Ch7: Chernobyl- The Biggest Nuclear Accident

Chernobyl (Ukraine, formerly Soviet Union) and Fukushima (Japan) serve as the two archetypal cases for large-scale nuclear accidents.
Both reshaped the public image of nuclear power and directly influenced regulatory frameworks, investment patterns, and social acceptance worldwide.

Scale of release
- $9$ tons of radioactive material (≈ $100 imes$ the Hiroshima bomb).
- $50$ different radio-isotopes, half-lives spanning from $2$ h to $24\,000$ y.
Root cause
- Human error: operators pushed the reactor to $120 imes$ its rated capacity.
- Control-rod fires produced large quantities of hydrogen gas and high-pressure water vapor.
Mechanical chain reaction
- Hydrogen gas ≈ highly flammable → surrounding fires.
- Steam over-pressure blew away a $40$ -ton concrete lid.
Immediate human impact
- $1\,000$ people injured, $31$ fatalities on site.
- $151\,000$ evacuated, many leaving belongings due to contamination.
Long-term impact
- Predicted up to $1\times10^{6}$ cancer cases over decades.
- Uncertainty remains because exact isotope mix (%) with $2$ -day vs. $20\,000$ -year half-lives is unknown—thus no clear timeline for environmental clearance.
- Fallout path: Belarus → Russia → Poland → Baltic region → Scandinavia.

Dual natural triggers: an earthquake + tsunami.
Floodwaters disabled backup diesel generators → loss of cooling water.
Three simultaneous core meltdowns (reactors $1$ – $3$ ).
Chemical side-reactions produced large volumes of H₂ → vented to avoid explosions, but venting also released $^{131}\text{I}$ (iodine- $131$ ) and other isotopes.
Explosions occurred in $4$ of $6$ reactor buildings, multiplying radiological releases.

Core concept: control rods.
- Inserted rods absorb excess neutrons → moderate or stop chain reaction.