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I would really like to give an explanation similar to this one.

Here's my current recipe:

(i) Mine uranium, for example take a rock from here (picture of uranium mine in Kazakhstan).

(ii) Put the rock in water. Then the water gets hot.

(iii) [Efficient way to explain that now we are done with the question]

This seems wrong, or the uranium mine would explode whenever there is a rainfall. Does one need to modify the rock first? Do I need some neutron source other than the rock itself to get the reaction started?

As soon as I have a concrete and correct description of how one actually does I think I can fill in with details about chain reactions et.c. if the child would still be interested to know more.

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closed as off-topic by Kyle Kanos, MAFIA36790, Danu, CuriousOne, Daniel Griscom Mar 19 at 13:57

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there are also fusion reactors, a bit hard for an 8 year old youtube.com/watch?v=O5_WvmQiqz0 – anna v Mar 18 at 13:36
    
I agree with @annav : even if you explain it as simply as possible and with as few details as possible, it's likely that a typical 8-year old won't understand it... Maybe wait a few more years ;) – Radu Murzea Mar 18 at 13:41
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When you get to the part where you want to explain the chain reaction, you can find an impressive visual aid by Googling for "mouse traps" and "ping-pong balls". – james large Mar 18 at 14:07
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Don't make another David Hahn! – Captain Man Mar 18 at 14:27
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You might find Randall Munroe's book Thing Explainer: Complicated Stuff in Simple Words interesting. He explains everyday things (including Nuclear Power) in the thousand most common words, so it doesn't take a large vocabulary to understand. Conveniently, the Nuclear Power Station (or Heavy Metal Power Station as it's called in the book) is available in the free Amazon preview so you can check it out. – Johnny Mar 19 at 5:34
up vote 20 down vote accepted

This seems wrong, or the uranium mine would explode whenever there is a rainfall.

A natural nuclear "reactor" probably existed at Oklo, Gabon

The natural nuclear reactor formed when a uranium-rich mineral deposit became inundated with groundwater that acted as a neutron moderator, and a nuclear chain reaction took place. The heat generated from the nuclear fission caused the groundwater to boil away, which slowed or stopped the reaction.


Does one need to modify the rock first?

No, you just need enough of the right kind of rock in close enough proximity.

Nowadays, on our planet, most of the right kind of rock (containing lots of U235) has turned into the wrongish kind of rock (mostly U238 and U234) by the natural process of nuclear decay.

So you need to separate out the right kind of stuff (nuclear fuel) from your rock. This is done by a complicated process (gas centrifuge).


Do I need some neutron source other than the rock itself to get the reaction started?

The rock produces neutrons. You usually need a moderator to slow your neutrons down, water will do.

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Note that the Oklo natural reactor was last active ~2 Gyr ago. Uranium-235, which can go critical when immersed in water, has a shorter lifetime than U-238, so all natural uranium is now "less enriched" in U-235 than in the past and a natural reactor is no longer possible on Earth. This falls under the umbrella of "you need to separate out the right kind of rock." The way it's done in practice --- centrifuge separation of UF$_6$ gas by uranium mass --- is far enough from "grinding up the rock" to warrant a place in the explanation. – rob Mar 18 at 12:17
    
@rob: I started to update my answer but realised I don't understand this stuff well enough. Are gas centrifuges needed if you are not making a bomb? I read that natural Uranium is adequate for power stations (but I don't know how the ore is processed) - maybe you could update my answer (I can make it community wiki if you like), or write another (I can delete this one if your answer would be a superset) – RedGrittyBrick Mar 18 at 15:12
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A natural reactor would use natural water ($\rm H_2O$); according to your link a reactor using natural uranium must use heavy water, $\rm D_2O$, or graphite to slow down the neutrons. (However you can't use pencil-lead graphite, which is usually contaminated with boron.) – rob Mar 18 at 17:04
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@RedGrittyBrick yes, for practical purposes. Natural uranium is less than 1% U-235. Fission reactors use uranium that's been enriched something like 5% U-235, while material for bombs is usually enriched to 80%, 90%, or more. So there's a huge difference in the degree of enrichment, but the process is the same, and the reason for it is the same. – hobbs Mar 18 at 18:26
    
@RedGrittyBrick gas centrifuges are the best method for enriching it; but the Manhattan project also put two other methods other methods into production during WW2: Electromagnetic separation via cyclotrons, and gaseous thermal diffusion although IIRC the amount of time the cascades for the latter needed to reach equilibrium was long enough that it was only used to produce moderately enriched uranium as an input material for the other processes. – Dan Neely Mar 18 at 20:23

Everything is made of tiny things called atoms. All atoms have a tiny center part called the nucleus. Some atoms have an unusual type of nucleus that, every once in a long while, randomly explodes, sending tiny pieces in all directions. Normally those tiny pieces just bounce around until they join another atom. However, if you have a bunch of the right kind of exploding nuclei together, the exploding pieces of one nucleus can hit other exploding nuclei, and make them explode immediately, then those pieces hit even more exploding nuclei, and you get a chain reaction, sort of like dominoes.

To make a nuclear reactor, you dig up a bunch of rocks with the right kind of exploding atoms, and you carefully remove many of the other atoms so the exploding atoms are close enough together to make a chain reaction, then you put them in water*. All the exploding nuclei produce a lot of heat, which boils the water. The steam turns a fan, which spins a magnet, and creates electricity. You have to be very careful that you don't put too many of the pieces with exploding atoms together, or the atoms will explode too fast, and reactor will get too hot.

*If you want to get into more detail, you could explain that the exploding bits are going so fast, that they usually pass right through the other atoms, cartoon-style, unless you have other atoms, like those in water (a moderator), for them to bounce off of and slow down. You could also explain that reactors use "control rods", which are made of atoms that easily absorb the exploding bits, and therefore slow down the chain reaction. So, if they push the control rods further into the reactor, the chain reaction slows down more.

If you want to include more terminology:

Rocks = Uranium ore

Removing all the other atoms = enrichment

Nucleus exploding = nuclear fission

Exploding atoms = radioactive atoms (often Uranium)

Exploding pieces = neutrons (and some other particles)

Fan = turbine

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I would add that different nuclei have different size, and exploding nuclei are usually the large ones. – HolyBlackCat Mar 18 at 14:35
    
You know... There may be a book that has a Nuclear Reactor explained in this form. I'll have to find my book and check. – Nelson Mar 18 at 18:02
    
@Nelson not quite this explanation, but yes Randal has a reactor in his book. You can see his explanation if you go to Amazon and use the look inside option (link above the book cover on the left side of the page); it's book page 3 (page 6 counting the un-numbered pages Amazon also shows). – Dan Neely Mar 18 at 20:18
    
Nuclear decay isn't an explosion. I suppose you are using this term to dumb it down but I think it might create confusion that could inhibit understanding more than enhance it. – JimmyJames Mar 18 at 20:57
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@JimmyJames Could you explain what feature of an "explosion" differs conceptually from nuclear decay in a way that inhibits an 8 year old level understanding of nuclear decay? I would also be interested in what you think would be more pedagogically appropriate. – Brionius Mar 18 at 21:15

RTG

The described approach mirrors https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator devices used to generate power in, for example, space probes - simply take some radioactive material and extract energy from its decay, without needing to control a chain reaction or something like that.

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Well, if you have a really adventurous kid you can follow the recipe of the Radioactive Boy Scout

...and became fascinated with the idea of creating a breeder reactor in his home. Hahn diligently amassed this radioactive material by collecting small amounts from household products, such as americium from smoke detectors, thorium from camping lantern mantles, radium from clocks and tritium (a neutron moderator) from gunsights. His "reactor" was a bored-out block of lead, and he used lithium from $1,000 worth of purchased batteries to purify the thorium ash using a Bunsen burner.[2][3]

Hahn posed as an adult scientist or high school teacher to gain the trust of many professionals in letters, despite the presence of misspellings and obvious errors in his letters to them. Hahn ultimately hoped to create a breeder reactor, using low-level isotopes to transform samples of thorium and uranium into fissionable isotopes.[4]

Although his homemade reactor never came anywhere near reaching critical mass, it ended up emitting dangerous levels of radiation, likely well over 1,000 times normal background radiation.

However, that bit about Tritium seems suspect and is probably wrong

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+1 for the interesting link, but why is the tritium part suspect? It is in fact used in gunsights, see en.wikipedia.org/wiki/Tritium_illumination – March Ho Mar 19 at 7:11
    
Tritium is extremely difficult to confine (being a gas), and when it is confined the decay energy cannot generally penetrate the container. That's why Tritium based equipment is generally safe unless you break the container and eat the contents. What is probably a more accurate account is here: dangerouslaboratories.org/radscout.html – Dirk Bruere Mar 19 at 10:28

You may need to explain the concept of a chain reaction, at least enough to explain why the rocks don't explode. Every fission event in Uranium generates neutrons. This occurs naturally at a slow rate, or can also be triggered by a Uranium atom getting hit by a neutron. The more densely packed the Uranium is, the more fission occurs (this is drastically simplified, skipping the issues of neutron speeds, but good enough to expand on later). Rocks with a lower Uranium density do heat up water, but not to the extreme that concentrated Uranium does. The larger the volume of Uranium, the more neutrons hit other Uranium atoms. The more Uranium there is per volume, the more neutrons hit other Uranium atoms. This is why we enrich Uranium for use in reactors.

Then you can show them this example of what chain reactions look like.

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