Dear all, I would like to learn more about what happens to CRs during the reactor cycle. At the beginning of the cycle large excess reactivity is present in the core and has to be compensated with burnable absorbers (lumped in rods and WABA coating), chemical shim and CRs. There are several things I am not sure about. I presume that CR are just gradually extracted from the core as the cycle progresses. But is it rally as simple as that? I heard that some rods remain in the core throughout the entire cycle. In my opinion this is done to counter some sort of excess reactivity that remains even towards the end of the cycle. (Things are even more confusing, since in a PWR different CR are present i.e. half- length and shut down rods. In BWR only one type of absorber crosses are present.) Eventually the absorber in the rods will start to burn-out. So I guess that CR are exchanged to prevent swelling. Therefore, I would like to ask what happens to a CR during the cycle.