In more general terms, thermal runaway happens when you have something that makes more heat as it gets hotter.. .. .One of the most common examples of thermal runaway is the common or garden household lightbulb. Over time, material evaporates from the filament, evenly to start with, but there are always some places where a little extra vapourises. Those bits then get thinner than the rest of the filament, have less metal in them to conduct the lectrikery and have a slightly higher resistance. By the wonders of Ohm's Law -- voltage equals current times resistance -- these thinner segments get a higher voltage across them than the filament next to them, and tend to get hotter still. Thus, even more filament gunk evaporates, the thin bit gets thinner still and... eventually, there's a bright flash as the last bit goes out in a blaze of glory.. .. .R