Energy From Thorium Discussion Forum

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PostPosted: Nov 29, 2014 8:43 am 
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I am talking about the Stirling Generator project for Plutonium powered generators, which presumably proceeded independantly of any terrestrial projects. (For one thing I doubt any terrestrial projects really need 30 year mission lives with no maintenance)


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PostPosted: Dec 03, 2014 3:45 pm 
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Modern high-reliability Stirling generators are not like little lawnmower engines with a piston rod and crankshaft.
They use elastic suspension, usually flat or convoluted springs, and a linear AC generator with no brushes.
The gases touch the displacer and piston. The magnetics extract the energy.
The displacer and piston resonate out of phase, sometimes at audio frequencies, sometimes with electronics on the generator(s) arranging the phase relationship.
Also, the seals are non-contact, (usually precision-machined though). A Stirling displacer and piston don't need to seal, especially if the seal-to-piston area ratio is large.
So these don't touch the cylinder walls.
I believe there are no lubricants, because no solid moving parts touch.

It's about as close to a no-moving-part system as a true heat engine could get.

Except for the seals, and maybe the generator coils, the parts could be stamped out of sheet metal.
A number of businesses have attempted to mass-produce these, by the way, and I think machining the seals was the production issue.
Another issue is that the most efficient working fluid is hydrogen, and if you ridiculously prioritize fire-safety, helium. Both are notorious escape artists.
My favorite home application was the cogenerator for gas water heaters.
Those engineers had figured out how to make the engine resonate at AC mains frequencies, so the thing just pumped power into the mains.
When it was on, of course.


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PostPosted: Feb 26, 2015 7:35 pm 
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The $50 million/year pays for more than just the work at Oak Ridge. It also pays for work at Los Alamos and at the Idaho National Laboratory. There are chemical cycles, irradiation work, and fabrication work segments spread across the labs.

Readers of this forum might be interested to know that NASA also cancelled exactly $50 million/year in funding for the Advanced Stirling Radioisotope Generator (ASRG) which was being worked on at NASA Glenn. The program was on track to produce two units ready for flight by 2017 using far less PU-238 and providing far more power for the weight.

In 2013 NASA Glenn also won an R&D 100 award for a uranium powered RTG design that could provide 350 watts or power. Two of them would give a deep space probe much wider bandwidth for radio transmission of data and power a lot more instruments.

DOE forced NASA to make a tough decision to keep the lab facilities open to provide PU 238 for conventional RTGs.

I'm enclosing a technical fact sheet on the ASRG for anyone who is interested.

NB: I was the deputy lead for project planning & controls for the ASRG program at NASA Glenn (as a contractor) 2013-2014. I don't work there anymore so I am speaking solely for myself here.


Attachments:
File comment: ASRG Fact Sheet - NASA Glenn 2013
ASRGfacts2_10rev3_21.pdf [627.36 KiB]
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Last edited by djysrv on Feb 26, 2015 7:49 pm, edited 1 time in total.
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PostPosted: Feb 26, 2015 7:40 pm 
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Welcome back Dan!

djysrv wrote:
In 2013 NASA Glenn also won an R&D 100 award for a uranium powered RTG design that could provide 350 watts or power. Two of them would give a deep space probe much wider bandwidth for radio transmission of data and power a lot more instruments.


Are you sure this wasn't a reactor? I can't think of any uranium isotope other than U-232 that could put out enough power to work as an RTG.


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PostPosted: Feb 26, 2015 7:57 pm 
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The conventional RTG works by harnessing decay heat and getting power from a thermocouple.

The ASRG works by a piston moving in a helium gas environment which exploits temperature differences between the cold of outer space and the decay heat from the PU 238. The tolerances between the piston and the piston wall are about 10 microns. The actual assembly isn't much larger than a large cuban cigar. You can hold the whole piston and its housing in one hand. The flight qualification criteria is that the system has to be able to run for 14 years trouble free.

Given the limits on propulsion from chemical rockets used to escape earth's gravity, a payload will take a long time to reach a deep space destination. For instance, the New Horizons Pluto mission, which will arrive on station in July 2015, was launched in 2006 http://www.nasa.gov/sites/default/files/files/NHMissionFS082114HiPrint.pdf It used Jupiter for a gravity boost.

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PostPosted: Feb 26, 2015 8:01 pm 
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The uranium design is a sodium cooled reactor. It was fueled by HEU, e.g., fuel enriched to 80%+ U235.

From the NASA press release . . .

"Another winning team co-developed KiloPower, a system for making electricity from uranium by nuclear fission in space. Uranium is more plentiful in the U.S. than the plutonium used by current systems. The equipment is also smaller than current equipment and produces more power — 500 to 1,500 watts over 15 to 30 years. The improvements will help flights too far from the sun to rely on it for power."

http://www.cleveland.com/brook-park/index.ssf/2013/11/two_nasa_glenn_innovations_mak.html

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PostPosted: May 18, 2016 7:52 am 
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Full-Scale Production of Plutonium Spacecraft Fuel Still Years Away

Quote:
"What we're shooting for is to get to an interim production level of around 400 to 500 grams [14 to 18 ounces] per year in 2019, and then full-scale, a kilogram and a half [3.3 lbs.] — if everything goes right — in 2023," Bob Wham, the Pu-238 project lead in the Nuclear Security and Isotope Technology division at Oak Ridge, said last month during a presentation with NASA's Future In-Space Operations (FISO) working group.


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PostPosted: May 19, 2016 8:19 am 
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Kirk,
How much 238-Pu might you expect from a 1GW LFTR? Assuming 92% fission efficiency for 233-U and 85% for 235-U, you get 12 kg of 236-U per ton of 232-Th. Most of the 236-U eventually becomes 238-Pu, but how long does that take? 236-U's cross-section for thermal neutron capture is about 5 barns. Compared to ~500 barns for 233/235 fission, that's quite a bottleneck. It might take centuries to come to equilibrium and realize that 12 kg.

The IAEA reports the resonance integral for 236-U at 345 barns. How would you incorporate that into your estimate?


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