Define thermal runaway.

• A. An orderly, controlled release of heat to maintain stability.
• B. An uncontrolled, self-accelerating reaction where heat generation exceeds removal, risking rapid temperature rise and vessel failure.
• C. A phenomenon where temperature drops due to cooling.
• D. A process where a reaction stops spontaneously.

Answer: (D)

Thermal runaway happens when a reaction’s heat generation outpaces the system’s ability to remove that heat. In exothermic reactions, the rate tends to climb as temperature rises (often described by Arrhenius behavior), so a small temperature increase can cause more heat to be produced, which raises the temperature again in a self‑accelerating loop. If cooling or heat removal is insufficient, this positive feedback can lead to a rapid, uncontrollable temperature and pressure rise, potentially causing vessel damage or failure. This is why maintaining adequate heat removal and keeping temperatures within safe limits is essential. The other descriptions describe controlled heat output, cooling outcomes, or the reaction stopping, none of which capture the dangerous, self‑amplifying nature of thermal runaway.