Mechanics of a Meltdown For all the potential hazards presented by nuclear power plants, there have been very few incidents that have caused human or environmental harm. However, those that have happened have often had disastrous consequences, for example, the accidents at Chernobyl in Ukraine and, just last year, at Fukushima Daiichi in Japan. Burns et al. (p. 1184 ; see the cover) review the state of knowledge on the chemical and physical processes following the nuclear reactor accident and how these results may inform decision-making during future events. Because a large portion of prior research has focused on radionuclide transport following leaks from nuclear waste repositories, and not active reactors, experiments at the more extreme conditions experienced in major nuclear core-melt accidents may have more predictive value.