to indicate that for 100 kWe range on the moon, solar comes out heavier than nuclear due the need to oversize and have storage for overnight power. The balance would be even more heavily in nuclear's favor on Mars, both due to added panel size and higher gravity to support against.

Not to nit-pick but a He3-D reactor wouldn't be aneutronic, because even if the primary reaction of helium-3 and deuterium is aneutronic, because of the plasma would be of higher then sufficent energy you would get lots of D-D side reaction as well producing neutrons.

You really have to start looking at he3-he3 or P-B11 which are really far down the road to get a reactor that has a very benign neutron environment.

Offline iamlucky13

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Re: Compact nuclear power sources for space exploration
« Reply #19 on: 03/07/2008 04:56 pm »
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neviden - 6/3/2008  12:49 PM

Quote
iamlucky13 - 6/3/2008  9:11 PM
NASA's current analysis (referenced in ESAS) seems to indicate that for 100 kWe range on the moon, solar comes out heavier than nuclear due the need to oversize and have storage for overnight power. The balance would be even more heavily in nuclear's favor on Mars, both due to added panel size and higher gravity to support against.
As far as I am aware there wouldn't be much of overnight power need if Moon base would be on the Peaks of eternal light as it is currently planned. I am not 100% certain if the sun always shines there, but it shines most of the time (and when it doesn’t, don’t use any power hungry things). ISRU could be used as a power storage if it was designed with that in mind.

You could build tents out of thin film solar panels on Mars. It is far worse energy wise, but it is at least possible to do it without nuclear reactor. Nuclear reactor would be nice, but is not that it is exactly cheap to design and build either.
Referring to the ESAS section on outpost power again, the estimates for a nuclear reactor pricetag are 30-50% higher than for solar. Both are rated approximately the same for feasibility, but the two factors that favor nuclear at this point were the overnight capability and the extensibility to a Mars mission, where perpetual light is definitely not an option and the panel size needs are higher. Maintenance is also expected to be higher on tracking solar panels or panels exposed to dust than on a reactor.

Anyway, none of it is set in stone yet, but if there's any leaning one way or the other, that's how I see it.

Offline publiusr

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Re: Compact nuclear power sources for space exploration
« Reply #20 on: 03/07/2008 08:10 pm »
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Sid454 - 3/3/2008  6:11 PM

I feel compact nuclear power sources in the 30KW range and larger are more important then even heavy lift vehicles for a mars mission.
ISRU and even just closed loop life support is going to need some serious KW let alone propulsion etc.
But also remember it's gotta break down into easy to assemble parts that weigh less then 25 to 100tons so for near term fusion is out of the question.

I don't see that has to be the case. Use the heavy Lifter to launch to launch the nuclear power source.

Offline kttopdad

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Re: Compact nuclear power sources for space exploration
« Reply #21 on: 03/07/2008 08:34 pm »
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iamlucky13 - 6/3/2008  2:11 PM
Right...Tokamaks (and stellerators) are huge. If Brussard's device actually is workable (far from proven), it might be a little smaller, but not overwhelmingly.

Dr. Bussard figured that a 2nd-generation pB11 reactor could be made about the size of a standard shipping container.  By the time we get to starting Mars missions, we should have spaceworthy BFRs available.  Of course, this assumes that Dr. Bussard knew what he was talking about.  :-)
"Do what you can, with what you have, where you are."  -T. Roosevelt

Offline Kaputnik

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Re: Compact nuclear power sources for space exploration
« Reply #22 on: 03/08/2008 05:07 pm »
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publiusr - 7/3/2008  9:10 PM

Quote
Sid454 - 3/3/2008  6:11 PM

I feel compact nuclear power sources in the 30KW range and larger are more important then even heavy lift vehicles for a mars mission.
ISRU and even just closed loop life support is going to need some serious KW let alone propulsion etc.
But also remember it's gotta break down into easy to assemble parts that weigh less then 25 to 100tons so for near term fusion is out of the question.

I don't see that has to be the case. Use the heavy Lifter to launch to launch the nuclear power source.

Yes, but if it's destined for the surface of Mars it hits a per-item mass limit- currently about 9t per piece and maybe up to 50t or so if radical new technologies for landing on Mars are developed.
"I don't care what anything was DESIGNED to do, I care about what it CAN do"- Gene Kranz

Offline alexterrell

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Re: Compact nuclear power sources for space exploration
« Reply #23 on: 03/08/2008 06:15 pm »
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iamlucky13 - 4/3/2008  1:26 PM



BTW, the Exploration Systems Architecture Study, section 4, briefly discusses power options for a lunar outpost and their extensibility to a Mars mission. Their preliminary conclusions put solar and nuclear systems on similar feasibility levels, but favored a nuclear system for its ability to operate equally effectively over a 14 day-long night and for its extensibility to Mars exploration. In particular, they suggest that a 100 kWe reactor could be developed with a 10-20 ton mass. They don't discuss a buried reactor, but instead suggest placing it approximately 2 km away from the habitat.

http://www.nasa.gov/mission_pages/exploration/news/ESAS_report.html

Thin film solar should come out at about 100We / Kg gross. However, they will generate electricity about 1/3 of the time and might support structures. (Alternatively, just lay them over a south facing hill. If the "cloth" is light enough, just accept the low capacity utilisation.)

At the poles, solar can work 80% of the time but needs to be on vertical axis towers - something like the masts of old square rig sailing ships. (see http://www.lpi.usra.edu/meetings/lpsc2004/pdf/1387.pdf)

At the poles, nuclear power could also be a useful source of heat, especially if the one of the tasks is to heat up volatile bearing regolith from 30K.

Offline Patchouli

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Re: Compact nuclear power sources for space exploration
« Reply #24 on: 03/08/2008 07:14 pm »
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Kaputnik - 8/3/2008  12:07 PM

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publiusr - 7/3/2008  9:10 PM

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Sid454 - 3/3/2008  6:11 PM

I feel compact nuclear power sources in the 30KW range and larger are more important then even heavy lift vehicles for a mars mission.
ISRU and even just closed loop life support is going to need some serious KW let alone propulsion etc.
But also remember it's gotta break down into easy to assemble parts that weigh less then 25 to 100tons so for near term fusion is out of the question.

I don't see that has to be the case. Use the heavy Lifter to launch to launch the nuclear power source.

Yes, but if it's destined for the surface of Mars it hits a per-item mass limit- currently about 9t per piece and maybe up to 50t or so if radical new technologies for landing on Mars are developed.

I guess we have another problem we got to figure out how to land 50T on mars as 9T just is nowhere big enough unless you wish to do lots and lots of low level assembly and I don't mean ISS attach the tinker toys type assembly as that needs a minimun of 20T to 25T sections but real construction.

We'll have to learn to weld on mars as well as bolt pressure vessels together from parts though a stripped inflatable with nothing inside it might be able to get under 9T but we'll have to talk with someone from bigelow on this.

But this would not solve the ascent vehicle problem which is going to have a mass of at least 15T  dry and 50tons fueled
and I'm just talking something that can get into mars orbit and rendezvous with the waiting MTV not something like Zurbin's ERV which goes all the way home.

Offline Kaputnik

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Re: Compact nuclear power sources for space exploration
« Reply #25 on: 03/08/2008 08:35 pm »
Landed mass on Mars is the real bottleneck. I've looked into this a little myself. The biggest LV fairings currently envisaged are around 16m (but 14m may be more likely). That gives maximum payload diameters in the range 12-14m. Using Viking heritage entry systems with a maximum entry vehicle density of 150kg/m2, it translates to 17-23t maximum entry mass. How much of this mass makes it to the surface as useable payload is dependent on the vehicle's design, but somewhere between 25% and 35% appears to be achievable; one would hope that something like 50% could be achieved but that's simply optimism on my part. Hence, payloads of between 4.25t and 9t seem possible; that's really quite small!
"I don't care what anything was DESIGNED to do, I care about what it CAN do"- Gene Kranz

Offline Patchouli

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Re: Compact nuclear power sources for space exploration
« Reply #26 on: 03/09/2008 02:21 am »
A mars lander would have a much larger mass fraction then a lunar lander because you have an atmosphere to help you slow down.
Of course the bigger you make these things the more mass efficient they get due to mathematics of scaling.

We only tried I think one design of reentry vehicle so far on mars the viking system so there is a lot of room for improvement such as biconic and lifting body.
Something lifting or even winged might have a much better mass fraction esp since it can be long and relatively narrow such as with DCY and the shuttle though no matter what you reenter with that finial descent will have to be under rocket power.

Offline Kaputnik

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Re: Compact nuclear power sources for space exploration
« Reply #27 on: 03/09/2008 05:35 pm »
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Patchouli - 9/3/2008  3:21 AM

A mars lander would have a much larger mass fraction then a lunar lander because you have an atmosphere to help you slow down.
Of course the bigger you make these things the more mass efficient they get due to mathematics of scaling.

We only tried I think one design of reentry vehicle so far on mars the viking system so there is a lot of room for improvement such as biconic and lifting body.
Something lifting or even winged might have a much better mass fraction esp since it can be long and relatively narrow such as with DCY and the shuttle though no matter what you reenter with that finial descent will have to be under rocket power.

True, the atmosphere allwos us to contemplate higher mass fractions, but in practise a lot of mass must be allocated to the heatshield, backshell, and parachutes- not to mention the landing apparatus itself. This leads to a low overall useful payload fraction- for example, on the MERs, this was 22.5% (although the airbag and tetrahedron system is not very efficient).

If a larger useful payload is to be attempted, it will likely depend upon a lifting body or biconic design, or maybe an inflatable heatshield- or even one pieced together from separate pieces.

The final propulsive element of Mars descent is a relatively minor operation, in terms of preopllat mass. For example, Viking was only travelling at about 60m/s when the thrusters kicked in, so the total delta-v needed to null that speed and ensure a soft landing is quite small.
"I don't care what anything was DESIGNED to do, I care about what it CAN do"- Gene Kranz

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Re: Compact nuclear power sources for space exploration
« Reply #28 on: 03/09/2008 05:55 pm »
Is there sufficient atmosphere on Mars that jet engines or propellers can provide say a quarter of the slow down delta-v?

Offline Kaputnik

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Re: Compact nuclear power sources for space exploration
« Reply #29 on: 03/09/2008 06:07 pm »
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A_M_Swallow - 9/3/2008  7:55 PM

Is there sufficient atmosphere on Mars that jet engines or propellers can provide say a quarter of the slow down delta-v?

Delta-v for which phase of descent? The entry aeroshell- which has to be there anyway- does a good job down to about Mach 3, and parachutes are a very low mass way of slowing from there to less than 100m/s. So for the final decceleration we need the lowest mass solution to kill off tha final bit of speed. I doubt that anything is more mass-efficient than a rocket motor because the mass of the engine itself is nice and low and the propellant supply can also be quite small.
"I don't care what anything was DESIGNED to do, I care about what it CAN do"- Gene Kranz

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Re: Compact nuclear power sources for space exploration
« Reply #30 on: 03/09/2008 06:29 pm »
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Kaputnik - 9/3/2008  8:07 PM

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A_M_Swallow - 9/3/2008  7:55 PM

Is there sufficient atmosphere on Mars that jet engines or propellers can provide say a quarter of the slow down delta-v?

Delta-v for which phase of descent? The entry aeroshell- which has to be there anyway- does a good job down to about Mach 3, and parachutes are a very low mass way of slowing from there to less than 100m/s. So for the final decceleration we need the lowest mass solution to kill off tha final bit of speed. I doubt that anything is more mass-efficient than a rocket motor because the mass of the engine itself is nice and low and the propellant supply can also be quite small.

Do rockets have to be switched off during the last few feet of the descent?

100 m/s = 224 mph is well within the operating speed of helicopters and ordinary aircraft.  Given an aircraft on a rocky place like Mars there are plenty of other journeys it can take.

Bringing this back on topic, the nuclear powered aircraft may finally get built.

Offline Kaputnik

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Re: Compact nuclear power sources for space exploration
« Reply #31 on: 03/09/2008 07:43 pm »
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A_M_Swallow - 9/3/2008  8:29 PM
Do rockets have to be switched off during the last few feet of the descent?

100 m/s = 224 mph is well within the operating speed of helicopters and ordinary aircraft.  Given an aircraft on a rocky place like Mars there are plenty of other journeys it can take.

Bringing this back on topic, the nuclear powered aircraft may finally get built.

Yes they do, so the spacecraft must be built to withstand the final drop to the surface. I think this may be to avoid 'sandblasting' and damage by blow-back of the exhaust products.

I'm sure that some form of jet engine or rotor blade could well kill off the last bit of velocity, but the timeframe for doing this is very, very tight, so high thrust/weight systems are favoured- so much so that SRMs can be a good choice (e.g. Pathfinder & MER) despite their lower isp. With a jet engine, I don't know how that could work anyway- what is being combusted? With a propeller or rotor, in addition to the question of an energy source, there is the matter of deployment as well, adding complexity and, crucially, time.
Finally, remember that the atmosphere on Mars is of a similar density to that of Earth at about 100,000ft. Not many aircraft can operate there and none can operate at a suitable speed to effect a landing. If you are determined to use a non-rocket descent system, a balloon is far more promising IMHO.

And getting back on track... we need to increase the payload fraction that reaches the surface. Unless that payload happens to itself be some form of aircraft, the aeroshell/parachute/rocket system currently in use will take some beating.
"I don't care what anything was DESIGNED to do, I care about what it CAN do"- Gene Kranz

Offline clongton

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Re: Compact nuclear power sources for space exploration
« Reply #32 on: 03/09/2008 11:13 pm »
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A_M_Swallow - 9/3/2008  2:55 PM

Is there sufficient atmosphere on Mars that jet engines or propellers can provide say a quarter of the slow down delta-v?
Jet engine? There is no oxygen in the Martian atmosphere to speak of so such an engine couldn't function. So it doesn't even matter what the propellant is, you couldn't ignite it.
Chuck - DIRECT co-founder
I started my career on the Saturn-V F-1A engine

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Re: Compact nuclear power sources for space exploration
« Reply #33 on: 03/10/2008 01:20 am »
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clongton - 10/3/2008  1:13 AM

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A_M_Swallow - 9/3/2008  2:55 PM

Is there sufficient atmosphere on Mars that jet engines or propellers can provide say a quarter of the slow down delta-v?
Jet engine? There is no oxygen in the Martian atmosphere to speak of so such an engine couldn't function. So it doesn't even matter what the propellant is, you couldn't ignite it.

Magnesium can actually burn the carbon dioxide in Mar's atmosphere.
http://www.angelo.edu/faculty/kboudrea/demos/burning_magnesium/burning_magnesium.htm


NASA document "Powdered Magnesium—Carbon Dioxide Rocket Combustion Technology for In Situ Mars Propulsion"
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080002287_2008000948.pdf


Propellers can be electrically powered.  Possibly from a nuclear reactor.

Offline clongton

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Re: Compact nuclear power sources for space exploration
« Reply #34 on: 03/10/2008 01:38 am »
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A_M_Swallow - 9/3/2008  10:20 PM

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clongton - 10/3/2008  1:13 AM

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A_M_Swallow - 9/3/2008  2:55 PM

Is there sufficient atmosphere on Mars that jet engines or propellers can provide say a quarter of the slow down delta-v?
Jet engine? There is no oxygen in the Martian atmosphere to speak of so such an engine couldn't function. So it doesn't even matter what the propellant is, you couldn't ignite it.

Magnesium can actually burn the carbon dioxide in Mar's atmosphere.

Propellers can be electrically powered.
What are the combustion residues of the magnesium/carbon dioxide burn? Even if they were benign, the density of the atmosphere is so small that I can't envision such an engine functioning efficiently at all. And even though my first reaction to electrically powered propellers is to say that it's not a jet engine, the atmospheric density difficulty would also prevent it being useful for landing a spacecraft from orbit. An airplane with huge propellers and huge wings can fly in the atmosphere, but it would also be very light, far lighter than any landing spacecraft.

If the landing is going to be powered in any way, it will be rocket power.
Chuck - DIRECT co-founder
I started my career on the Saturn-V F-1A engine

Offline A_M_Swallow

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Re: Compact nuclear power sources for space exploration
« Reply #35 on: 03/10/2008 02:21 am »
Some of your questions are answered by the report whose URL I edited into by previous post.
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080002287_2008000948.pdf

Mg + CO2 = MgO + CO
Mg + CO = MgO + C
Both rockets and jet engines can use this reaction.  A rocket would use liquid CO2, a jet engine gas CO2 directly from the atmosphere.

Edit added second burn equation.

Offline Kaputnik

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Re: Compact nuclear power sources for space exploration
« Reply #36 on: 03/10/2008 08:47 am »
Getting back on topic...

I've been looking around for more numbers and facts around nuclear power sources, and was interested to see some information on RTGs on WIkipedia. I hope somebody caa verify if this is correct.
From the figures, it seems as though the best power/mass ratio achieved with any RTG flown to date was the MHW-RTG used for Voyager, at 10w/kg. This is very surprising, because the same table included the proposed SRG and the MMRTG.

10w/kg doesn't sound like much, but scale it up to say 25Kwe and you're talking 2.5t for a super-reliable, long-lived, constant heat and power source with no shielding worries. Pu238 is probably very expnsive, of course, but on paper it sounds like a viable option.
"I don't care what anything was DESIGNED to do, I care about what it CAN do"- Gene Kranz

Offline meiza

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Re: Compact nuclear power sources for space exploration
« Reply #37 on: 03/10/2008 01:01 pm »
The modern RTG:s try to minimize the nuclear material, but have more other hardware so total power to mass is lower.

Offline Kaputnik

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Re: Compact nuclear power sources for space exploration
« Reply #38 on: 03/10/2008 10:00 pm »
That makes sense- I've just been reading about the stock of Pu238 being down to the last few kgs. So obviously you'd want to minimise hte mass of fuel needed.

Assuming Pu238 supply was not the constraining factor, how practical would a large-scale RTG be? It would probably be worth using a Stirling cycle at the sort of scale I'm thinking of (25Kwe or more).
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Offline madscientist197

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Re: Compact nuclear power sources for space exploration
« Reply #39 on: 03/12/2008 10:49 am »
Assuming you could get enough material, a giant sterling engine RTG would be a lot safer (less gamma rays, neutrons etc.) than a nuclear reactor for lunar and mars bases.
John

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Re: Compact nuclear power sources for space exploration
« Reply #40 on: 03/12/2008 08:24 pm »
Quote
Patchouli - 8/3/2008  9:21 PM

A mars lander would have a much larger mass fraction then a lunar lander because you have an atmosphere to help you slow down.
Of course the bigger you make these things the more mass efficient they get due to mathematics of scaling.

We only tried I think one design of reentry vehicle so far on mars the viking system so there is a lot of room for improvement such as biconic and lifting body.
Something lifting or even winged might have a much better mass fraction esp since it can be long and relatively narrow such as with DCY and the shuttle though no matter what you reenter with that finial descent will have to be under rocket power.

I'm sure it's impractical given that the Martian atmospheric pressure is about equal to that at 100,000 feet here on Earth, but the image of a quarter-mile-wide para-sail deployed above a descending lander paints quite a picture in my mind.  A final flair right before landing would reduce the final impact like a parachuter landing in the stadium at half-time.  Quite a sight!   :cool:
"Do what you can, with what you have, where you are."  -T. Roosevelt

Offline Kaputnik

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Re: Compact nuclear power sources for space exploration
« Reply #41 on: 03/19/2008 12:10 pm »
SRGs have got me quite interested now. Cheaper to develop than reactors, safer, easier to package with no shielding worries.
The big problem with them is getting hold of Pu238. I've heard that the manufacturing of this fuel has been stopped. Is it possible to re-start this, and is the stuff inherently very expensive? How much would we be talking about per kg, assuming quite large scale production?
"I don't care what anything was DESIGNED to do, I care about what it CAN do"- Gene Kranz

Offline Gene DiGennaro

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Re: Compact nuclear power sources for space exploration
« Reply #42 on: 03/19/2008 04:09 pm »
When I worked on RTG projects for Teledyne Energy Systems back in 1998, we were told then that Pu-238 was in short supply. I believe we were told that NASA could buy the stuff from Russia back then. I bet that has changed now. When I left there in 2000, they were developing an RSG. I don't know what became of the project.  Why couldn't something like the ill-fated SP-100 or Topaz reactors be resurrected for use?

Offline khallow

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Re: Compact nuclear power sources for space exploration
« Reply #43 on: 03/19/2008 04:26 pm »
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Kaputnik - 19/3/2008  6:10 AM

SRGs have got me quite interested now. Cheaper to develop than reactors, safer, easier to package with no shielding worries.
The big problem with them is getting hold of Pu238. I've heard that the manufacturing of this fuel has been stopped. Is it possible to re-start this, and is the stuff inherently very expensive? How much would we be talking about per kg, assuming quite large scale production?

Unfortunately, I come bearing criticism. While this is a very acronym friendly environment, I think we need to explain new acronyms like SRG (Stirling radioisotope generator). But I also bring good news. With the power of Google to augment my mediocre facilities, I found this environmental statement that briefly discusses some of the options for producing plutonium 238. What's interesting is that it dates from before 2007 and even cites a plan for establishing production of plutonium 238 dating from January 2001!

The method of production above seems to be to take neptunium 237, radiate it with protons, and extract the resulting plutonium 238. In turn. neptunium 237 appears to be a common byproduct of nuclear fissioning. I read here that out of 25 tons of spent nuclear fuel, the annual waste from a 1 GW fission plant, about 10 kg are neptunium 237. So plenty of stock to get plutonium 238 from.

Given that, it puzzles me why news stories came out about the supposed plutonium 238 shortage in recent weeks. Just implement one of these long standing plans and you can have up to (for the prefered "consolidation alternative") 5 kg of plutonium 238 per year. That seems to be adequate for the next decade or two of US needs for space probes at current usage rates. But if we're going for a more ambitious approach, we may need far greater production of plutonium 238. I gather that won't be much of a problem unless we need hundreds of kilograms or more of plutonium 238 per year.

Finally, the stirling engine concept looks pretty good. The design being proposed has been run constantly for several years (which is good enough for a lot of applications). That's long enough as I understand it to see minor radiation degradation in thermoelectric couples in regular RTGs. The worst problem I see is that it is a source of vibration. Even if the stirlings should work flawlessly for decades (or centuries as the case may be), we still have to worry about whether the rest of the vehicle can handle the resulting vibration. I'll need to know more about the vibration environments of existing deep space probes to determine whether this is really a problem.
Karl Hallowell

Offline iamlucky13

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Re: Compact nuclear power sources for space exploration
« Reply #44 on: 03/19/2008 04:46 pm »
It comes down to money. There just isn't enough demand at the moment to get the funding to setup the production chain. As the pdf you linked to notes, there is still currently a supply available from the milliwatt RTG's in nuclear warheads being dismantled.

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Re: Compact nuclear power sources for space exploration
« Reply #45 on: 03/19/2008 04:56 pm »
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khallow - 19/3/2008  12:26 PM

The method of production above seems to be to take neptunium 237, radiate it with protons, and extract the resulting plutonium 238. In turn. neptunium 237 appears to be a common byproduct of nuclear fissioning. I read here that out of 25 tons of spent nuclear fuel, the annual waste from a 1 GW fission plant, about 10 kg are neptunium 237. So plenty of stock to get plutonium 238 from.

You irradiate the neptunium with neutrons, not protons, to get Pu-238.  Also, Np is NOT being extracted from spent nuclear fuel due to the US ban on reprocessing dating back to the 1970s.  There is Pu-238 formed in conventional reactors, but it is mixed with Pu-239, Pu-240, and Pu-241 and is inseparable and worthless.

The Np supply in the US, which is fairly small, has to be irradiated in dedicated reactors to form enough Pu-238 to make the separation worth the trouble.  The problem is very much still there.

Offline TyMoore

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RE: Compact nuclear power sources for space exploration
« Reply #46 on: 03/19/2008 05:46 pm »
Pu-238 is fine as an isotope heat source for RTG's but it is far easier and cheaper (safer too) to use ordinary fissile fuels to create a fission reactor for a nuclear power source. A fission reactor can be turned on and off, even throttled a bit for power production--while the isotope heat source cannot. Once formed, an isotope heat source is driven by the rate of decay and nothing else.

The Topaz reactors used liquid lithium-potassium as a coolant--and transfers the heat to thermionic/thermoelectric modules nearby--in this instance, Topaz cannot do better then 5% or so efficiency.

There have been many studies of Potassium Rankine turbines--which uses the heat of the reactor core to boil potassium into a vapor and run the resultant vapor through a turbine. I think 20% efficiency can be readily achieved. Something like that with a Topaz derivative reactor ought to be able to provide 20-25KWe in a relatively small package--with plenty of heat left over to provide for water distillation for waste treatment...

Personally I think that if one were to 'think bigger' and look at something like the Pratt and Whitney "TRITON" motor--then a compact Tungsten-Cermet core could be made to easily handle a high pressure helium Brayton-Cycle gas-turbine loop that could produce Megawatts of electricity in a dedicated power generation configuration. The single largest component would be the radiator--which if it were built from lunar (or Martian) derived materials would substantially decrease the required material to be transported.

Megawatts of power would enable a substantial base to be grown into a colony--ISRU would no longer be a problem...


Offline khallow

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Re: Compact nuclear power sources for space exploration
« Reply #47 on: 03/19/2008 07:37 pm »
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vanilla - 19/3/2008  10:56 AM

Quote
khallow - 19/3/2008  12:26 PM

The method of production above seems to be to take neptunium 237, radiate it with protons, and extract the resulting plutonium 238. In turn. neptunium 237 appears to be a common byproduct of nuclear fissioning. I read here that out of 25 tons of spent nuclear fuel, the annual waste from a 1 GW fission plant, about 10 kg are neptunium 237. So plenty of stock to get plutonium 238 from.

You irradiate the neptunium with neutrons, not protons, to get Pu-238.  Also, Np is NOT being extracted from spent nuclear fuel due to the US ban on reprocessing dating back to the 1970s.  There is Pu-238 formed in conventional reactors, but it is mixed with Pu-239, Pu-240, and Pu-241 and is inseparable and worthless.

The Np supply in the US, which is fairly small, has to be irradiated in dedicated reactors to form enough Pu-238 to make the separation worth the trouble.  The problem is very much still there.

Thank you, I just looked at the difference between the isotopes and assumed that it had to be via proton capture.
Karl Hallowell

Offline Kaputnik

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RE: Compact nuclear power sources for space exploration
« Reply #48 on: 03/21/2008 09:46 am »
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TyMoore - 19/3/2008  7:46 PM
Pu-238 is fine as an isotope heat source for RTG's but it is far easier and cheaper (safer too) to use ordinary fissile fuels to create a fission reactor for a nuclear power source. A fission reactor can be turned on and off, even throttled a bit for power production--while the isotope heat source cannot. Once formed, an isotope heat source is driven by the rate of decay and nothing else.

I don't know that I understand what you're saying here. An RTG is a nuclear power source. I haven't found out yet what the costs of Pu238 might be if production were restarted, but I would argue that certainly in terms of development cost RTGs/RGSs win against reactors. Secondly, I would also think that they are much safer- they cannot go critical, they can survive re-entry intact, and there are virtually no shielding issues.
I'm sure it all depends on the power requirements, of course- trying to get more than a few kW from an SRG will probably become impractical, heavy, and expensive.
"I don't care what anything was DESIGNED to do, I care about what it CAN do"- Gene Kranz

Offline TyMoore

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RE: Compact nuclear power sources for space exploration
« Reply #49 on: 03/21/2008 08:37 pm »

Actually, all isotopes of plutonium are fissile--even Pu-238. So a large RTG could actually go critical--if it were poorly designed! The reason why Pu-238 is not used at all for weapons is that a 10-15 kg core would produce 10 KW or so of heat--and it would literally be glowing red hot! The weapon core would eventually melt the weapon it was installed in!

As far as safety--in terms of transportation of nuclear materials (i.e., launching them into space) then a compact nuclear reactor can be 'safed' or even 'inerted' by incorporating nuclear 'poison' rods which must be removed before the reactor can be 'switched on.' A couple of cadmium rods in a small reactor core will keep the thing from going critical under any circumstance...

Also, look at the number of curies of radioactive materials being transported--because of Pu-238's short half-life of about 87.7 years, versus U-235's 700 million years, an RTG carrying 25 kg will have the same bulk radioactivity as around 211,000 tons of U-235 before the reactor is activated. Once the reactor activates, then fission products will begin to accumulate, of course...

And control is a real issue too--again, a fission reactor can be throttled, shut down, and restarted. An isotope heater cannot--once the isotopes a created and fabricated into heater units, their rate of heat production is set by the decay rate, period. For small devices like RTG's for deepspace missions where the total Pu-238 loading is relatively low (a few tens of kilograms) the total heat output is still only a few kilowatts--manageable for launch platform cooling systems. Once you start thinking in terms of hundreds of kilowatts of heat--then launch cooling is going to be a big problem. A reactor on the other hand will generate essentially no heat until the moment it is activated--and by then a mission can already be well on its way.



Cost of Pu-238...not really sure. The US has about 9kg in inventory with plans to purchase an additional 1kg from Russia. So just a W.A.G: I'd say it is probably more than 10 times the cost of Gold on a weight basis. That should be in the 'ball park.'

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