What is control rods (CR)?
Control rods is a rod made of chemical elements capable to absorbing many neutron without fissioning themselves.They are used in nuclear reactor to control the rate of fission of Uranium and Plutonium.
Why control rods is important?
Control rods like a 'heart' in nuclear reactor.Because too few fission events can slow down and automatically stop the chain reaction.Too much fission can overheat the core and lead to a meltdown.That's why control rods is much important here.
Materials of control rods
- Silver,Ag
- Indium,In
- Cadmium,Cd
- Boron,B
- Hafnium,Hf
Nuclear engineers and technicians precisely control the amount of fission taking place by inserting control rods (upper left) into the fuel assembly(red box). The rods are made of a substance that readily absorbs neutrons, like graphite or cadmium. When things get too hot, technicians lower a few control rods into the core. The rods sop up some of the ricocheting neutrons, and the fission process slows down. The reverse is also true: control rods are removed to rev up the fissioning.
When control rods are lifted from the fuel assembly, neutrons (from the natural decay of uranium) bounce around and bombard other uranium atoms, causing them to split. This process gives off more neutrons and causes more splitting. This is a chain reaction. The heat generated from all this fissioning is converted into steam, which turns a turbine, which turns a generator that produces electricity.
REMEMBER, If the reaction gets too hot, the control rods are re-inserted to absorb neutrons. With fewer neutrons around, there is less bombardment and fissioning. The core cools; energy output slows down.
CR effectiveness
CR effectiveness is depends on the how many ratio of the flux at the location of the rod to the average flux in the reactor. Figure 1 show, when a reactor has one CR,the CR is must be place in center part of reactor core.The CR has a maximum effect when if it is placed in the reactor where the flux is maximum. At point A, if additional rods are added to this simple reactor, the most effective location is where the flux is maximum.
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Figure 1:Effect of control rod on radial flux distribution |
The exact value of reactivity that each control rods depends upon the reactor design. The reactivity caused by control motion is referred to as control rod worth.
Type of CR
- Integral CR worth ( 'S' shape)
Figure below show the result of a value of rate of change of control worth as a function of control rod position.
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| Figure 2: Integral CR Worth |
Function of integral CR worth curve is to define the ρ change due CR movement between two position. The integral CR worth is the total reactivity worth of the rod at the particular degree of withdrawal and is usually defined to be the greatest when the rod is fully withdrawn.
Differential CR Worth (Bell shape)
For figure tell us it has very low values at top and bottom of the core and a maximum at the center of the core. The curve has bell shape because of CR worth related to n flux and n flux max.Also,n flux max is highest in center of the core.
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| Figure 3: Differential CR Worth |
At the bottom of the core, where there are few neutrons, rod movement has little effects,so change in rod worth is very little.The effect become greater, when the rod approach the center on the core. Basically, from center to the top inverse of the rod per inch will applied here.
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| Figure 4: PWR fuel with control rod CLUSTER |
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| figure 4: BWR fuel with CROSS ROAD design |
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