Here are two basic concepts that should be understood by every electrical engineer in the nuclear power industry: redundancy and diversity. They are closely related, but not the same thing . . . .
There are so many critical pieces of equipment in a plant that, oftentimes, just a single failure of one of them could be catastrophic. If a containment isolation valve failed to close during a large-break loss-of-coolant accident, it would defeat the containment building’s purpose and allow radioactivity to escape to the public. That’s why there is always a backup isolation valve in series with the first; if the first fails, the second will operate to perform the isolation function instead.
Most such safety-related equipment requires electricity to operate properly. Not only is maintaining power during and after an accident or event (such as an earthquake) critical to the operation of isolation devices that keep radioactivity contained, but power is needed to run the pumps that cool the core.
Power is critical for ensuring that the fuel rods don’t melt, especially during an emergency. That’s why the NRC requires that all nuclear power plants have backup power. Not only must they have backup power, but they must have redundant sources of backup power. It is here where we enter the realms of redundancy and diversity. They are critical to a plant’s licensing basis because they are required in its design basis.