
NASA to test prototype Kilopower nuclear reactor - iliis
http://www.world-nuclear-news.org/ON-NASA-to-test-prototype-Kilopower-reactor-1711174.html
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curtis
This is similar to the "Stirling radioisotope generator" [1] but it is an
actual nuclear reactor (albeit a small one). Both systems are a response to
the same fundamental problem, the dwindling supply of Plutonium-238 [2]. The
Stirling radioisotope generator uses Pu-238 more efficiently than
thermoelectric RTGs, and the nuclear reactor from the article dispenses with
Pu-238 altogether.

[1]
[https://en.wikipedia.org/wiki/Stirling_radioisotope_generato...](https://en.wikipedia.org/wiki/Stirling_radioisotope_generator)

[2]
[https://en.wikipedia.org/wiki/Plutonium-238](https://en.wikipedia.org/wiki/Plutonium-238)

~~~
yborg
The problem with Stirling engines is they have moving parts. It's highly
unlikely a Stirling cycle generator on the Voyagers would have operated for
40+ years as their RTGs have.

~~~
raverbashing
Yup, I wonder how they can guarantee the engine will keep turning for 10 years
even. Maybe if everything is sealed in a (very long) lived lubricant that
stands the harshness of space?

Still hard to imagine

~~~
jacquesm
Stirling engines have a pair of seals that are extremely hard to make durable,
and to my knowledge only Whisper Systems has cracked this problem to the point
where you won't lose working gas (or seals) in a timespan much shorter than
those 10 years. It's a stupidly hard engineering issue, without it there
likely would have been far more adoption of the Stirling cycle for production
machinery.

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GlenTheMachine
This is a very cool technology, but frankly I have a hard time imagining that
it will see much use. NASA sees public protests and scare mongering whenever
it tries to fly an RTG, which is based in the radioactive decay of (usually)
plutonium, and does not use a nuclear chain reaction. They did fly one on Mars
Science Lab, because it was the only way to get enough power for the rover.
But it takes a PR hit every time it does so. RTGs are therefore only used when
no other technology will work.

It think it's likely that the public relations nightmare NASA would have to go
through to fly an actual reactor will be through the roof.

~~~
jlebrech
they should be indoctrinating kids at school that nuclear is safe, rather than
other "stuff" they tell them nowadays

~~~
TeMPOraL
Totally. You're probably catching downvotes from people offended by
"indoctrination", but a) indoctrination is what schools do, that's their
purpose, and b) nuclear _is_ actually pretty safe.

I agree with the sentiment. Anti-science sentiments proliferate partly because
science doesn't spend much on public outreach, especially compared to people
who want to make a buck off scaring others about new technologies. NASA has
been doing more and more PR work over the past few years, and that's great -
but IMO in this case, they really need to get someone who looks and sounds
confident, and who would go on national TV and say "Yes, we are totally
sending a nuclear reactor to space, why wouldn't we?".

~~~
matt4077
In your zeal to defend nuclear energy, you're come around full circle,
defending something indefensible, with arguments far removed from actual
science.

Nuclear energy may be safe, if done under near-perfect condition, with
extremely large budgets to understand risks, and plans to mitigate incidents.
There is truth to the argument that nuclear power saves lives compared to coal
power.

But nuclear power isn't inherently safe. It is, by its very nature, extremely
dangerous. Actual nuclear scientists understand that. See, for example,
Feynman's work on nuclear safety during the Manhattan project.

Strapping a nuclear reactor to a rocket is almost by definition a dirty bomb.
Depending on the height and mode of an eventual launch failure, the result
could be anything from Tchernobyl to a less-dramatic yet more deadly dispersal
of nuclear material in the upper atmosphere.

The dose-response relationship of radiation exposure is largely linear,
meaning the latter event might just increase your risk of brain cancer by
0.005%. Yes, you may consider it negligible. But statistically, it would kill
half a million people.

To blithely state that "nuclear is safe, hoho, why shouldn't we strap
Plutonium to a rocket, you environmental nincompoops" has nothing to do with
science, and gives science a bad name. Maybe NASA could come up with a way to
protect a reactor during a missile launch. But it wouldn't be easy, and saying
"why wouldn't we?" on TV would not inspire confidence in their abilities, but
doubts in their sanity. The right thing to say on TV is "We're sending a
reactor into space, and here are the mechanisms we've come up with to make it
safe..."

~~~
TeMPOraL
I wasn't aiming for the full pro-nuclear argument in a single comment, because
it's a well-trodden road. Nuclear safety issues have been discussed _ad
nauseam_ here and elsewhere.

But maybe I should have stated my main assumption explicitly: people at NASA
know what they're doing. They're not incompetent morons, they can do the math.
They already have absurd amounts of procedures in place to ensure a launch
failure doesn't hurt anyone. They're not just trying to strap a bunch of
random nuclear isotopes on a rocket and hope for the best.

> _Strapping a nuclear reactor to a rocket is almost by definition a dirty
> bomb._

No, it isn't. A dirty bomb is meant to create a large-scale radioactive
contamination that's dangerous to humans, and U-235 is probably the last thing
you'd like to put in it. It's an alpha emitter with a half-life so long it
doesn't matter, so the only effect you'd get is heavy metal poisoning.

(That's beyond the fact that dirty bombs are speculative devices, and from
what I read, they actually turned out not to be a real issue in practice.)

> _Depending on the height and mode of an eventual launch failure, the result
> could be anything from Tchernobyl to a less-dramatic yet more deadly
> dispersal of nuclear material in the upper atmosphere._

That's the kind of thinking that I suggest we need to fight with a serious
public outreach effort. Chernobyl was a total clusterfuck, but the only
connection between it and the NASA project is the word "nuclear" in newspaper
headlines. Chernobyl was an _active_ reactor, and its explosion contaminated
the area with _reaction products_ that had very low half-life and underwent
beta and gamma decay, making them really dangerous, _unlike plain U-235_.

> _To blithely state that "nuclear is safe, hoho, why shouldn't we strap
> Plutonium to a rocket, you environmental nincompoops" has nothing to do with
> science, and gives science a bad name._

I'm not saying that. I'm saying that there's so much baseless fear around
nuclear energy as a whole, that it's high time we stopped treating it as
reasonable. And right now, the biggest danger around nuclear energy is the
cost in lives of it not being used instead of fossil fuels.

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ChuckMcM
Okay, this is awesome. 10kW is a serious enough amount of energy to do some
fun stuff. Things like rovers that move at km/h rather than m/h and all day
operation. It also solves the 'base load' problem on deep space missions which
would like to use more power for radio communications.

~~~
hutzlibu
"It also solves the 'base load' problem on deep space missions which would
like to use more power for radio communications."

Not really, since "moving parts" ...

~~~
dgoodell
One of the primary uses are deep space probes. The stirling engine has exactly
two moving parts and uses non contact seals. In theory the lifetime should be
limited by the deformation of the piston at elevated temperatures. Still,
you’re right. They certainly have to do a lot of reliability testing to be
able to show that these stirling engines will last 10+ years before they use
them in a mission. They have several stirling engines that have been running
for multiple years in simulated conditions for just this purpose at NASA GRC.

~~~
hutzlibu
But on deep space missions, aren't you aiming for 50+ years, or ideally
potentially unlimited if you want to hit alpha centauri?

Voyager 1 is going for 50 years ...

~~~
ChuckMcM
Your point about moving parts is good, it is absolutely the place to look for
possible failure. That said, it is also possible to build such things in a
durable way. For example, at the science museum in London there was a display
with a Stirling engine that over 75 years old and still works fine. I expect
the challenge will be to keep the materials within the range of temperatures
and pressures that do not cause structural change in the material.

~~~
scrumbledober
Has that stirling engine had zero maintnence in the last 75 years? If so that
is quite impressive and gives much more confidence that NASA can pull this off
with the benefit of three quarters of a centruty of technological advancement.

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nickhalfasleep
Thermo-acoustic stirling engines have long been a niche product. Seems like a
great environment for them to shine, and far better efficiency than RTG's.

~~~
jessriedel
What's the advantage over direct thermoelectric conversion?

~~~
curtis
> _What 's the advantage over direct thermoelectric conversion?_

Stirling engines can approach 50% efficiency [1], whereas thermocouples are
usually less than 10% [2].

[1]
[https://en.wikipedia.org/wiki/Stirling_engine](https://en.wikipedia.org/wiki/Stirling_engine)

[2]
[https://en.wikipedia.org/wiki/Thermoelectric_generator#Effic...](https://en.wikipedia.org/wiki/Thermoelectric_generator#Efficiency)

~~~
jessriedel
Cool, thanks! It's really surprising to me that inserting a mechanical
intermediary has such a large efficiency advantage.

~~~
brians
Why does mechanical stuff suck? Because of friction of moving parts. Too much
energy is lost to heat. For a thermal engine, if you can arrange to put the
moving parts mostly on the hot side, that’s no longer loss.

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yummybear
I thought one of the advantages of the normal RTG is that it is non-
mechanical. I would think a mechanical engine has a lot higher fail rates?

~~~
wefarrell
The problem with normal RTGs is they're dependent on Pu 238 which is in short
supply.

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iliis
More details in this german article: [https://www.golem.de/news/kilopower-ein-
kernreaktor-fuer-rau...](https://www.golem.de/news/kilopower-ein-kernreaktor-
fuer-raumsonden-1712-131418.html)

I'll summarize some of the interesting points:

\- The nuclear core (75kg of enriched uranium/molybdenum [1]) is designed to
not go critical, even if it accidentally falls into the sea and is surrounded
by water (which is a good neutron reflector). It only starts when you surround
it with a neutron reflector made of beryllium (an even better neutron
reflector, mainly due to less absorbtion). Combined with the fact that the
reactor only gets nasty when it's been running for a while (and thus is
already far away from earth) it is a lot safer than plutonium fueled RTGs.

\- It would be very useful to reach far away destinations (like the orbit of
Uranus, Neptun or Pluto) using ion drives, as they need to run for years and
solar panels aren't effective far away from the sun.

\- While there have been other attempts at developing nuclear reactors for
space, most of them didn't go far. They could use an existing research reactor
(Flattop [2]) for this project which already has all the required permissions
to run, so a lot of paperwork could be saved for the Kilopower experiments.

\- The Kilopower reactor is the first to use heatpipes instead of pumps for
the heat transport and stirling engines for the energy generation. The first
experiment was thus to show that the cyclic heat draw of the stirling engine
would be safe, because usually nuclear reactors reach an equilibrium between
heating up (and thus expanding slighty which slows down the reaction) and
cooling down (which accelerates the reaction).

\- Instead of the planned eight 125W Stirling engines, they're currently using
two 70W ones from the Advanced Stirling Converter Project [3]. The other ones
will be simulated using simple heatsinks.

\- Theoretically it could run for hundreds of years (after 500 years less than
1% of the uranium will be used), but the Stirling engines will break much
sooner than that.

[1]
[http://www.iaea.org/inis/collection/NCLCollectionStore/_Publ...](http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/33/034/33034319.pdf)

[2]
[https://en.wikipedia.org/wiki/Flattop_(critical_assembly)](https://en.wikipedia.org/wiki/Flattop_\(critical_assembly\))

[3] [https://tec.grc.nasa.gov/rps/stirling-research-
lab/advanced-...](https://tec.grc.nasa.gov/rps/stirling-research-lab/advanced-
stirling-convertor/)

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hutzlibu
My first thought was "deep space mission". But because of moving parts
involved, this is probably not the use-case.

So a bit OT, I was wondering if anyone has yet thought about the solution of
still using solar panels for them, but to also use a mirror to focus more
light in the direction of the probe or a laserbeam?

I mean, theoretical I don't see why not, appart from being more expensive?
Andd you could also offset the laser/mirror cost, because you only need them
later on ....

(but to be on the safer side, I still would add a RTG)

~~~
monob
Inverse square law. Every n AU you go out from the earth you will need n __2
are of mirror to focus on a solar panel to get the same power. That assumes
you can even build a structure that can keep that shape.

Say you have a mission to Uranus, you will need a mirror with 400 times the
area of the solar cells to get as much power as they would in earth orbit.

~~~
imdsm
Does this mean that if you go towards the sun, you would be able to generate
more solar electricity with less surface area? I'm not thinking dyson sphere
style, but perhaps some sort of electricity space-tanker, that sails toward
(or around?) the sun, collecting solar energy, and then returns to earth with
batteries packed to the brim with energy?

Dibs on Solar Harvester

~~~
TeMPOraL
Yes, it means exactly that - though soon you'll start hitting into issues with
solar panel efficiency and thermal management. It's _very hard_ to keep things
cool in space.

The space electricity tanker idea is theoretically possible, but might not
make any economical sense. For it to be worth it, it would have to store _a
lot_ of energy, so that it could ship back more than the cost of moving it
around, while also being competitive with simply building more bigger
collectors further away (and closer to the industry). But maybe a highly
eccentric orbit, with a very low perihelion, would work.

Not totally sure if the math works out on this, but I could see moving the
energy-intensive industry closer to the Sun, and having it use beamed energy
to move resources and products to itself and back.

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Gravityloss
From
[https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/201600...](https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160012354.pdf)

2-5 W / kg.

Solar cells seem to be about 150 W/kg.

This is relevant for outer system exploration (beyond Jupiter) or maybe for
night power on planetary / moon surfaces.

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giarc
Anyone know the approximate size of that thing? Hard to tell from the picture.

~~~
GlenTheMachine
Hard to tell what the weight is, of course. Judging from the size of other
objects in the picture (the lift point and the connectors) it's very roughly
around four cubic feet. Which is actually pretty awesome; the equivalent solar
panels would be significantly larger.

~~~
jacquesm
According to a NASA pdf the thing weighs 1150 Kg or thereabouts.

[https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/201700...](https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170002010.pdf)

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fsloth
I first read 'NASA to test KILLpower nuclear reactor' and thought are they
starting to use Space X naming conventions...

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jlebrech
the cold of space and the heat of a nuclear reactor would be the perfect
combination for a peltier reaction. not very good for a rover though.

~~~
gruturo
cold of space? Unfortunately, no - space is an extremely effective insulator,
so it's extremely difficult to get rid of heat. You need large heat sinks
(which adds weight) and they have piss-poor efficiency in space. So your cold
side would get hot very quickly

~~~
jlebrech
are you saying it would be better on a planet with an atmosphere to use as
heat conduction medium?

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tobyhinloopen
put it in a car

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gambiting
1kW isn't driving you anywhere though(even if we ignore all the dangers of
having a nuclear reactor in a car). A typical automotive engine produces
100-300kW.

~~~
rijoja
But it would be perfect for a well isolated home wouldn't it?

~~~
gambiting
Hmmmm a normal boiler for a 4-person family home is about 8-10kW so again,
probably not. At least not for heating. But for normal power consumption
should be ok, especially if paired with some storage device to account of
periods of higher consumption(running a 3kW kettle for couple minutes for
example).

~~~
rijoja
Well yeah not the average household obviously but if you really wanted to get
rid of your dependency on the grid it would be somewhat possible. As opposed
to say drive a at least 70 HP ~ 50 KW car. Not a proposal for everybody of
course but for lets say political idealists.

As far as the 3kW kettle is concerned having a battery to provide for these
surges is entirely viable.

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jimmcslim
Strap it to an EmDrive and head for the stars!

