The second reports on Chernobyl; INSAG-7

The primary design cause of the accident, as determined by INSAG-7, was a major deficiency in safety features, in particular, the "positive scram" effect due to the control rods' graphite tips that actually initially increased reactivity when control rods entered the core to reduce reactivity. There was also an overly positive void coefficient of the reactor, whereby steam-generated voids in the fuel cooling channels would increase reactivity because neutron absorption was reduced, resulting in more steam generation, and thereby more voids; a regenerative process. To avoid such conditions, it was necessary for the operators to track the value of the reactor operational reactivity margin (ORM) but this value was not readily available to the operators.