A third fault lay in the heart of the reactor’s emergency protection system, the last line of defense against an accident. If the operators faced a situation calling for an emergency shutdown—a major coolant leak or a reactor runaway—they could press the “scram” button, activating the ultimate stage of the unit’s five-level rapid power reduction system, known in Russian as AZ-5. Pushing this button would drive a special bank of twenty-four neutron-absorbing boron carbide control rods—as well as every one of the remaining 187 manual or automatic control rods that remained withdrawn at the
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