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 with simple and consistent feedback. What does that mean? It means that if temperature in the 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 generator can have a positive temperature coefficient which leads to complicated control, necessitating many safety circuits to ensure proper operation and shutdown. Their failure mode is they go prompt critical, and no containment vessel can contain the explosion that would occur, so they are built without one.