How do control rods in nuclear reactors work?
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This is from what I remember from my time at Naval Nuclear Power School and as a qualified reactor operator on A4W reactors.
In order for uranium to break apart and release energy (fission) and subsequently release more neutrons, it has to absorb a thermal neutron (a neutron at relatively low energy). In order for a neutron to become thermal, it interacts with water molecules and imparts energy on the water molecule. A good way to imagine this is to picture what happens when you sling a pool ball at a stationary pool ball sitting on the table; the one that you whipped almost stops and the formerly stopped one now has a bunch of energy. The fast neutron (pool ball you threw) will collide with the oxygen (I *think*, I can’t remember if it was hydrogen) part of water (H2O) and slow down.
With all that said, the goal of the control rods is absorb those neutrons instead of them being absorbed by the uranium. Hafnium is a very common component in control rods due to the large neutron absorption cross section (the ability and probability of something to absorb neutrons).
When the control rods are inserted into the core, it is said that we are adding negative reactivity, because the number of neutrons available to become absorbed by uranium goes down over time because they are absorbed by the hafnium which doesn’t release new neutrons. When the control rods are withdrawn, we add positive reactivity. We control reactor power, in a way, by positioning the control rods at some point in the core. (This is grossly over simplified, as steam demand actually drives reactor power, but I won’t get into that here). In the event of an emergency when the reactor is at power and needs to be shutdown immediately, the reactor operator has the ability to “SCRAM” the reactor, or literally drop the control rods to the bottom of the core.
I hope this helps a bit.
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