If an abnormal situation is detected with the reactor at power, requiring rapid shutdown, all the control rods are dropped into the core to shut down the reactor, the safety rods are then withdrawn (when it is deemed safe to do so). Safety rods provide a re­serve of negative reactivity which can be dropped into the reactor core if an abnormal situation is detected while the reactor is shut down.

In most magnox reactors this function is provided by a group of black rods, 10-20 in number, specially designated for this duty, and these rods are normally held fully withdrawn at all times. At Dungeness A, safety rods were originally provided, but these have now been incorporated into the bulk groups and the satetv function, while shut down is provided by the regulating rods and trim rods. At Wylfa, the last of the magnox series, no safety rods are provided be­cause the complement of other rods is deemed to be adequate to ensure safety at all times.

In AGRs the safety rod function is provided by some of the grey rods while the reactor is shut down. Prior to start-up the safety group are lowered to the same height as the remaining grey rods.

Safety rods are not formally claimed to provide protection in any faults, and the reactivity margin at shutdown does not depend on safety rods.

Secondary shutdown rods

In magnox reactors with steel pressure vessels some bulk rods, typically 25-40 in number and designated secondary shutdown (SSD) rods, are fitted with a me­chanical latch which releases the rod from its sup­porting wire or chain in the event of a rapid gas depressurisation, to ensure a rapid shutdown of the reactor. In all other respects the rods operate as bulk rods, and in normal operation an SSD rod is in­distinguishable from other bulk rods.