[HN Gopher] Building a Reactor
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Building a Reactor
Author : beefman
Score : 112 points
Date : 2024-05-30 21:49 UTC (3 days ago)
(HTM) web link (www.usnc.com)
(TXT) w3m dump (www.usnc.com)
| amelius wrote:
| > The reactor itself, the nuclear core, is not a complicated
| mechanical contraption. The core has no moving parts or complex
| mechanisms. It's just an arrangement of special materials, that
| permits nuclear reactions to occur. There are moving parts to
| transfer the heat and control the reactions, but it's basically a
| pile of bricks.
|
| I could say roughly the same thing about a smartphone.
| willis936 wrote:
| Yes, but a smartphone has billions of microswitches and
| articulated features. The smallest feature of a fission core is
| about the same size as a smartphone. The level of
| sophistication of the tooling needed is miles apart. If we were
| to start from scratch we could make a fission core long before
| we could make a smartphone.
| Maakuth wrote:
| And we did, half a century earlier!
| amelius wrote:
| However, the smartphone is evolutionary not revolutionary.
| numpad0 wrote:
| Nuclear reactors are definitely simpler. The simplest reactor
| is just a pure enough and large enough blob of a fissile
| material. A Turing machine is more complex.
| acidburnNSA wrote:
| Arguably, the complexity of getting the blob of fissile
| material should count as complexity. I'd argue that the
| simplest reactor therefore is the kind you can make with
| natural material, so a beryllium, heavy water, or graphite
| moderated natural uranium reactor like CP-1. Even then there
| was much complexity in getting pure enough graphite.
| grecy wrote:
| The article even mentions there were naturally occurring
| nuclear reactors on earth a while ago.
|
| So it's possible to for a blob of fissile material to just
| form naturally.
| acidburnNSA wrote:
| Well, that was 2 billion years ago. Since U-235 has a
| shorter half-life than U-238, the natural uranium
| enrichment was far higher back then. And indeed that was
| a water-moderated reactor. [1]
|
| Some people have postulated that the moon itself was
| formed in a nuclear fission excursion [2], and/or that
| there is or was a nuclear reactor in the center of Earth
| [3]. These are both not commonly believed.
|
| [1] https://en.wikipedia.org/wiki/Natural_nuclear_fission
| _reacto...
|
| [2] https://phys.org/news/2010-01-moon-nuclear-
| explosion.html
|
| [3] https://arxiv.org/abs/1308.5934
| tekla wrote:
| The Chicago pile was demonstrated in 1942. Transistors weren't
| even invented yet.
|
| You are incredibly off on complexity
| simmerup wrote:
| Not to pile on but there have been natural reactors in nature,
| but zero natural smartphones
| krallja wrote:
| > pile
|
| I see what you did there
| bobthepanda wrote:
| I mean our brains are basically meatsack computers.
| barfbagginus wrote:
| Yep you definitely need Unix Theory to build and run a nuclear
| reactor. I'm saying you're right!
| 12_throw_away wrote:
| Not sure how any this description could apply to even the
| simplest microprocessor-containing device. Seems like fabs are
| sufficiently advanced tech so as to be indistinguishable from
| magic ... so much so that we forget how amazingly, absurdly,
| not-simple they are?
| preisschild wrote:
| Somewhat related, a random nuclear reactor generator :)
|
| https://whatisnuclear.com/random.html
| dtgriscom wrote:
| Rats. I thought the "Start Company" button would draw up custom
| incorporation papers for a company building my random nuclear
| reactor.
| dtgriscom wrote:
| > To split atoms and release nuclear binding energy, we need to
| launch neutrons at the fuel atoms. ... 1 kg of natural Uranium
| generates just 10 such neutrons per second.
|
| Not saying that's wrong, but my seat-of-the-pants guess would
| have been ten orders of magnitude higher than this. (Citation?)
| credit_guy wrote:
| https://en.wikipedia.org/wiki/Isotopes_of_uranium
|
| Natural uranium is a mix of 0.7% U-235 and 99.3% U-238. U-235
| has a half-life of 700 million years and a spontaneous fission
| (SF) probability of 7e-11. U-238 has a half-life of 4.5 billion
| years and a SF probability of 5.4e-7. So on average it takes 6
| times longer for a U-238 to decay than U-235, but when it does
| it has a probability to do it by spontaneous fission that is
| 7000 times higher. Add to that that U-238 is 141 times more
| plentiful in natural uranium, and you get that you can
| completely ignore U-235 when it comes to spontaneous fission.
|
| 1 kg of Uranium is about 4.2 moles. Each has 6e23 atoms
| (Avogadro's number), that's a total of about 25e23 atoms. Every
| year about 1 in 4.5 billion of those will decay one way or
| another. That's 5.5e14 decay events. The probability of
| spontaneous fission is 5.4e-7, so basically 10 million of these
| decay events are SF. There are 32 million seconds in a year, so
| I am getting about 0.3 SF events per second. On average a
| fission event generates about 2.5 neutrons, so that's close to
| 1 neutron per second.
|
| They are getting 10 per second. They are nuclear engineers and
| I'm just a guy who knows how to multiply and how to read
| wikipedia. Most likely their number is correct.
| ffhhj wrote:
| Interesting. How much Uranium do we have on Earth?
| archgoon wrote:
| Depends on what you mean. If you mean "how much is
| commercially viable" versus "how many kilograms of the
| stuff are there" you get very, very different answers.
|
| For just "how much stuff is there", we can look at the
| concentration in the earths crust, and we get about 10^17
| kilograms of the stuff (10^15 if you want U-235).
|
| If you just say "Sure, but we can't get most of that, how
| much can reasonably get extracted at current prices and
| technology" then you go down to about 10^9 kilograms.
| the8472 wrote:
| Most of Earth's uranium is already engaged in
| commercially useful activity such as sustaining the
| geomagnetic field and the geological carbon cycle.
| credit_guy wrote:
| For all practical purposes the quantity is infinite. The
| so-called "proven" reserves currently stand at 6 million
| tons [1]. But the quantity of uranium in seawater is
| staggering, of the order of 5 billion tons. It is estimated
| that, if needed, one could extract uranium from seawater at
| about 10 times the current cost of getting it from mines.
| That would add less than 1 cent to the price of 1 kWh of
| electricity generated by nuclear power plants. For
| comparison the average retail price of 1 kWh in the US is
| about 17 cents. Of course, nobody is seriously thinking of
| getting uranium from seawater because there are much
| cheaper ways to get it from mines. But any talk of uranium
| lasting only a few decades, or centuries is non-sense.
|
| [1] https://world-nuclear.org/information-library/nuclear-
| fuel-c...
| worldvoyageur wrote:
| > that's close to 1 neutron per second.
|
| > They are getting 10 per second. They are nuclear engineers
| and I'm just a guy who knows how to multiply and how to read
| wikipedia. Most likely their number is correct.
|
| Nicely done! It gets complicated, well beyond my current
| understanding. However, U238's first decay product is
| Thorium234, which has a half life of 24 days. Thorium234 in
| turn decays into Proactinium, which has a half life of 1.17
| minutes. There are 16 more decays until finally arriving at
| Pb206 (lead), which is stable.
|
| These decays must give room for lots more neutrons to emerge.
| acidburnNSA wrote:
| Those decays don't emit neutrons. Only spontaneous fissions
| make neutrons. The reactions you're talking about emit
| alpha particles, beta particles, and gamma rays.
| kryptiskt wrote:
| Some of the neutrons from spontaneous fission will hit other
| uranium atoms and cause non-spontaneous fission. Maybe enough
| to amplify it ten times?
| lven wrote:
| I worked on this and host it on my site as well where it's not
| parceled up into bits and pieces. Have to thank Bartosz
| Ciechanowski. Learned a lot from his code and approach.
|
| https://lvenneri.com/nuclear_reactor_explainer
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