The rods consisted of the 7.5m boron portion and a 4.5m graphite portion. There is a 1.5m water gap on either side of the graphite part when pulled out.

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Regular water is slight neutron absorber. When they were trying to raise the power they had pulled out almost all the control rods. When they hit AZ5 it pushed the water out of the bottom of the control rod tubes. Displacing the all the water with graphite greatly accelerated the reaction locally in the bottom of the reactor. This caused the rods to get jammed due to massive thermal expansion. The reaction accelerated uncontrolled once this happened.