What conditions lead to thermal runaway in reactors and how is it prevented?

Runaway occurs when heat generation exceeds heat removal capability, causing accelerating temperature rise. Conditions: highly exothermic reaction, insufficient cooling capacity, loss of stirring/mixing, accumulation of reactants, and operating near critical temperature. Prevention: proper reactor sizing with adequate cooling, emergency cooling systems, controlled reactant addition rates, temperature monitoring and interlocks, relief systems sized for runaway scenario, and inherently safer design (limit inventory, dilute conditions). HAZOP and reaction calorimetry identify risks.