A term used in nuclear engineering. As a verb, it refers to the act of raising or lowering the control rods in a nuclear reactor. As a noun, it refers to the distance control rods were moved.
Ex. Petty Officer Brockman shimmed rods for fifteen seconds, which resulted in a .5 inch shim.
Note: This example does not provide an accurate rod speed. This was intentionally done because actual control rod speeds are classified.
Note: currently working on a node for control rods that will explain why shimming is necessary.
So why is shimming necessary?
As the control rods are withdrawn from the core, fewer and fewer neutrons are absorbed by the rods and more neutrons are absorbed by the 235U fuel. At the point where the number of neutrons produced by the fuel is equal to the number of neutrons absorbed by the fuel is known as criticality. This is a good thing. A reactor must be critical to produce power. So, whenever you see a movie that exclaims, "The reactor's gone critical!" you know that they don't know what they're talking about. The term they were looking for was prompt critical.
Once the control rods have been pulled to criticality, then they must be pulled, or shimmed further. The reactor is still not yet at a point where it can produce electrical power. Reactor power is still at a level insufficient for producing steam. More shimming is required to raise the temperature of the reactor.
Once temperature has been raised to the normal operating range, occasional shimming is still required. Temperature will slowly drift because of the buildup or burning off of neutron absorbing poisons such as 135Xe.