A nuclear power plant or nuclear power station (NPP) is a thermal power station in which the heat source is a nuclear reactor.
Initially, nuclear reactors have been created for atomic weapons. However, a method to use the nuclear power for peaceful purposes has been found.
The nuclear reactor and weapon work identically: energy is released during a nuclear chain reaction. The minimum amount of fissile material needed to start a reaction is called the critical mass. The reactor covers that the accident on the NPP, if it happens, will not have such destructive consequences as an atomic bomb explosion.
The potential hazard of a power reactor lies in the accumulation of radioactive substances during the operation. To prevent these substances from entering the environment, the principle of defense-in-depth protection is incorporated into the NPP design: a number of installed consecutive physical barriers reliably keeps radioactive substances within specified limits.