Energy From Thorium Discussion Forum

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PostPosted: Nov 08, 2011 11:43 pm 
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The Voyagers are coming up on 35 years since launch. Lack of power is expected to shut them down in another 10 years or so. About half of the power loss is a result depletion of 238Pu. On the other hand, the extra weight of 241Am (60 kg instead of 15 kg) would have had to have been made up elsewhere.


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PostPosted: Nov 09, 2011 2:09 am 
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The 238Pu should be at around 75% power at 45 years aging. Still much higher power per mass than Am241.

Lars


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PostPosted: Nov 09, 2011 4:00 am 
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Sr90 could be a cheaper substitute for shorter durations. China may come up as the next supplier of Pu-238, like everything else.


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PostPosted: Nov 09, 2011 7:43 am 
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Strontium-90 does look like a pretty good RTG. Beta minus decay only, so no gammas. Strontium-90 does decay into yttrium-90 which is a short lived very hard beta minus emitter, that determines the thicker shielding required (on the plus side this does make more heat). It is going to be quite having for a space probe, but pretty good for remote terrestrial and marine applications.

How much shielding would be needed if something effective such as tungsten is used as shielding material? Remember there are no gammas, just a really hard beta emitter (Y-90).


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PostPosted: Nov 09, 2011 9:31 am 
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Cyril R wrote:
How much shielding would be needed if something effective such as tungsten is used as shielding material? Remember there are no gammas, just a really hard beta emitter (Y-90).

The betas cause Bremsstrahlung (in the form of X-rays) when they hit high-Z atoms: http://en.wikipedia.org/wiki/Bremsstrahlung
Quote:
the bremsstrahlung produced by shielding the beta radiation with the normally used dense materials (e.g. lead) is itself dangerous; in such cases, shielding must be accomplished with low density materials, e.g. Plexiglass (lucite), plastic, wood, or water;[10] because the rate of deceleration of the electron is slower, the radiation given off has a longer wavelength and is therefore less penetrating


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PostPosted: Nov 09, 2011 3:04 pm 
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Thanks Jaro, so that explains why they're using such lower Z materials for beta ray shielding. I was wondering about that. I learn something new every day!

I guess for a radiothermal generator with beta, you'd want the aluminium shielding for high thermal conductivity. What would that weigh, for a Sr-90 source, compared to a Pu238 source?


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PostPosted: Nov 11, 2011 1:57 am 
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Boron Carbide among solids and a solution of boron salts as liquid may be useful.


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PostPosted: Nov 14, 2014 2:15 pm 
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The spatial probe lander Philae has landed on the Comet 67P/Churyumov–Gerasimenko. That's a great achievement but it seems that the robot has some problems with lack of energy. Unfortunately it landed in a position where it receives only 1.5 hour of sun every 12 hours and it needs much more than that. They will try to make the robot move to have a better sunshine. Let us hope they will succeed. Otherwise the robot will hibernate and it may be able to "wake up" when the comet will be much closer to the sun.

With a Radioisotope Thermoelectric Generator (RTG) they wouldn't have had this problem. I guess that maybe the robot was too small ( weight : 100 kg), they didn't have enough weight to put a RTG. Or it was too costly, or they had enough confidence that the solar option would work whereas a RTG would have been more complicated to put in place.

Anyway I hope that it was not for political or anti nuclear reasons.


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PostPosted: Nov 18, 2014 3:52 pm 
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fab wrote:
With a Radioisotope Thermoelectric Generator (RTG) they wouldn't have had this problem. I guess that maybe the robot was too small ( weight : 100 kg), they didn't have enough weight to put a RTG. Or it was too costly, or they had enough confidence that the solar option would work whereas a RTG would have been more complicated to put in place.

Anyway I hope that it was not for political or anti nuclear reasons.

Yup, likely due to mass concerns. I recently attended a talk by the manager for NASA Ames' Small Sat Integrated Product Team, Bruce Yost. I asked him after that talk if they're developing RTGs for cubesat scale missions since NASA has plans to use them beyond Jupiter. He confirmed that they are developing them with the work mostly spearheaded by NASA Glenn, I think. There's some discussion starting on page 23 of NASA's 2014 Small Spacecraft Technology State of the Art report. They're looking at "beta-voltaic, alpha-voltaic, thermophotovoltaic, piezoelectric, and mechanical conversions" methods of power generation.

As a depressing side note: he declined to mention this effort in the list of current R&D during his talk because of the use of the "N-word" as he put it. And this was in a room predominately filled with space exploration enthusiasts, most of whom recognize the value of nuclear power, in space the least. Now I wish I had asked him my RTG cubesat question more publicly, during the presentation.


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PostPosted: Nov 18, 2014 11:54 pm 
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you see the problem solar power is having during the comment landing that just happened. It bounced into the shadows and went dead. A nuclear powered craft would be sending data by now.


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PostPosted: Nov 19, 2014 12:04 pm 
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Philae had an average power demand of roughly 15W (32W solar panel would lead to that conclusion) - assuming you keep the battery pack to allow for load transients.
That would require roughly 430g of 238Pu - and a total assembly weight measuring three-four kilogrammes, assuming you stick with an RTG instead of a SRG.
That assumes that an RTG would work in an environment quite different from floating in deep space (you might have heat radiation issues if you aren't careful, a long boom to get it away would unbalance the spacecraft during descent and landing).

3-4kg is a significant amount when your entire probe weighs only 100kg and the power system in total weighed about 12kg.

Also the mission is considered a success as the probe functioned properly for the first sixty hours - that was all that was required to return the data required to justify the initial expenditure on the mission. The solar panels were, I understand, a light thin film type added as an attempt to extend the mission at minimal weight cost.


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PostPosted: Nov 19, 2014 2:37 pm 
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I like solar panels for space probes, its one of those niche applications where solar does compete well with alternatives for most missions.

But what really strikes me is that a civilisation capable and bold enough to land a probe on a comet can't keep its panties dry on nuclear power.


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PostPosted: Nov 19, 2014 6:48 pm 
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Cyril R wrote:
But what really strikes me is that a civilisation capable and bold enough to land a probe on a comet can't keep its panties dry on nuclear power.


So true. At a recent family gathering I got into a conversation with one of my aunts. When talking about what I do I mentioned that I sit next to a bunch of computer equipment. She asked me if I was concerned about all the radiation the electronics give off. She'd rather eat cold food than use a microwave oven and, if I understood correctly, only uses a cell phone with a speaker (she won't put it to her head). I explained that the sun shining through the window puts out more radiation than the electronics that I share a room with but I doubt it changed her mind.

I like solar power for things like space probes. Not a fan of solar power on the utility grid though.

One thing I've been pondering is the practicality of radio-thermal power for terrestrial use. RTGs in space is a great idea, but why not also here on Earth? I know that cost is an obvious concern, safety is another. Seems to me that while radioisotopes are not children's toys we deal with things of equal hazards all the time, like lead, mercury, high voltages, great heights, powerful machines, and so on. We treat them with due care so we don't end up dead.

I don't want to derail the thread with my pondering of terrestrial uses of RTGs. I just thought I'd share that thought for others to ponder on their own. I know that public perception of such devices would prevent their use even if Pu238 were free and unlimited. I'm just dreaming of a possible world where I'd never have to charge a battery or fill another fuel tank, all because of nuclear power.

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Disclaimer: I am an engineer but not a nuclear engineer, mechanical engineer, chemical engineer, or industrial engineer. My education included electrical, computer, and software engineering.


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PostPosted: Nov 19, 2014 8:36 pm 
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238Pu is absurdly expensive.

Most terrestrial RTGs/SRGs would use 90Sr because its cheap and the shorter half life is not really a problem.


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PostPosted: Nov 20, 2014 2:48 am 
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E Ireland wrote:
238Pu is absurdly expensive.


Space grade photovoltaic panels are also absurdly expensive. Question I have is, how much would the relative prices between the two have to change to where it makes economic sense to use RTGs instead of solar panels? Other than the political and economic obstacles what else would prevent more widespread use of RTGs? Weight was mentioned, are there other challenges?

What practical means are there to produce or mine radioisotopes for RTGs other than a fission reactors? I see that tritium has been used in RTGs, and the isotope can be extracted from seawater. I realize it's not as ideal of a heat source as Pu238 but the lack of a need for a reactor to get it has to be worth something.

Just how bad would it be to use naturally occurring radioisotopes for RTGs? Seems obvious to me that uranium and thorium would be bad ideas or they'd be using them. Question is how many orders of magnitude bad?

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