the NRC requires 7 days of fuel for the back-up generators on site; and 28 days worth readily accessible. If the nuclear power generator(s) shut down, the core of nuclear fuel and the storage pools for spent fuel still need back-up electrical power from elsewhere to continue to circulate water to keep the zirconium cladding on the fuel rods from overheating and melting, as happened at Fukushima.

The robustness of the tanks and their mountings, of the piping running from the tanks to the generators, and of what do if roads to the nuclear plant are severed, are other issues.