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I've been reading up on nuclear reactors, and understand explanations of how it works, how water is heated to steam, which turns the turbines, etc.. I understand all of the safety features, and how control rods are used, and what they do with spent fuel.

However, what I can't figure out (after lookin at many websites and videos) is where the reaction actually starts. The fuel rods contain Uranium pellets, and then they're put in the reactor, where the reaction starts and neutrons start hitting each other to create heat? So is this happening all over the reactor, or just in each fuel rod assembly?

Also, you can hold a Uranium pellet in your hand (ideally wearing a glove) and it's not dangerous, so what starts a reaction? A lot of people say the reaction can start by itself, so why do uranium pellets not suddenly heat up and start spreading radiation by themselves? Is it because they haven't been enriched?

What if you dropped a Uranium pellet on the floor?

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You are missing the concept of critical mass. In a small amount of uranium, like a single pellet, some fraction of the atoms will decay spontaneously every second. When enough of this is put in proximity, then the emissions of some of the decaying atoms kick other atoms just right so that they fission too. As a result, more atoms fission than you would predict from just the probability of a single atom doing so.

The main difference between a small pile of uranium and a large one is that in a large one you get a chain reaction. Power plant reactors rely on this chain reaction mechanism to get the large amounts of output power. The control rods control how much the emissions of fissioning atoms can hit other atoms, thereby controlling the overall reaction rate.

In reality, this description is over simplified. There can also be moderators envolved that sortof convert some of the fission results into stuff that can kick other atoms to fission when without the moderator they would not. However, that is a aside to this question.

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Moderators do something more specific than "sort of convert fission results into..." They thermalize (i.e., slow down) neutrons. Thermal neutrons are much more likely than fast neutrons to interact with fissionable nucleii. – james large Dec 2 '15 at 19:06
    
@james: Yes, what you say is more specific. I was deliberately trying to stay away from the details of moderators, hence "sortof convert". – Olin Lathrop Dec 2 '15 at 21:21

Uranium pellets are not dangerous if they haven't been inside of a working reactor (I wouldn't advise using them in a school chemistry class, though). During fission, there starts a buildup of various highly radioactive products (Strontium, Iodine, Caesium) and also transmutation of Uranium-238 (always present, even in enriched uranium) into various isotopes of Plutonium.

Now, after some time within the reactor, pellets are not safe to take into your hand.

The remaining question of yours: how does a chain reaction start in the reactor? A neutron source is used to create enough neutrons to initiate fission, and control rods are withdrawn in a specific order to enable rapid multiplication of neutrons until the reactor reaches a given power level. That's the basics, there are many books if you decide to be a reactor operator.

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I believe uranium is rather poisonous, though (quite aside from radioactivity). As you say, don't use them in school chemistry - the radioactivity danger from all these very low level sources is the risk of ingestion of shards and residue which can stay in the body a long time. – WetSavannaAnimal aka Rod Vance Sep 17 '13 at 23:56
    
@WetSavannaAnimalakaRodVance, it's not really "aside from the radioactivity". Alpha radiation is seriously harmful to your health when it happens inside your body. If you ingest or inhale uranium compounds, some of the uranium atoms actually end up inside your living cells. – james large Dec 2 '15 at 19:14
    
@jameslarge Sorry if I'm not clear. The alpha hazard from ingested shards is what I'm talking about in my last sentence. I do believe that my "quite aside from radioactivity" stands: I'm pretty sure uranium and many of its salts are highly chemically toxic just from the havoc their chemistry plays with biology, although I'm unsure of the details. – WetSavannaAnimal aka Rod Vance Dec 3 '15 at 22:12

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