Molten Salt Nuclear Reactors have a very high negative temperature coefficient leading to a safe and stable control. This is another beauty of the molten salt design. The temperature coefficient is highly negative, leading to a safe design enabling simple and consistent feedback. What does that mean? It means that when the temperature of the fissile core rises, the efficiency of the reaction goes down, leading to less heat generated. There is no risk for a thermal runaway. In contrast, graphite moderated generators can have a positive temperature coefficient which leads to complicated control, necessitating many safety circuits to ensure proper startup, operation and shutdown. Their worst failure mode is they go prompt critical, and no containment vessel can contain the explosion that would occur, so they were built without one. There have been several major accidents in graphite moderated reactors, with the Windscale fire and the Chernobyl disaster probably the best known.