
NASA completes full-power tests of small, portable nuclear reactor - neverminder
https://www.engadget.com/2018/05/02/nasa-completes-full-power-tests-small-nuclear-reactor/
======
syntaxing
For those curious, this uses a nuclear reactor to power a Stirling
engine.Stirling engines are a good choice because there is no mass transfer to
create the work (i.e. no fuel needed, just heat). It only needs a temperature
differential and sealed gas/liquid to work. I highly recommend to read more
about Stirling engines since they are of one the most efficient form of work
generation with just a temperature differential.

~~~
cathhhhji
Stirling engines are not efficient. Using that heat to make steam to turn a
turbine is much more efficient. But it would be harder to make that portable.

~~~
telchar
Yes they are. An ideal stirling engine has the Carnot level efficiency, i.e.
maximum theoretical efficiency for a heat engine.

[https://blog.mide.com/thermodynamic-theory-of-the-ideal-
stir...](https://blog.mide.com/thermodynamic-theory-of-the-ideal-stirling-
engine)

~~~
coryrc
Except the ideal one doesn't exist and multistage stream turbines are more
efficient in practice.

~~~
hwillis
Definitely not at that scale. Small turbines have low efficiency because tip
speed increases quadratically as they get smaller.

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DuskStar
For those interested in learning more, I've found the Beyond NERVA blog to
have really good articles! Their most recent post (and the 3rd in a series on
Kilopower) [0] is on this set of tests, but there's also posts on more modern
Nuclear Thermal Rocket designs [1] as you may have guessed from the blog's
name.

[0] [https://beyondnerva.wordpress.com/2018/05/02/krusty-we-
have-...](https://beyondnerva.wordpress.com/2018/05/02/krusty-we-have-fission-
kilopower-part-iii/) [1] [https://beyondnerva.wordpress.com/2018/01/19/leu-
ntp-part-tw...](https://beyondnerva.wordpress.com/2018/01/19/leu-ntp-part-two-
cermet-fuel-nasas-path-to-nuclear-thermal-propulsion/)

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stcredzero
I think someone should develop a 2 module design for this, where the Stirling
engine is separated from the fission reactor with insulated pipes. Why? Weight
savings. Insulated pipes can be made very efficient and relatively light.
Radiation shielding can't easily be made light. By separating the modules into
two sections, martian soil can be more easily used to provide the radiation
shielding. It could even be built out of bricks, which we already know can be
made out of martian soil.

~~~
simcop2387
That might be a good second phase setup for a larger base, but this current
one would be good for initial setup since it could be ready without having to
produce anything on planet. That makes it good for rovers or other unmanned
setups.

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ChuckMcM
They nailed all their milestones. This is tremendously important work with
regards to a future outpost anywhere and possible outposts on Earth. (Imagine
how much fuel you wouldn't have to ferry down to Antartica each year to
prepare for winter operations, for example).

Now to ruggedize the heck out of it and make it easily transportable.

~~~
jabl
Not Antarctica but pretty close to the other end, this thing is being towed up
there at the moment:
[https://en.wikipedia.org/wiki/Akademik_Lomonosov](https://en.wikipedia.org/wiki/Akademik_Lomonosov)

(it's a fairly traditional PWR, though, so nothing new technology-wise per se)

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Sir_Substance
Conspicuously not mentioned: what kind of design the reactor uses. Thanks
engadget, 10/10 reporting.

Based on the wikipedia article, it looks like a passive decay heater attached
to a bunch of sterling engines. Pretty much a souped up RTG that trades more
energy output for having moving parts. Neat.

~~~
dukoid
Wouldn't "passive decay" mean it's not based on fission -- but the article and
video both mention fission...?

~~~
Sir_Substance
Passive decay is still fission, it's just not artificially sped up fission :)

it's probably more complex than that. Is high speed natural decay of an
artificially created or enriched heavy element natural or artificial? I'm not
sure how that gets classified.

~~~
dukoid
My bad, I had assumed fission was splitting atoms by adding neutrons, but
actually fission is just the "splitting" part, so both cases are indeed
fission....

~~~
darken
The fission process releases neutrons, so you effectively "add neutrons" by
concentrating self-fissioning material in a small space.

To put another way: If you increase the size of a sphere of fissionable
material, its rate of "natural" decay stays the same, but more of these decays
lead to neutrons triggering additional fission reactions. So doubling the size
of the sphere leads to >>2x fission events due to neutrons having more fissile
material to hit on the way out.

A large fission reactor is just a scaled up version of this with control rods
to adsorb extra neutrons to control secondary "induced" fission.

------
philipkglass
Some background on this: NASA deep space missions have historically used
radioisotope thermal generators powered by the decay of the exotic plutonium
isotope Pu-238. This isotope has a good balance of lifetime (87.7 year half
life) and specific energy (0.5 watts/gram). It is non-fissile -- no risk of
criticality. It also decays solely by alpha emission, so there are no problems
with shielding the rest of the systems from e.g. gamma or neutron radiation.

American plutonium 238 was formerly produced using Savannah River Site
reactors that primarily produced materials for nuclear weapons. With the
retirement of those reactors in 1988, and the American nuclear weapons program
going to maintenance mode, NASA lost the side-benefit of Pu-238 production
using the weapons infrastructure. Deep space missions requiring RTGs had to
subsist off of historical Pu-238 stockpiles and additional material purchased
from Russia. But the Russian supply has run out too now -- apparently they
aren't producing more of it either.

American plutonium 238 production efforts resumed in 2013:

[http://www.spacesafetymagazine.com/aerospace-
engineering/nuc...](http://www.spacesafetymagazine.com/aerospace-
engineering/nuclear-propulsion/nasa-pay-entire-pu-238-production/)

With NASA now paying the full cost, including fixed startup costs, Pu-238 is
extraordinarily expensive:

 _NASA and DOE have estimated the rebooting will cost between $75 million and
$90 million over five years. According to NASA officials, the agency expects
to have 1.5 to 2 kilograms produced per year, starting 2018._

The high cost and limited supplies of Pu-238 have spurred the search for
alternatives to Pu-238 RTGs. A few years ago I remember reading that the
European Space Agency was going to try to design its own RTGs using the less
powerful but more abundant americium 241 instead of plutonium 238. But a quick
search just now doesn't show any concrete development effort.

This Kilopower reactor is an alternative to RTGs for some mission profiles --
in fact offers more power than RTGs _and_ uses cheaper nuclear materials.
(Highly enriched U-235 isn't cheap in an absolute sense, but it's far less
expensive than Pu-238. And the security of supply is effectively backstopped
by its use in reactors for the US Navy.) It can offer ample power for very
deep space missions at a cost significantly less than using years' worth of
Pu-238 production.

Improved photovoltaic cells have also enabled missions further from the Sun
than they could have supported in 1988. The Juno mission, which entered
Jupiter orbit in 2016, relies on PV instead of RTGs. It seems plausible that
further evolution will eventually push PV's reach out to Saturn missions. But
PV is not _currently_ plausible for missions beyond Jupiter, and it is reliant
on slowly-evolving battery technology for surface missions on Mars. Martian
missions using PV also face significant problems from cell-obscuring dust.
This reactor seems too large to be conveniently integrated in a Martian rover,
but enabling non-surface missions to avoid Pu-238 use may mean more can be
reserved for future rovers akin to the RTG-powered Curiosity.

~~~
petra
The major fear that comes up with these is: what if the missile blows up while
still in earth? How are these risks managed?

~~~
MertsA
To simplify the other more detailed response, It's just Uranium at that point.
Even highly enriched Uranium isn't that radioactive by itself, it's once you
start operating the reactor that it starts generating highly radioactive
waste.

The new fuel rods going into it are totally safe from a radioactivity
perspective. Here's a picture of a man holding a fuel rod bundle with nothing
more than gloves on.
[http://nuclearstreet.com/images/img/dw037.jpg](http://nuclearstreet.com/images/img/dw037.jpg)

~~~
jessaustin
One can't help but notice from his shirt and his hair that the picture was
taken in the 1970s. Lots of awful practices still existed then; who's to say
this isn't a picture of that?

~~~
philipkglass
Here's a photo series from 2005 at a Swedish nuclear fuel facility showing how
uranium turns into fuel pellets and fuel rod bundles:

[https://www.iaea.org/newscenter/multimedia/photoessays/train...](https://www.iaea.org/newscenter/multimedia/photoessays/training-
nuclear-watchdogs-0)

At all stages the material can be handled with no more protection than gloves.
It is roughly as dangerous as handling lead fishing weights until the fuel
actually attains criticality.

~~~
IdeaHamstir
It still produces a fair bit of alpha, so avoid particulate ingestion.

~~~
MertsA
I don't know why people downvoted this, it's entirely correct. Although I
guess the larger concern than the radiation would be the heavy metal poisoning
but in any case it's still fine, just treat it like you would any other heavy
metal.

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mkoryak
I wonder how safe these are. Could I get one in my basement and stop paying
nstar for the rest of my life? Would my neighbors complain much?

~~~
jabl
The black helicopter people might arrive and have a nice chat with you when
they discover you ordered a bunch of HEU from aliexpress.

~~~
reaperducer
Funny you should use the phrase "black helicopter." During the Cold War, the
military used tiny nuclear reactors to power automated remote sensing stations
in the Arctic. We called them "Power Pigs" and they were delivered by
helicopter to very remote places.

They were fascinating little devices, and because of where they were placed,
used the outside air/ice/snow/rocks for cooling.

Don't know if they are still being used, or what happened to the old ones, but
they were very clever technology.

~~~
jarvist
I guess these are the Strontium-90 RTGs made by the Soviets. There's about
1000 out there still - some have been vandalised, some washed out to sea, some
sitting rusting in a lighthouse outhouse etc.
[http://bellona.org/news/nuclear-issues/radioactive-waste-
and...](http://bellona.org/news/nuclear-issues/radioactive-waste-and-spent-
nuclear-fuel/2003-11-two-strontium-powered-lighthouses-vandalised-on-the-kola-
peninsula)

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api_or_ipa
> Lower power Kilopower systems, like the one kilowatt version, can power a
> basic toaster,

Toaster is a bad example, a laptop uses around 30 watts so the 1kw version can
power several dozen laptops.

------
baybal2
I wonder, can you make a stirling with plutonium pentafluoride working fluid?
Eg, cold chamber with neutron poison, hot chamber out of a neutron moderator
material?

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JTbane
Pretty amazing that the simple Stirling engine can be the best choice for
generating kilowatts of power in a hostile environment.

~~~
jjoonathan
The magic comes from the nuclear half, not the Stirling half.

~~~
dgoodell
The free piston stirling engines required also operate via magic and few
people understand them well enough to make one suitable for this purpose. The
engines required for a reactor like this haven’t been designed yet and it
would take years of effort before They would be designed and approved for
flight use.

For this test they used a couple of undersized (<100W) stirling engines and
the rest were just thermal simulators.

~~~
jjoonathan
Stirling Engines exceed their competition by percentage points. Nuclear energy
exceeds its competition by orders of magnitude. I stand by my statement that
the magic comes from the nuclear half of this relationship -- so long as we
accept a definition of magic related to, you know, energy production, rather
than effort or hipster points.

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chiph
How do they manage to not have the reciprocating mass (the piston) throw off a
space vehicle's trajectory? IIRC the Space Shuttle used magnetic tape because
a hard drive would have acted as a gyroscope.

~~~
Doxin
Keep in mind that in the space shuttle era hard drives were a good deal more
massive than today. Presumably a modern hard drive wouldn't present much of a
problem.

In any case a rotating mass isn't too big of a problem, it's changes in
rotational speed that'll mess with your heading -- not your trajectory though.
The piston itself shouldn't be too much of a problem either because any motion
it makes one way it shortly after makes the other way balancing out the forces
over time.

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ausjke
can this be used for car/boat/etc-fuel somehow one day, then all gas-station
will be run out of business and middle-east-plus-texas-oil-field all file for
chapter-11?

not sure how to prevent radiation though.

~~~
larkeith
There's a few problems with making Highly-Enriched Uranium commercially
available, even before we get to radiation.

Thorium reactors [1], on the other hand, might be more feasible due to their
lack of weaponization potential, cheaper fuel, and reduced waste. However,
Thorium reactors are much more complex, and will likely never approach the
small scale of KiloPower; they also have significant startup costs, remain
commercially unproven, and have a more dangerous fuel cycle (in terms of
radioactivity).

[1] [https://en.wikipedia.org/wiki/Thorium-
based_nuclear_power](https://en.wikipedia.org/wiki/Thorium-
based_nuclear_power)

~~~
IdeaHamstir
They have completed designs using low enriched Uranium. They're not quite as
elegant or as small, but they should work fine.

[https://fas.org/nuke/space/leu-reactor.pdf](https://fas.org/nuke/space/leu-
reactor.pdf)

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forapurpose
What are the other uses for this? Civilian power? Military applications?

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ajuc
> For example, cutting off the system's cooling or ramping it up to its
> maximum level both resulted in a core temperature change of just 15 degrees
> or less.

Why use cooling at all then?

~~~
Symmetry
You can't extract energy from a heat engine without cooling. Useful work is
done in the process of moving thermal energy from a high temperature location
to a low temperature location.

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netzone
What would the effects be of releasing nuclear waste in space? Could we sent
up all of our nuclear waste, put a rocket on it and just let it disappear into
the distance?

~~~
dukoid
How about just sending it to the moon instead? Perhaps then we could use the
moon as a giant space ship by blowing up the nuclear waste?

~~~
practice9
It's a fascinating idea from science fiction perspective, but moving the Moon
anywhere will probably have devastating effects on the Earth's biosphere

~~~
dukoid
I think that's just short term thinking: When the sun expands into a red
giant, the gas drag will slow down the moon. I think that means it will crash
into the earth if it's not removed before.

~~~
craftyguy
Regardless of whether or not what you say is likely to happen (I think not,
but anyways), if humans can even last long enough to see the Sun as a red
giant, we'll have larger problems on our hands. In any case, we'll likely have
destroyed ourselves or evolved into something else by that time.

~~~
killjoywashere
The Moon orbits the Sun, and is only mildly perturbed by the Earth. Gas drag
would decay the Moon's orbit, but it's not at all clear to me it would crash
into the Earth. It seems far more likely it would simply fall into the Sun.

~~~
craftyguy
The Earth and Moon system orbit the Sun, but the Moon is very much in orbit
around the Earth.

A more likely scenario is that the Earth's orbit will move outwards as the Sun
expands, since stars in the red giant phase(s) lose a lot of mass. It's
plausible that the Earth/Moon will not be destroyed, physically. But in that
case all life would have been obliterated due to the immense temperatures
associated with having the surface of the Sun within a few million miles (or
less) of Earth.

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ianai
On resiliency “cutting off the system's cooling or ramping it up to its
maximum level both resulted in a core temperature change of just 15 degrees or
less”

------
BrownArch3310
Lot's of insights just by reading the comments! haha

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forkandwait
So... Are plans available anywhere?

~~~
GCU-Empiricist
Umm. . . it probably wouldn't be cost effective for you: the NRC is pretty
brutal in it's regulations. I'm very gun-ho on nuclear power, but that is one
bureaucracy I don't mind charging ~$4-500/hr for people to review submitted
designs for approval and then painfully inspecting operating plants.

~~~
forkandwait
I would still like to look at the plans (though it's charming you thought I
might try to build it...)

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rbanffy
I wonder if it can operate in zero g...

~~~
zaroth
That is in fact precisely what it is designed for.

~~~
linuxguy2
Isn't it actually designed for working on Mars? Convection in the sodium heat
pipes wouldn't work in zero G.

> generate heat that is carried to the Stirling converters via passive sodium
> heat pipes.

[https://en.wikipedia.org/wiki/Kilopower](https://en.wikipedia.org/wiki/Kilopower)

~~~
asteli
The wiki page you linked specifically mentions deep space applications as a
design goal.

I'm not sure about the sodium heat pipes but the alcohol vapor heat pipes you
see in electronics typically use an internal wick to return the working fluid.

~~~
rbanffy
Worst case scenario is you put the reactor next to the toilet, in the rotating
part of the spacecraft, to solve the problem with convection...

These two things are enormously simplified when there is gravity.

~~~
mcguire
" _Worst case scenario is you put the reactor next to the toilet..._ "

Please don't give any ideas to our collective employers.

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Thorazul
They should team up with Tesla to develop storage batteries for the reactors
in case a reactor fails you would still have some temporary emergency power.

~~~
rmah
Tesla doesn't make the batteries, Panasonic, Samsung and Hitachi does. IIRC,
Panasonic is the firm building batteries at Tesla's new gigafactory facility.
Tesla simply slaps their name on the battery packs.

