
Traveling Wave Reactor - jonbaer
https://en.wikipedia.org/wiki/Traveling_wave_reactor
======
yborg
Nuclear designs are like quarterbacks, the best one is always the one "in
development". The problems with all of these next-generation designs are not
going to be with the physics, but the engineering. These reactors will be
operating for 40+ years, and are complex collections of machinery that have
wear-out and failure mechanisms that may not be discovered until years after
full operation commences - when this machinery is now highly radioactive.
Remember the pebble-bed reactor? Sound idea in principle, the Germans found
out that its main problem was inherent in the engineering of the fuel - it
turned out to produce large amounts of radioactive dust in the reactor.

The description of this reactor, which involves robotic fuel handling
equipment operating within the containment constantly moving around fuel
elements immersed in molten sodium metal seems doomed to create difficult
repair problems as these mechanisms will fail and now need to be
replaced/repaired within the hot zone of the reactor. It's quite easy to build
a system which technically works, but ends up being uneconomic. To my
knowledge there has yet to be a successful long-term operating power
generation design of any kind using sodium coolant; the Monju plant in Japan
was a failure largely because of problems with the coolant loop (including a
leak and serious fire). Until these basic engineering issues are proven to be
solved, the fuel physics seem kind of beside the point.

~~~
api
Elon would say that nuclear power has a "high PITA factor."

PITA factor is "Pain in the Ass" factor and refers to technologies that look
great on paper but explode into a fractal of non-obvious design and
operational difficulties when put into practice. He used the term to refer to
hydrogen as rocket fuel for example.

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chkaloon
Looking at TerraPower's website, they're talking about a prototype by the
mid-2020s, and they've already been working for 11 years. Would be nice to see
this sped up. The Manhattan Project lasted only 4 years. We spend much more
time on the ethical, safety, and proliferation considerations than we did back
then, of course, and with good reason. But I wonder if this project could use
a little Elon Musk hell-or-high-water drive to move it along. I'll probably
get down-voted for that, but seems like a discussion worth having.

~~~
Reedx
It's shocking to me that the best solution we have for climate change gets
downvoted on a site that's generally pro-science. But it seems to happen with
every discussion about nuclear energy.

Also surprising how even Bill Gates gets ignored on this issue. He's talked
repeatedly about why nuclear[1][2][3], yet doesn't get a lot of traction.

1\. [https://www.gatesnotes.com/About-Bill-Gates/Year-in-
Review-2...](https://www.gatesnotes.com/About-Bill-Gates/Year-in-Review-2018)

2\. [https://youtu.be/d1EB1zsxW0k?t=520](https://youtu.be/d1EB1zsxW0k?t=520)

3\. [https://ourworldindata.org/what-is-the-safest-form-of-
energy](https://ourworldindata.org/what-is-the-safest-form-of-energy)

~~~
pfdietz
It's shocking because you are laboring under the misconception that nuclear is
the "best solution".

~~~
Reedx
It's the best known solution we have today.

The misconception is that wind and solar are able to replace fossil fuels.
They can't as of now, that's just the unfortunate reality. They're a good
supplement, but they don't produce 24/7, don't recycle well, take up tons of
space, and batteries are orders of magnitude too expensive to be a complete
solution. See the video I linked to where Gates explains this.

~~~
pfdietz
Batteries are orders of magnitude too expensive if you expect to cover
seasonal differences with batteries. But no one in their right mind would do
that. You are engaging in a kind of strawman argument I call the Argument from
Bad Engineering.

Instead, batteries would be used for short term leveling (diurnal, perhaps a
few days). Beyond that, leveling would be by demand dispatch, transmission,
dispatchable demand, and long term chemical storage (for example, hydrogen or
ammonia). In the shorter term, covering the last 10% with natural gas would be
workable, even if the CO2 had to be captured and sequestered.

~~~
Reedx
I didn't set up that strawman. They're too expensive for a lot more than that.

Seriously, watch Bill's response to this and I'd be interested if you or
anyone has a better answer:
[https://youtu.be/d1EB1zsxW0k?t=520](https://youtu.be/d1EB1zsxW0k?t=520)

~~~
ZeroGravitas
Gates claims Tokyo needs 23GW for 3 days a year during a monsoon.

Tokyo has less cars per household than many Japanese cities but there's still
about 3 million of them.

If those cars each had a 50KWh battery then that would be enough power to run
the city for about 1/3rd of a day.

But since replacing all the ICE cars with electrics is forecast to save money
and lives and help make cheap renewables even cheaper, it seems like they
could work something out to tap into that when necessary and have solved a
fair chunk of the problem without spending any money (in fact saving quite a
lot which they can put towards this problem).

~~~
pfdietz
Gates appears to think providing that 23 GW with nuclear, at $10,000/kW, is
better than providing backup combustion turbines at $400/kW.

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zamadatix
After watching the Bill Gates Netflix documentary I was wondering what HN
would think of the nuclear safety debate in regards to TerraPower's traveling
wave design.

~~~
cagenut
I am very wishful but its still stuck in the wishful thinking phase. Obviously
the 'trade war' derailing it is a travesty, but I think its important at this
point (post SR15) to recognize that wishful thinking for future tech is a form
of soft/self-delusional climate change denialism. The graphs are very stark at
this point. We need to turn the corner on emissions in 2020/2021\. That means
anything still in the R&D phase is not going to be part of the first decade of
the solution. This is basic project management not even an opinion on nuclear
safety/proliferation.

~~~
DennisP
Yes, but if you believe that going from our current level of wind/solar to 80%
wind/solar is a lot easier per gigawatt than going from 80% to 100%, then it
still makes sense to develop advanced nuclear for the last part, while still
rolling out wind/solar as fast as possible right now.

Not to mention, fast reactors like the TWR can destroy the nuclear waste that
people claim to be so worried about.

~~~
baq
Solar has a really bad eroi problem and I say it as an enthusiast and an owner
of a microinstallation on my roof. New nuclear is absolutely necessary but it
just can’t come online for a million reasons.

~~~
choeger
Eroi is a red herring. The question should be energy returned on manual labor.

~~~
TeMPOraL
How is EROI a red herring? Energy returned on manual labor may be interesting
wrt. profitability, but EROI is literally whether there's any sense of doing
it in the first place.

~~~
choeger
The thing is: if you can output (through highly automated processes) a device
that yields even just 1% more energy than it required in production over its
lifetime, you have the opportunity for an exponential growth in energy
production at your hands. Higher EROI might indicate a steeper growth, but you
can achieve the same by just scaling up production.

------
baybal2
An alternative: mix californium and natural uranium and you get a solid state
subcritical reactor with no need for regulation.

Though, there will be no way to turn it off until Cf burns out.

~~~
pfdietz
This is a completely terrible idea.

Let's suppose we have a just barely subcritical reactor with k = .99. Then,
you're going to need about 1% Cf in the core to sustain a chain reaction.

The core of a 1 GW reactor contains about 100 tonnes of fuel, and fissions
maybe a tonne of fuel per GW(e)-year. So, at k = .99, it would consume 10 kg
of Cf per year. At current market prices this would cost $270 B/year, assuming
even that much Cf were available.

It would be BY FAR cheaper to provide the neutrons to drive a subcritical
reactor by means of an accelerator, rather than with californium. 1 GeV
protons on a uranium target will produce about 60 neutrons per proton, from
spallation and fast fission. This sort of scheme has been repeatedly
investigated, but has no sufficient advantage over conventional reactors to
justify the extra cost and complexity.

~~~
baybal2
Hmm, I think you have a miscalculation about neutronics here, and
fission/capture ratios. Cf neutrons are in their majority come nicely in
1MeV-2MeV, so you have _much_ more fission going per neutron, and much less
thermal neutron captures.

If you have an intense neutron emitter dispersed in fuel + moderation +
reflectors you should be able to go with much lower k to generate a meaningful
amount of power.

You should me able to go down to single grams of Cf per megawatt. Yes, not
that economical, but at least possible for things like reactors for space use
and such. At that price, that will be cheaper than current RTG material.

Correct me if I am wrong.

~~~
pfdietz
Correcting away!

The energy of the Cf neutrons doesn't matter much, since after that first
generation the neutrons will be ordinary U fission neutrons.

The comment about moderators/reflector is silly, since the purpose of those is
to make k higher. One you know what k is, they are irrelevant.

The ratio of Cf burned to (Cf + U) is 1 - k, so unless k is very close to 1
you are going to use a hell of a lot of Cf. But if Cf is very close to 1, you
need to control it anyway as fuel burns up, so you might as well just go
critical. Cf buys you nothing.

~~~
baybal2
Hi, I checked my math and found it completely unsound. It will still have to
be very, very close to criticality, though with a bit lower critical mass.

It will also require regulation over the years as Cf burns out, and u238 fast
fission products themselves product less neutrons than fission products of
thermal neutron reactors.

