178 control rods began to move in the active zone of the reactor. They were 7 meters long, moved at a speed of 40 centimeters per second, and were made of boron, which absorbed neutrons and reduced the rate of the reaction. The tips of the rods, however, were made of graphite, and the graphite tips appear to have tipped the already highly unstable reactor toward catastrophe. As the rods began to descend into the core of the reactor, the tips replaced neutron-absorbing water in the top part of the active zone, thus not decreasing, but further increasing, the rate of the reaction. This was the
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