The Sensitivity of Positive Scram Reactivity to Neutronic Decoupling in the RBMK-1000

The sensitivity to the axial neutron flux distribution of the positive reactivity that may have been introduced on initiation of scram in Chernobyl-4 has been evaluated. It is found that the scram reactivity is positive and its size is remarkably insensitive to a wide range of axial flux distortion provided the flux shape is concave, which is a characteristic of neutronic decoupling of the core. In contrast, the scram reactivity is negative when flux shapes are convex, i.e., those that are a characteristic of strong neutronic coupling. This indicates that unless there were a significant number of control absorbers present in the core just before the accident to provide a convex flux shape, the chances that some positive scram reactivity was inserted to initiate the power pulse are high.