Right now, most nuclear reactors are light water reactors. Light water reactors are very prone to meltdowns when their power supply is cut off. These reactors are extremely hard to maintain when their active cooling systems shut down and they release massive amounts of radiation into the surrounding area.
Molten salt reactors are more efficient and could even be used as smaller reactors to power individual cities. These reactors are also less prone to meltdowns. It is estimated that a molten salt reactor can run for 120 days without an operator. Also, designs for molten salt reactors have a fail-safe in case the power goes out. When electricity is cut off from the reactor, the fuel is dropped into a cooling tank directly below it.