-
#180
by
jnc
on 13 Aug, 2012 01:48
-
Pu238 decay produces almost exclusively alpha particles ... A small fraction of the decays produce neutrons from spontaneous fission events, and there are some gamma rays produced further down the decay chain. Both neutrons and gamma are not so easily shielded against but the intensity is fairly low.
The first decay product is U-234, which has a half-life of like 250K years, so the radiation levels from its decay (via whatever path), and its descendants in turn, will be pretty low.
Noel
-
#181
by
randomly
on 13 Aug, 2012 02:45
-
The first four decays in the chain are alpha decays, U234->Th230->Ra226->Rn222, then some beta decays start showing up. However a very small percentage of decays are spontaneous fissions which throw out neutrons. Also there are other radioactive isotope impurities in the PU238 as well which contribute beta and gamma decays. All in all Pu238 is by far the best isotope for RTGs from a standpoint of half-life, decay energy, and minimum shielding requirements. I hope Congress stops giving lip service to starting up Pu238 production and actually funds it for once.
-
#182
by
jnc
on 13 Aug, 2012 03:13
-
The first four decays in the chain are alpha decays, U234->Th230->Ra226->Rn222 ... However a very small percentage of decays are spontaneous fissions which throw out neutrons.
Right, but my point (which I guess I didn't make clearly enough) was that the 250K-year half life of U-234 applies to all the decay modes, including the spontaneous fission one.
(I.e., to belabour the point to death, if x% of U-234 decays are via the fission mode instead of the alpha mode, the half-life of that x% fraction of the U-234 is also 250K-years.)
And with that long a half-life (about 3K times as long as that of the Pu-238), the radiation levels from the decay of U-234 will be 'pretty low' (1/3K that of the Pu-238, for all decay modes together, so the fission one will be much smaller than that).
I hope Congress stops giving lip service to starting up Pu238 production and actually funds it for once.
Amen to that!
Although I read somewhere that NASA still has enough for a decade or so, at the moment, though...
Noel
-
#183
by
randomly
on 13 Aug, 2012 06:57
-
I was actually referring to the spontaneous fissions of the Pu238. The fraction of decays that are spontaneous fission is very small, but with something like 2 million billion decays a second in the MMRTG it doesn't take much.
Pu238 is just wonderful stuff, 1/2 watt per gram, 88 yr half-life, incredibly easy to shield.
Now with MSL launched there is something like 16.8 kg of Pu238 left, which is only about half what the Cassini probe used. I believe under the current Pu238 supply issues no probe or spacecraft proposal launching past 2018 will be accepted if it requires Pu238. The ASRG can extend the limited supply of Pu238, but with moving parts it's going to be considered risky for very long duration missions. The Pu238 shortage is definitely crimping future missions from here on out.
-
#184
by
rds100
on 13 Aug, 2012 07:04
-
Is Pu238 the same stuff that is used in nuclear bombs? If yes, then there should be nearly unlimited supply of Pu238, by disassembling and recycling old bombs.
-
#185
by
QuantumG
on 13 Aug, 2012 07:18
-
Is Pu238 the same stuff that is used in nuclear bombs? If yes, then there should be nearly unlimited supply of Pu238, by disassembling and recycling old bombs.
No. That's 239 or 241.
-
#186
by
randomly
on 13 Aug, 2012 07:25
-
No. bombs use Pu239. not the same at all. Weapons grade plutonium is 93%-98% Pu239, most of the rest is Pu240, and a few tenths of a percent of Pu241. Although Pu238 has a bare critical mass of around 15kg it's essentially impossible to make a bomb with it because of the enormous amount of heat being generated. It would just melt from it's own decay heat. Pu238 is actually one of the most effective ways of denaturing Plutonium to render it unusable for weapons.
Also you want relatively pure Pu238 to keep radiation other than Alpha decays to a minimum. Pu238 is made by putting Neptunium 237 targets inside a reactor and bombarding them with neutrons. It used to be relatively 'cheap' because it could be made in all the bomb grade plutonium production reactors used for making weapons material in the US and Soviet Union. But after the end of the cold war those reactor facilities were shut down.
-
#187
by
rds100
on 13 Aug, 2012 07:33
-
Well, if it is so easy to produce and there is a good market demand i am sure someone (the Chinese) will start producing it and selling it. It's another question if the US would want to buy it from the Chinese
-
#188
by
randomly
on 13 Aug, 2012 07:42
-
The Chinese are pursuing nuclear power and nuclear research very aggressively while it stagnates in the west. It wouldn't surprise me if the US is buying reactors from China in 10-20 years. I expect they very much will set up Pu238 production at some point. I have no idea if they would be willing to sell it though.
-
#189
by
jnc
on 13 Aug, 2012 14:32
-
I was actually referring to the spontaneous fissions of the Pu238.
Ah, got it now. Sorry!
there is something like 16.8 kg of Pu238 left
All busily decaying with a half-life of 88 years!! Athough I guess that's long enough that not that much will have decayed before it is 'used' in RTG's.
The Pu238 shortage is definitely crimping future missions from here on out.
Yeah, wasn't thinking about the planning impact - without a
guaranteed supply, one can't really rely on it when planning, I can see.
Noel
-
#190
by
Bogeyman
on 14 Aug, 2012 17:45
-
How does the Rover Break? What keeps it from rolling away on a slope?
-
#191
by
Blackstar
on 14 Aug, 2012 18:03
-
-
#192
by
savuporo
on 14 Aug, 2012 22:23
-
-
#193
by
randomly
on 15 Aug, 2012 02:56
-
Thanks Blackstar and Savuporo. Wonderful resources
-
#194
by
Bogeyman
on 15 Aug, 2012 17:53
-
Repost:
How does the Rover Break? What keeps it from rolling away on a slope?
-
#195
by
Kaputnik
on 15 Aug, 2012 18:46
-
Not an expert, but I believe the gearing coupled with the resistance of the electric motor itself is ample friction to act as a brake. Sojourner used a 2000:1 gearing system and whilst I don't know what is used on Curiosity, it is evolved from the same basic system so is probably also a very high ratio.
Interestingly, Curiosity has a means of disengaging the drive system to allow the wheel to rotate freely. This means that if it suffers a failed wheel, like Spirit, it won't have to drag it along behind it. This bodes well for a long and productive roving mission.
-
#196
by
Bogeyman
on 15 Aug, 2012 19:40
-
Thank you!!
-
#197
by
savuporo
on 15 Aug, 2012 22:33
-
Not an expert, but I believe the gearing coupled with the resistance of the electric motor itself is ample friction to act as a brake.
Even if it weren't any regular H-bridge driving the motor allows "shorting" of the leads which puts a hard brake on the motors.
-
#198
by
Plopper
on 16 Aug, 2012 09:14
-
Q1: What kind of multitasking is the rover capable of?
For example, while the SAM is analyzing a sample (for up to 3 hours), can the arm move away and let the APXS make an analysis on the ground while at the same time a new weather reading is made? Can SAM work while it's driving? Or is it strictly one thing at a time.
Can its Mastcams film (10 fps/s, 3D, I read) the arm while it is moving?
I can imagine that the electric output is a restraint, but also that they are very careful.
Q2: What is the energy consumption of different operations?
What is the rank of these operations in terms of energy consumption:
-Driving X feet.
-The complete operation of the arm to take a drilled sample.
-Radio transmissions.
-The scientific instruments performing each one full analysis.
Does any one type of operation stand out as a real energy hog?
As for the computer discussion here recently, I guess that they have no need for more computational power. And also that they need to save energy, they might for example have preferred a 133 MHz over a 200 for that reason.
-
#199
by
Plopper
on 18 Aug, 2012 12:02
-
What is that concave feature on the ridge of mount Sharp (or on the mound), which I've circled?
The enclosed image cut comes from the panorama here:
http://www.planetary.org/blogs/emily-lakdawalla/2012/08171906.htmlAlso, I wonder if there is any panorama around with a bar at the bottom which indicates the direction in degrees in which one is viewing? I'd like to pin point where in the panorama above, for example, Glenelg is located. I would now guess that it's slightly to the left of where the shadow of the mastcam points.
