This is the easiest perturbation to understand since there is a simple relationship between cause and ef­fect. It can readily be assumed that gas flow remains constant, as noted in a preceding paragraph. First, consider the disturbing influences which can cause re — activitv дк to deviate from zero and thereby cause a change in neutron power.

A change in the position of one or more control rods in the reactor core will directly change the value of reactivity. Rod movement may be intentional, such as that carried out either automatically or manually for temperature trimming purposes or to remove/ reinstate a rod for on-load control rod actuator main­tenance; or it may be a fault, such as a fault in an electrical control system causing a rod or rods to drive into or out of the core, or a contactor failure or supply failure releasing a rod to run into the core under gravity. Divergent faults such as rod run-out are covered by fault studies and protection is provided to ensure reactor safety, but prompt action by the control engineer may arrest the fault and keep the reactor safely on load. Such faults are rare, rod run — in being more likely, although convergent and safe, and prompt action by the reactor control engineer (assisted where appropriate by the auto control system on reactor gas outlet temperature) might keep the re­actor on load.