How do control rods in nuclear reactors work?
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Think of the uranium rods as emitting ping-pong balls. The balls bounce against the other rods, and when a ‘pong’ happens you now have 2 ping-pong balls and the process continues.
The control rods are like putting your hand in between the rods. Depending on how far in you put your hand, few or a lot of the balls will hit your soft hand instead of the hard surface of the rods and fall down into the pit below (away from the all the rods) where they stop doing anything.
Thus you can control how frenetic the ping-pong action is by how far in your put your hand.
Watch the last episode of Chernobyl on HBO Max. It does a really good job is explaining it.
I guess these control rods are like nuclear bouncers, keeping the party from getting too wild!
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They control the rate of the nuclear chain reaction in the reactor itself. The chain reaction, once started, pretty much goes on by itself, but if you let it go uncontrolled, it only grows more intense and hotter. Nuclear chain reactions are propagated by atoms (the atoms of the fuel like uranium) being hit by neutrons that are produced by fission of previous atoms. The control rods are made of materials that absorb neutrons, which means they prevent the chain reaction from happening. If you have all the control rods inserted in a reactor, the reaction chain cannot be sustained and the reactor shuts off. Since there’s many control rods, removing or inserting rods is the way in which the rate of the chain reaction is controlled, and thus also its power output and heat output.
Fission creates neutrons that fly around and cause more atoms to split. Control rods absorb those free neutrons so they can’t cause a chain reaction. This causes the reaction to slow and, if there are enough control rods, stop completely.
The way neutrons and different materials behave together is an interesting science.
Hope this makes sense
When the uranium fissions, it generates a few fast moving neutrons. There are a few things that can happen to those neutrons. They can go right through some materials, can be absorbed into the nucleus of another atom, or they may bounce off the nucleus of other atoms. Different material have different probabilities for what happens when a neutron approaches it, and sometimes what happens is also affected by how fast that neutron is travelling.
In the reactor, the fuel pellets or rods are incased in material that appears essentially invisible to neutrons (zirconium alloy). The bundle is surrounded by material that neutrons tend to bounce around in, like billiard balls on a billiard table. This material is called a moderator, because it moderates, or slows down, the speed of the neutrons. Common material for this is water, or graphite. Finally, if the neutron is going the right speed and passes back into a fuel bundle and encounters a Uranium 235 nucleus, it will be absorbed, and that atom fissions, starting the whole process all over again.
The whole process is a balancing act of creating sufficient fissions to create sufficient neutrons, so that the reaction keeps occurring. Obviously, you don’t want it to run away on you, so that’s where control rods come into play. They are made of material that likes to simply absorb neutrons and hang on to them, no matter how fast the neutrons are travelling. If you insert enough control rods, there will be fewer and fewer neutrons available to find a uranium atom and cause another fission, and the reaction slows to a halt. As you withdraw the rods, the fission reaction speeds up. So, with lots of rods, you can control the fission reaction throughout the reactor.
As a side benefit, you can use Cobalt as your control material, and make Cobalt 60, which has many uses.
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.
Nuclear fission happens when a neutron smashes into the core material which breaks off neutrons from the atoms. The neutrons being broken off is where you get the immense release of energy in the form of heat. Water is used tp cool the materials. That cooling water is used to generate steam. That steam turns a turbine which converts it into electricity.
Hot rock make hot water, hot water make steam, steam make iphone and tesla juice.
The control rods absorb neutrons thus interrupting the self perpetuating cycle of nuclear fission.
They absorb the neutrons. When control rods are inserted into the reactor the absorption of neutrons slows down the reaction. How at the same time this is where heat control comes into play. Cooling is an extremely important part of reaction control.
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