Sequence of Events: Chernobyl Accident Appendix 1

Overview

• Date: 26 April 1986
• Event: Destruction of Chernobyl 4 as a result of a power transient during a safety system test.
• Cause: Poor safety culture, inadequate reactor design, lack of operator awareness, and non-compliance with operational procedures.

Accident Description

Test Objective

• Objective: Determine if the slowing turbine could power core cooling pumps during a loss of station power, until diesel generators started.
• Previous Test: Had failed due to rapid power drop-off; new test used improved voltage regulators.

Issues and Errors

• Coordination Lapses: Poor exchange of information between test and reactor safety teams.
• Safety Precautions: Inadequate precautions were included in the test program.
• ECCS: Emergency Core Cooling System shut off for the test, reflecting lax safety attitude.

Sequence of Events

1. 25 April 1986

   • 01:06: Scheduled reactor shutdown started.
   • 03:47: Power reduction halted at 1600 MWt.
   • 14:00: ECCS isolated; power maintained at 1600 MWt due to grid demands.
   • 23:10: Power reduction recommenced.

2. 26 April 1986

   • 00:05: Power decreased to 720 MWt.
   • 00:28: Power unexpectedly dropped to 30 MWt due to control system issues.
   • 01:00: Power stabilized at 200 MWt; ORM violation occurred.
   • 01:03: Additional cooling pumps engaged.
   • 01:23: Test commenced; turbine stop valves closed.
   • 01:23:40: Scram button pressed; control rods inserted.
   • 01:23:43: Power excursion rate signals activated; power exceeded 530 MWt.
   • 01:23:49: Pressure increase signals and equipment failures.
   • 01:24: Severe reactor shocks logged.

Explosion and Immediate Impact

• Explosions: Two recorded, likely from steam and hydrogen build-up.
• Core Exposure: Destruction exposed core to the atmosphere.
• Casualties: Two immediate deaths from explosions.

Consequences of the Accident

• Radioactive Release: Plume of smoke and radioactive materials released into the air.
• Firefighting Efforts: Extensive firefighting efforts involving over 250 firefighters to control fires.
• Materials Dumped: 5000t of materials including boron carbide, lead, sand, and clay to control fire and radioactivity.
• Reactor Cooling: Cold nitrogen system installed by 5 May to cool the reactor.

Timeline

• April 25-26: Detailed timeline provided to track actions and decisions leading up to the accident.
• Post-Accident: Measures taken to mitigate consequences and provide cooling.

Additional Notes

• Xenon Poisoning: Contributed to difficulty in power stabilization.
• Operating Reactivity Margin: ORM violations were significant.

References

• Various INSAG reports and expert testimonies provide detailed analysis and context for the incident.