Nuclear safety and security called a nuclear meltdown. After shutting down, for some time the reactor still needs external energy to power its cooling systems. Normally this energy is provided by the power grid to which that plant is connected, or by emergency diesel generators. Failure to provide power for the cooling systems, as happened in Fukushima I, can cause serious accidents. Nuclear safety rules in the United States "do not adequately weigh the risk of a single event that would knock out electricity from the grid and from emergency generators, as a quake and tsunami recently did in Japan", Nuclear Regulatory Commission officials
Nuclear power of the fission reactor power-level, principally the nuclear engineering reliance on a negative Void coefficient of reactivity, a form of Passive nuclear safety. The chain reaction can also be manually stopped completely by inserting control rods into the reactor core. While again depending on the reactor design, a mixture of both passive and active Nuclear reactor safety systems, the most well known being the Essential service coolant system, operate to remove decay heat. Alongside this, a number of further Defense in depth (nuclear engineering) principles are relied upon, such as physical containment barriers. Some serious nuclear and radiation accidents have