Directly below the Can is the Fuelsalt Drain Tank (FDT) shown in green in this view. In the bottom of the Can is a fuse valve shown in gray. The fuse valve is merely a low point in a drain line. At normal operating temperatures, the fuelsalt in the fuse valve is frozen, creating a plug. But if the Can overheats for any reason, the plug will thaw, and the fuel salt will drain to the FDT. Since the drain tank has no moderator, fission will stop almost immediately. This drain is totally passive. There is nothing an operator can do to prevent it.