This accident can be caused by a fault of the operator or of the control system. The number of control rods which can be moved simultaneously is limited for safety reasons. For the same reason is limited the maximum speed of withdrawal. The maximum rate of insertion of reactivity is determined by the operational requirements, and in particular by the Xe override. The control system must be able to compensate the rate of increase of Xe after a power reduction, even in the most unfavourable configuration. This requirement can be fulfilled by moving more rods at a lower speed or one rod only at higher speed. In the case of accidental withdrawal, it must be assumed that the maximum number of control rods which can be moved simultaneouly are withdrawn at the maximum speed, and that their configuration is the one giving the maximum reactivity increase. In the study of this configuration the insertion depth and the effect of other control rods which may be inserted in the reactor must be considered, as well as the burn-up stage of the fuel elements surrounding the control rods.

When a scram occurs it may be necessary to suppose that one of the rods, at least, involved in the accident does not respond to the signal and continues to move out of the core. The maximum withdrawal speed is usually the one foreseen for reactor operation, but in some cases a higher speed may be possible. Rods inserted from below the core can fall out of the reactor, and the gas pressure in the reactor may shoot a rod out. Usually proper mechanical design can insure that these events are either impossible or extremely improbable.