shim rod

Shim rods are a type of control rod used in nuclear reactors to manage reactivity changes over time, primarily due to fuel depletion. Unlike safety rods, which are for rapid shutdown, or regulating rods, used for fine adjustments, shim rods compensate for the slow decrease in reactivity as fuel is consumed during operation.1

These rods can control a significant amount of reactivity but are operated slowly. They are often used in conjunction with regulating rods, being moved to allow the regulating rods to return to a more useful position. Shim rods are crucial for maintaining the reactor's power level over extended periods, ensuring stable operation as the fuel burns. In some cases, an undesired withdrawal of shim rods can lead to a reactivity insertion, necessitating the rapid action of safety rods to shut down the reactor.12