Good news. U.S resuming Pu238 production for space missions.

[AJ Dual]

Presumably, birdman already knew.

The ASRG is mentioned too.

http://news.discovery.com/space/nasa-plutonium-production-space-fuel-130314.htm

[RocketMan]

Will they have enough to make another Illudium Q-36 Space Modulator?  If not, what's the point?

[AJ Dual]

I wasn't to know what's under the ice of Europa.

[Gewehr98]

All these worlds are yours, except Europa. Attempt no landings there.

[Cliffh]

This month's PopMech has a couple of articles of some interest; one on space law regarding the claiming & mining of asteroids and the other on "How to build a starship" - 2,500' long, for a 90 year trip to Proxima Centari leaving in 2112.

[Sergeant Bob]

All these worlds are yours, except Europa. Attempt no landings there.

I see what you did there!

[Devonai]

While this might seem like good news, it really overshadows NASA's continued failure to derive free, limitless Zero Point Energy from an extradimensional source using easily generated yocto-wormholes.

[AJ Dual]

All these worlds are yours, except Europa. Attempt no landings there.

We'll have to query the monolith/Dave/HAL and ask if high velocity penetrometers count as "landing"...

[kgbsquirrel]

Didn't we have a discussion regarding the Thorium fuel cycle being a better producer of Pu238? Don't quite remember and no time to search as I'm about to step out.


And the obligatory cultural touch-stone quote...

"My God, it's full of stars!"

[Tallpine]

Yeah, where is my atomic powered space ship that Heinlein promised 

[RevDisk]

All these worlds are yours, except Europa. Attempt no landings there.

Just makes you want to hit it with a chunk of iron the size of a school bus at a significant fraction of C.

[birdman]

Didn't we have a discussion regarding the Thorium fuel cycle being a better producer of Pu238? Don't quite remember and no time to search as I'm about to step out.


And the obligatory cultural touch-stone quote...

"My God, it's full of stars!"

No, thorium makes it effectively impossible to make pu-238.
Basically, Pu-238 is created by neutron absorption by Np237, which is created by neutron absorption by U236, which is usually the result of neutron absorption BUT NOT FISSION of U235 (which is rare). And by a few other paths, but all of them low probability (including alpha decay of Cm-242)

(See image below from Wikipedia)

In a thorium reactor, the main things present are Th-232, U-233, U-232 and such, meaning it would require many successive absorption events to get to Pu-238.

[kgbsquirrel]

No, thorium makes it effectively impossible to make pu-238.
Basically, Pu-238 is created by neutron absorption by Np237, which is created by neutron absorption by U236, which is usually the result of neutron absorption BUT NOT FISSION of U235 (which is rare). And by a few other paths, but all of them low probability (including alpha decay of Cm-242)

(See image below from Wikipedia)

In a thorium reactor, the main things present are Th-232, U-233, U-232 and such, meaning it would require many successive absorption events to get to Pu-238.

And that chart be saved....

So the dumb question, if we're flinging this stuff into space, ostensibly never to be seen again, why aren't we bothering with Strontium-90 instead? Thought that stuff was a lot easier to make.

[Regolith]

And that chart be saved....

So the dumb question, if we're flinging this stuff into space, ostensibly never to be seen again, why aren't we bothering with Strontium-90 instead? Thought that stuff was a lot easier to make.

From what I can tell, Strontium-90 would require heavier shielding to protect people working on the craft before it's launched. Pu-238 mostly emits alpha radiation, while Srt-90 produces beta and occasional gamma radiation. Pu-238 also has a longer half life and produces a LOT more decay energy: 5.593 MeV vs. 0.546 MeV for Sr-90.

[AJ Dual]

From what I can tell, Strontium-90 would require heavier shielding to protect people working on the craft before it's launched. Pu-238 mostly emits alpha radiation, while Srt-90 produces beta and occasional gamma radiation. Pu-238 also has a longer half life and produces a LOT more decay energy: 5.593 MeV vs. 0.546 MeV for Sr-90.

Yeah, I think it's more just the decay energy, the Watt's per pound. Pu has more. If it were better, I think the NASA types would put up with lots more precautions if it meant more power for instruments etc.

Just the whole brutal launch weight issue.

[kgbsquirrel]

Power generation is nice, half-life is about 25% though. Extra shielding for technicians can be removed just prior to launch, though it's gamma decay is considered "negligible". Only real question is would the extra lifting capacity for more isotope material be more or less expensive than the cost difference between producing Pu238 versus Sr90. On the flip side, if you designed the instrumentation of the craft to operate at the expected power output around say year 50, accounting for both decay of the isotope and the thermocouples, then that huge amount of extra power at the beginning could be diverted to something else. Say an ion thruster?

Pure Stronium-90 - 0.921 Watts/gram - Half-life 28.8 years
Pure Plutonium-238 - 0.560 Watts/gram - Half-life 87.7 years

Stronium Titanate - 0.536 Watts/gram
Plutonium(IV) Oxide - 0.540 Watts/gram


ETA: Oooo... Americium 241. Needs about 18mm of lead shielding and much lower thermal output, but it's half-life is absurd.

Pure Americium-241 - 0.140 Watts/gram - Half-life 432.6 years - Price: $1500/gram

It's the most common byproduct in nuclear waste. I was just imagining some 20 kilogram (not counting shielding) rods of Am241 for a personal RTG. Have it trickle charging a large battery bank, and replace the thermocouples every decade or so, and I'd have a personal home RTG that would last not just the rest of my life, but the lives of several generations of my progeny.

[Regolith]

Pure Americium-241 - 0.140 Watts/gram - Half-life 432.6 years - Price: $1500/gram

It's the most common byproduct in nuclear waste. I was just imagining some 20 kilogram (not counting shielding) rods of Am241 for a personal RTG. Have it trickle charging a large battery bank, and replace the thermocouples every decade or so, and I'd have a personal home RTG that would last not just the rest of my life, but the lives of several generations of my progeny.

One 20 kg rod would put out, what, 2.8 kW?

You'd need about 10-15 or so to run a house no problem. It'd only cost you ~$450,000,000 for the fuel, plus whatever it costs to put together. 

[Nick1911]

One 20 kg rod would put out, what, 2.8 kW?

You'd need about 10-15 or so to run a house no problem. It'd only cost you ~$450,000,000 for the fuel, plus whatever it costs to put together. 

Now figure it out for the thorium fuel cycle, at current market rates for thorium.

We're on earth.  We can use modulating reactors.  We can easily reject heat.  All the limitations of space that make RTGs appealing don't apply terrestrially.

[kgbsquirrel]

Birdman! You have a new assignment. Design us a self-regulating low-maintenance thorium fuel reactor that can power the average American house.

[birdman]

Birdman! You have a new assignment. Design us a self-regulating low-maintenance thorium fuel reactor that can power the average American neighborhood house.

Fixed, and already done.

A reactor capable of powering a home (a few kW) is capable of powering a neighborhood.  Once you have enough fuel together to form a critical mass, you can easily cool that mass enough to make a few hundred kW at a minimum,so that's kind of the "minimum" reactor size.

At LANL we designed one that was 10-20kWe and was about 2ft across, 5ft long including power conversion and shadow shield.

Of course, the biological shield for that would be 4-6ft thick.

[Tallpine]

Fixed, and already done.

A reactor capable of powering a home (a few kW) is capable of powering a neighborhood.  Once you have enough fuel together to form a critical mass, you can easily cool that mass enough to make a few hundred kW at a minimum,so that's kind of the "minimum" reactor size.

At LANL we designed one that was 10-20kWe and was about 2ft across, 5ft long including power conversion and shadow shield.

Of course, the biological shield for that would be 4-6ft thick.

So, roughly 12' x 12' x 15' ?   

[birdman]

So, roughly 12' x 12' x 15' ?   

Unless you want to die, yes.  Neutrons are a bitch to shield

[kgbsquirrel]

Unless you want to die, yes.  Neutrons are a bitch to shield

Couldn't I just put this thing in a concrete vault about 40 feet underground?

[birdman]

Couldn't I just put this thing in a concrete vault about 40 feet underground?

Well, yes, it's 4-6ft of just about anything.  As long as the casing stays under vacuum, and you cover it with a minimum of 4-6ft (10-20 would be better) of dirt/concrete, you would be okay.

The best one I did was for the interstellar precursor mission, it was a 300kWe output system weighing less than 1000kg using a radiation enhanced MHD cesium rankine cycle.  5-10yr life at full power.

That one was about the size of a 55gal drum (minus the radiators), so you could turn it down to 4-5kW and power your house for 300 years.
Of course, it would cost about a billion dollars.

[kgbsquirrel]

So here's the big question: Is it possible to build one of these things for less than $10k? (Not counting permits from the EPA and bribes to congresscritters)