Is an explosion of an atomic bomb an uncontrolled chain reaction?

Yes, the explosion of an atomic bomb is the result of an uncontrolled chain reaction. In a nuclear weapon, a fissile material, such as uranium-235 or plutonium-239, undergoes rapid fission when a critical mass is achieved, leading to an exponential increase in energy release. Unlike controlled nuclear reactions in power plants, where the reaction is carefully regulated, an atomic bomb allows the reaction to proceed without any control, resulting in an immense and instantaneous explosion.