A gray building model with a chimney. - Model: Building model - Material: Plastic - Features: Building with a chimney - Color: Gray - Condition: Unpainted - Scale: Approximately 1/6666 Accident Overview • Date and Time: April 26, 1986, 1:23 AM (local time) • Location: Chernobyl Nuclear Power Plant, Unit 4, Soviet Union (present-day Ukraine) • Reactor Type: RBMK-1000 (graphite-moderated, water-cooled) • Type of Accident: Uncontrollable power excursion → Steam explosion → Graphite fire • International Nuclear and Radiological Event Scale (INES): Level 7 (the worst ever) Reactor Building per Unit • Height: Approximately 70 m (Equivalent to about a 20-story building) • Width and Depth: Approximately 100 m × 100 m • Weight: Hundreds of thousands of tons --- ■ Reasons for the Accident (Technical + Human Error) ① A dangerous experiment was being conducted A safety test was being conducted to see "how much emergency power could be supplied by the inertia of the turbine during a power outage." However, this experiment was conducted under extremely dangerous conditions: • Inexperienced shift personnel at night • Suspension of safety devices that were originally prohibited • Operation with extremely reduced power --- ② Defects in the reactor design itself The RBMK reactor had significant flaws. • Positive void coefficient • A dangerous design where power increases when coolant decreases • The tips of the control rods were made of graphite • When the emergency shutdown button (AZ-5) was pressed, the power momentarily surged --- ③ The decisive moment (01:23:40) • Operators pressed the emergency shutdown button (AZ-5) • → Power went out of control during the insertion of the control rods • → Hundreds of times the rated power in a few seconds • → Steam explosion → Building destruction → Ignition of graphite Thank you for watching. 2WS