Modern nuclear weapons have what are called use control measures, which prevent their unintended detonation. For example, the warhead of a missile must meet certain conditions before it can explode, so that it cannot-or at least it is highly unlikely to-detonate in its silo or on board its submarine.
Furthermore, modern nuclear weapons have been carefully designed so that when they are in an inactive state, the materials required to combine and create a nuclear explosion are separated from each other. Nuclear weapons incorporate a total of six safety devices to prevent unintended detonation.