I guess what I meant is that the overall concepts are understandable. Nuclear fuel gets hot, boils water, drives a turbine. For transmissions, different sized gears allow things to turn at different rates.
But as soon as I dive into the details, I get lost. How exactly can you control the nuclear decay? How exactly does do the gears in the transmission move around and combine with eachother to create a specific gear ratio? These concepts probably are probably pretty simple for a lot of people, but they just make my head spin.
That's what the control rods are for. The uranium in one fuel rod in isolation decays at whatever natural rate, which would warm water but not boil it, and placing the rods near each other allows for the decay products (high energy particles) to interact with other fuel rods and induce more rapid decay.
The control rods slot in between the fuel rods, and absorb the decay products without inducing further nuclear decay. Usually these are graphite rods.
> How exactly does do the gears in the transmission move around and combine with eachother to create a specific gear ratio?
It really depends on the specific transmission, a manual transmission is using the shift lever movement to move the gears into place. An automatic transmission most likely uses solenoids to move things (a solenoid is basically a coil of wires around a tube with a moveable metal rod inside, when you put current through the wire, the metal rod is pulled into the tube, you attach the larger thing you want to move to the end of the rod (sometimes with a pivot or what not), and use a spring, another solenoid, or gravity, etc to make the reverse movement. A solenoid by itself gives you linear movement, if you need rotational movement, one way to do that is have a pivot on the end of the solenoid rod, then a rod from there to one end of a clamp on a shaft, then when the solenoid pulls in its rod, the shaft will rotate (this is the basic mechanism for pinball flippers).
> The control rods slot in between the fuel rods, and absorb the decay products without inducing further nuclear decay. Usually these are graphite rods.
AFAIU graphite rods increase fission by slowing (not capturing) neutrons which in turn have a better chance of propagating further fission, because .. physics.
Quite nifty actually - without the moderator, the fuel wont burn.
I think you're right; I misinterpreted the term 'graphite-moderated reactor' to mean something it doesn't. Graphite will slow the neutrons so they react more. Also, the Chernobyl reactor design has graphite tips on its control rods, which I misremembered as the primary substance of the rod.
The primary substance of the control rods is (usually) a neutron absorber, and most reactors with control rods have a passive safety system, so gravity and springs will force the control rods in to significantly slow the reaction unless actively opposed by the control system.
The Chernobyl rods had graphite ends so that when fully retracted, the reactor output was higher than if there was simply no neutron absorber present; unfortunately, this also meant that going from fully retracted to fully inserted would increase the reactivity in the bottom of the reactor before it reduced it, and in the disaster, this process overheated the bottom of the reactor, damaging the structure and the control rods got stuck, and then really bad things happened.
Long story short, most control rods don't have graphite. ;)
But as soon as I dive into the details, I get lost. How exactly can you control the nuclear decay? How exactly does do the gears in the transmission move around and combine with eachother to create a specific gear ratio? These concepts probably are probably pretty simple for a lot of people, but they just make my head spin.