Nuclear accident history — identify the worst radioactive pollution event The world’s worst radioactive pollution from a nuclear reactor disaster occurred at which location?

Introduction / Context:
Understanding major environmental disasters helps frame modern safety standards and risk management. The worst nuclear-power-related radioactive release in history is a well-known landmark event.

Given Data / Assumptions:

• Comparing famous incidents: Chernobyl (1986), Three Mile Island in Pennsylvania (1979), and others.
• Metric is overall radioactive pollution and environmental impact.

Concept / Approach:
The Chernobyl Unit 4 accident in the then U.S.S.R. (now Ukraine) involved a catastrophic power excursion and steam explosion followed by a graphite fire, ejecting large quantities of radionuclides into the atmosphere, contaminating broad regions and prompting long-term exclusion zones.

Step-by-Step Solution:

Verification / Alternative check:
International assessments (e.g., UNSCEAR and IAEA reports) document the magnitude of releases and health/environmental consequences, confirming Chernobyl’s severity.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing nuclear weapons tests or other incidents with civilian reactor accidents.

Final Answer:
Chernobyl (undivided U.S.S.R.)