TD what is a control rod please? And what does the acronym SCRAM stand for?
Reactor control rods act as a sort of throttle in the sense that they moderate the reaction. Here's an easy way to visualize what's happening using Uranium 235 as an example:
A neutron (Small grey circle on the left) strikes a U-235 atom causing it change into U-236 momentarily and then split into two smaller atoms. (Barium and Krypton in this example.) These smaller atoms require a lot less energy to hold them together, so the excess energy is released as heat. The split also releases two or more extra neutrons, which may strike other U-235 atoms resulting in more heat and more neutrons. This doubling is called neutron multiplication and it can quickly get out of control.
In commercial reactors, the control rod mechanism is a bundle of about 20 rods composed of materials which absorb these extra neutrons, which prevents them from traveling into adjacent fuel pellets and rods. This in turn lessens the thermal output of the reactor.
In English, a SCRAM is an emergency shutdown of a reactor. The control rods are completely inserted and water based solutions laced with the same sort of materials as the control rods themselves are injected into the reactor coolant.
There isn't any real agreement today over the etymology of the word. In the very early days of nuclear research, scientists would experiment on table tops (I kid you not) and if something went wrong, the best thing to do was to run, which is one possible explanation for the term. Other explanations are possible acronyms the word could have stood for, like Safety Control Rod Activation Mechanism, but again, there's no real consensus on this.