

Making Plutonium-238 in large enough quantities is an ongoing challenge - jonbaer
http://www.popsci.com/nasa-can-make-3-more-nuclear-batteries-and-thats-it

======
curtis
I've been wondering if tritium could be used as an alternative power source
for the nuclear batteries used on space missions. Of course tritium has to be
"manufactured" just like plutonium-238, but on the other hand it can be made
from lithium using a fusion reactor [1].

But wait, you're thinking, fusion reactors based on current technology consume
more energy than they can produce. That's true, but also irrelevant if your
business model is producing tritium rather than energy. This is because the
going rate for tritium (according to Wikipedia) is something like $30,000 per
gram [2].

I'm not sure what the licensing requirements are on fusion reactors, but I
think they're a lot less onerous than licensing a new fission reactor. I don't
think there's any licensing requirement at all for a small fusion reactor like
a Farnsworth fusor.

Disclaimer: This is just idle speculation because I haven't done the math.

[1]
[http://en.wikipedia.org/wiki/Fusion_power#Power_Production](http://en.wikipedia.org/wiki/Fusion_power#Power_Production)

[2] [http://en.wikipedia.org/wiki/Tritium#Self-
powered_lighting](http://en.wikipedia.org/wiki/Tritium#Self-powered_lighting)

~~~
daeken
Nuclear 'batteries' are radioisotope thermoelectric generators (RTGs). They
require a pretty substantial amount of heat to produce electricity. I don't
know about temperatures, but tritium produces a decay energy of 0.018590 MeV,
versus plutonium-238's 5.593 MeV. Assuming you can scale it up directly, the
4.8kg of plutonium-238 in Curiosity would be equivalent to 1444kg of tritium.
It's just way, way less energy dense.

Edit: This explanation is way, way off. See response from 'throwaway_yy2Di

~~~
throwaway_yy2Di
There's two other differences you'd need to account for:

* One Pu-238 nucleus weighs 238 amu; a triton only weighs 3 amu

* Tritium decays 7 times faster -- a 12.32 year half-life, vs. 87.7 years for Pu-238

Put together, the specific power (watts/kg) of pure tritium is about twice as
high. That's chemically just hydrogen gas, which I'd guess isn't practical.
The solids with the highest hydrogen density, like polyethylene ([C2H4]n),
would be less power-dense, but not by much.

But I think there's some slack here. The ESA is designing their future RTGs to
use Americium-241, which is drastically worse (in terms of these figures), yet
still usable.

~~~
daeken
Ah hah -- great points, thanks! Edited my original post to note that my
analysis was woefully incomplete.

------
IndianAstronaut
We may have to reconsider some aspects of non-proliferation for the sake of
space exploration. Not just for nuclear batteries, but also for things like
Project Orion which tried nuclear propulsion.

~~~
mikeash
Orion is a really cool concept and it's a lot of fun to think about, but
realistically any space propulsion system that horribly contaminates the
atmosphere and fries every satellite anywhere near Earth is not an option.

~~~
Igglyboo
You wouldn't use the nukes until you're safely out of earths atmosphere which
is easily attainable by having two separate propulsion systems.

~~~
mikeash
You lose much of the advantage of Orion, since Orion is extremely heavy, and
you're doing the hardest part of the work without it. You also have to wait
until you're pretty far away from Earth, not just out of the atmosphere,
before you start blowing up nukes unless you want to destroy all electronics
in sight.

~~~
venomsnake
Not if it is assembled by nanobots on the moon.

------
badloginagain
Doesn't Plutonium-238 occurs naturally along the Thorium decay chain? I
believe that it's fairly easy (relatively speaking) to capture 238 from the
Thorium nuclear reactor process.

A Thorium-based nuclear reactor prototype has already been successfully run
for years at Oakridge National Laboratory- seeing the value of Plutonium-238,
it might justify the cost of building a full grade Pu238-extraction nuclear
reactor.

~~~
mfisher87
I've consistently heard the following two claims about thorium reactors:

1\. They offer a number of advantages in safety, efficiency, input materials,
byproducts. No comparative disadvantages.

2\. The United States is not actively pursuing any plans to implement thorium
reactors.

Is the above really true, or are there some comparative disadvantages to a
Thorium reactor vs a traditional nuclear reactor? If the above is true, there
must be some conflict of interest (imagine that, a conflict of interest in the
domain of energy!) causing #2. Is there a legal issue?

~~~
protonfish
From what I have read, the best traditional fission plant designs are very
safe, efficient, and produce few byproducts. It's the older plants that have
problems but we have learned from them and made improvements.

Compared to the most state-of-the-art plant, thorium reactors are not
significantly better and we have years of first-hand experience with
traditional uranium reactors and very little with liquid fluoride thorium
reactors so the incentives to start at square 1 with an new technology are
small.

So the good news is if we ever got over our irrational fear of nuclear power,
we have time-tested plans for safe, clean and efficient power plants ready to
be built.

------
cpcarey
The Nature article that PopSci links to goes way more in depth and is a great
read: [http://www.nature.com/news/nuclear-power-desperately-
seeking...](http://www.nature.com/news/nuclear-power-desperately-seeking-
plutonium-1.16411). Also brings up discussion of the need of much more
powerful energy sources (potentially nuclear) for proposed human space
exploration missions.

------
Faint
If people (politicians, and public) would just get rid of their prejudices and
we could just develop a nuclear reactor for outer system exploration and be
done with it...

Fuel would not be such a pain to manufacture, there'd be oodles of more power
available (more bandwidth for communications, nuclear electric propulsion for
faster travel times), and the reactor would be just as safe, if not safer to
lift to orbit (unused nuclear fuel is not radioactive, Pu-238 is, although
it's so well sealed that it's practically impossible to free it to environment
on an accident).

------
nickhalfasleep
I wonder if for activities near the gas giants the massive Magnetospheres
could be tapped for energy as the satellites move through them. However over
time you'd be trading orbital elevation for energy.

Another great side effect of RTG's is the "waste" heat keeps your electronics
from freezing in the deep of space.

------
kumarski
The problem is that the production methodologies for Pu-238 are ridiculous and
have a ridiculous number of volatile critical dependencies in both the
creation and harvesting process.

"This has been the method of choice at the Department of Energy's Savannah
River Site (SRS) production reactors. However, there are several shortcomings
with this method of production. First, the production efficiency is quite
limited, i.e., to approximately 13% efficiency. This is seen from the fact
that the Pu-238 produced in the target after only 2.12 day's half-life decay,
itself becomes a target for production of higher isotopes of plutonium, thus
reducing the Pu-238 purity by producing Pu-239 and Pu-240. Second, this
process produce a hazardous Pu-236 by-product. As noted on the decay chain,
above, there is a η→2η or γ→η reaction that results in the production of
Uranium-236 (U-236) and Pu-236. These reactions increase with exposure to fast
neutron flux. Pu-236 decays to U-232, which has a hazardous gamma-ray energy
emitting daughter product. Even a few parts per million U-232 increase the
radiation exposure hazard to personnel dramatically. Lastly, Np-237 must be
chemically purified before target fabrication. This is seen from the fact that
Np-237 decays to Protactinium-233 (Pa-233), which in turn has a strong gamma-
ray emission with its beta decay to U-233 (half-life 27 days). Therefore, the
Np-237 was stored in solution at SRS and chemically processed immediately
before fabricating targets. Solution storage of Np-237 may not be practical at
an alternate production site."

Source:
[http://www.google.com/patents/US6896716](http://www.google.com/patents/US6896716)

The extent to which this should worry our interstellar concerns is huge. The
voyager 1 went several billion kilometers away from the Sun. You know it's
crazy, it's so far away that it takes 15-20 hours for the signal to reach us.

The voyager 1 is probably going to go out in like 2020, but that's because of
the Plutonium- 238.

There's 2 sites in the USA that can produce it.

Hanford in Washington State and Savannah River Site in South Carolina.

The Russians have Mayak, and thank goodness they kept making the stuff.

Hopefully we can pump some steroidal funding into High Flux Isotope Reactors.

I've been watching this issue since 2006. In 2009, someone finally spoke up
about it and published some papers on it.

[http://www.nap.edu/openbook.php?record_id=12653](http://www.nap.edu/openbook.php?record_id=12653)

Any NRE's or Particle Accelerator Designers on HN?

~~~
cocoablazing
238Pu is always going to be problematic because of the fact that the target
material is only available in the US in weapons-grade form. USG moved all of
it to secure storage at INL, and are very cautious about any program to
transport it for 238Pu production. Hopefully they can get ATR moving and keep
missions supplied.

------
jameshart
Was lucky enough to get to see Dave Lavery (Program Executive for Solar System
Exploration at NASA) speak about Curiosity a year or so back; on the Rover's
radioisotope thermoelectric generator, he mentioned that these devices come as
a complete package from the DoE (so Nasa had little say in the power
characteristics, for example, which constrained what they could do with it),
and that technically they are 'on loan' to Nasa.

The DoE has no plans, so far as I'm aware, to go and get the one on Curiosity
back when Nasa's finished with it.

~~~
barkingcat
That's the real mission for those unmanned boeing test spacecraft - they are
the precursors and work-in-progress test craft for the classified military
project to reach Mars 10 years before current estimates of civilian efforts
(including NASA) in order to retrieve the generator from the rovers.

------
tomswartz07
Realistically, any form of power generation could be used in space. It's
really only a question of whether it's financially and practically feasible.
I've been interested to see if Liquid Flouride Thorium Reactors (LFTRs) ever
become a thing.

From what I've read about LFTRs, they can produce a whole lot of power- the
most efficient system produces 1000-MW(e) from 700kg of fuel. That lasts
approximately 12.5 years, give or take.

Obviously, thats on an Earth-based system and most spacecraft probably don't
need anywhere close to 1GW.

~~~
john_b
> _" Realistically, any form of power generation could be used in space. It's
> really only a question of whether it's financially and practically
> feasible."_

Are you referring only to nuclear power sources? Because a literal reading of
your comment ("any form of power generation") is obviously silly (e.g. steam
power). And questions of financial and practical feasibility are two of the
most important considerations for nuclear power systems, so one shouldn't
dismiss those concerns so easily.

~~~
Retric
Closed loop steam power is actually not that unreasonable in space.

Concentrated solar cells have been considered for a range of space operations.
They generate a lot of waste heat, which opens the way for a range of heat
engines. Currently, non concentrated solar is a clear winner but serious
effort went into studying combined systems.

Long term several space mining concepts involve using concentrated sunlight to
heat and refine asteroids. This would tend to make quite a bit of waste heat
and steam power is one method to utilize that waste heat.

------
nickhalfasleep
Another aspect of this that the article fails to mention is that NASA has
tried to come up with more efficient thermo-acoustic energy converters than
the existing seebeck-effect RTG's for radioactive sources [1], but the
projects went terribly over budget and got cancelled.

[1] [http://www.planetary.org/blogs/guest-blogs/van-
kane/20131208...](http://www.planetary.org/blogs/guest-blogs/van-
kane/20131208-the-asrg-cancellation-in-context.html)

------
nickhalfasleep
So I thought, maybe we could beam energy to satellites?

Using:
[http://www.pseudonomen.com/lasers/calculators/index.html](http://www.pseudonomen.com/lasers/calculators/index.html)

If I beam 1 GigaWatt to Jupiter from Earth at closest approach, with the same
beam divergence as current moon-shooting laser technology, they could gather
at 0.10655 W/m^2.. so terrible. Truly RTG FTW.

------
Zikes
The article mentions other batteries that can use the existing Plutonium 238
more efficiently, but is it not possible to design a battery/generator that
runs on an alternative nuclear fuel source?

~~~
Crito
Other fuels are possible, however Plutonium 238 is preferred for these
purposes for a number of reasons:
[http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_gen...](http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator#Fuels)

~~~
Zikes
That's a great explanation, thank you.

------
Someone1234
Linkbait headline. As this article itself (last line) alludes to, production
of plutonium-238 has restarted[0]. So we have enough for three more batteries,
but are producing more...

[0] [http://www.popsci.com/science/article/2013-03/first-time-
col...](http://www.popsci.com/science/article/2013-03/first-time-cold-war-us-
making-plutonium-238)

~~~
iwwr
Apparently, the rate of production is enough for just one deep-space mission
per decade.

~~~
toomuchtodo
Could we not reprocess uranium into plutonium from commercial reactors to
recycle the waste and use that for space probes?

~~~
cocoablazing
238Pu is only efficiently manufactured by activating 237Np.

------
c0rnelius
Maybe nasa can ask Iran for help.

~~~
lavamantis
It's a pity you're getting downvoted, I thought it was funny.

On a serious note, presuming a significant increase in trust among all
parties, wouldn't this idea radically change an ominous situation (Iran having
weapons-grade plutonium laying around) into one with peaceful, mutually
beneficial, trust-building cooperation between formerly antagonistic nations?
Have not stranger things happened?

~~~
cocoablazing
Iran was enriching Uranium, not producing Plutonium. Furthermore, this type of
Plutonium is not what you desire for nuclear weapons material.

~~~
anigbrowl
Iran has a plutonium reactor as well, although its exact capabilities and
output are a matter of debate:
[http://en.wikipedia.org/wiki/IR-40](http://en.wikipedia.org/wiki/IR-40)

~~~
cocoablazing
I'm skeptical that this small reactor rivaled the Uranium program. If your
rush 239Pu production, you end up with a product that is very difficult to
purify, raises the probability of a fizzle, and generates thermal problems for
the pit.

