The AZ-5 button in Chernobyl was the emergency shutdown (SCRAM) button of the RBMK-1000 nuclear reactor. When pressed, its intended purpose was to rapidly insert all control rods into the reactor core to halt the nuclear chain reaction and shut down the reactor in an emergency. However, due to a critical design flaw inherent in the RBMK reactor's control rods, pressing AZ-5 during the infamous 1986 accident at Chernobyl Unit 4 paradoxically triggered a catastrophic power surge, leading to the reactor's explosion.
The Purpose of AZ-5
In nuclear power plants, an emergency shutdown system is crucial for safety. The AZ-5 (Аварийная Защита 5 – "Emergency Protection 5") button was designed to activate this system in the RBMK reactor. Its primary function was to:
- Initiate a rapid shutdown: By fully inserting all 187 control rods into the reactor core.
- Absorb neutrons: The control rods, made of boron carbide, are designed to absorb excess neutrons, thereby slowing down or stopping the nuclear fission process.
- Prevent core meltdown: In a typical scenario, this action would quickly bring the reactor to a subcritical state, preventing overheating and potential meltdown.
The Fatal Flaw of AZ-5 at Chernobyl
The design of the RBMK control rods, specifically their tips and the sequence of insertion, contained a critical flaw that proved fatal during the Chernobyl accident.
- Graphite Displacement Effect: The RBMK control rods had 4.5-meter long graphite displacers (extenders) attached to their boron carbide absorbing sections. These graphite tips were initially located below the core when the rods were fully withdrawn.
- Water Displacement: When the AZ-5 button was pressed, the control rods began to enter the core from the top. As the graphite displacers (which are neutron moderators) entered the core, they displaced the neutron-absorbing water coolant in the lower sections of the fuel channels.
- Temporary Reactivity Increase: This displacement of water by graphite temporarily increased reactivity in the bottom part of the core, causing a localized surge in power. This effect was known as the "end effect" or "positive scram."
During the Chernobyl accident, this design flaw turned a safety measure into a catalyst for disaster. The operator pressed the AZ-5 button, initiating the insertion of control rods. As these rods started to enter the core, instead of immediately reducing power, they increased the reactivity at the bottom of the core. The power excursion rate emergency protection system signaled on, but by then, it was too late; the power quickly exceeded 530 MWt and continued to surge uncontrollably.
Sequence of Events Leading to Disaster
The pressing of the AZ-5 button was the final act in a series of unfortunate events and procedural violations during a poorly planned test at Chernobyl Unit 4.
- Preparation for Test: Operators were conducting a test to determine if the turbine could generate enough power to run the emergency water pumps during a power failure.
- Unstable Reactor State: The reactor was operating at a very low power level, making it inherently unstable and difficult to control. Xenon poisoning also played a role.
- Power Surge Begins: As the test progressed, the reactor's power level began to rise uncontrollably due to a combination of factors, including operator actions and the reactor's design characteristics.
- AZ-5 Pressed: In an attempt to prevent an uncontrolled power excursion, the operator pressed the AZ-5 button.
- Catastrophic Outcome: Instead of shutting down the reactor, the graphite tips of the descending control rods caused a massive, rapid increase in reactivity at the bottom of the core. This led to an extreme power surge that exceeded the reactor's design limits within seconds, causing fuel elements to rupture and steam explosions that destroyed the reactor building.
The table below summarizes the intended versus the actual outcome of pressing the AZ-5 button during the Chernobyl disaster:
| Feature | Intended Outcome (Normal Operation) | Actual Outcome (Chernobyl Accident) |
|---|---|---|
| Action Initiated | Rapid insertion of all control rods | Rapid insertion of all control rods |
| Core Reactivity | Immediate decrease in reactivity (subcritical) | Initial increase in reactivity (positive scram) |
| Reactor Power | Rapid decrease and shutdown | Catastrophic surge, exceeding 530 MWt, leading to explosion |
| Safety Result | Emergency shutdown, reactor stability | Reactor destruction, widespread disaster |
Legacy and Lessons Learned
The Chernobyl accident profoundly impacted nuclear safety worldwide. The AZ-5 button, and specifically the design flaw it exposed, became a symbol of the critical importance of robust and failsafe reactor design.
- RBMK Modifications: Following the accident, all remaining RBMK reactors underwent significant modifications to eliminate the positive scram effect. These included:
- Replacing the graphite tips with neutron-absorbing material.
- Increasing the number of control rods.
- Implementing faster insertion mechanisms.
- Establishing strict operating procedures to prevent low-power operation that could lead to instability.
- Enhanced Safety Culture: The disaster highlighted the necessity of a strong safety culture, rigorous training, independent oversight, and transparent communication in the nuclear industry. It led to international cooperation in nuclear safety standards.
The AZ-5 button, therefore, represents not just an emergency control, but a pivotal element in the Chernobyl disaster that exposed fundamental flaws in reactor design and operational safety practices, forever changing the global approach to nuclear power.
