First of all, the circular shape of those buildings is one of the most stable known to man. You would need an insanely powerful tornado to even damage the exterior of the building. If a tornado could destroy the building, all of the households in immediate danger of radiation poisoning would be long gone, but lets entertain your idea a little further.
The government is well aware of the fact that nuclear power plants could potentially kill a lot of people if they were to suffer a meltdown, and therefore have the highest safety standards possible. Keep in mind that these things are built to withstand an enemy attack. The nuclear material that is at the moment being used to generate power is located in the safest part of the buildings: underground surrounded by more steel and concrete than the president's whitehouse bunker. An earthquake could potentially cause damage to this protection, but that is because earthquakes send waves through the ground which carry much more force than wind going much, MUCH faster than record tornados. Even if the whole building were to come crashing down, the reactors (and the generators which keep them cool) would be safe.
As for the material that is nuclear waste or the material that has not been used yet, they are stored in these:
They are designed to contain material that will be dangerous for the next 10,000 years. A fucking meteor would have a tough time busting it open.
Sparknotes: A tornado-induced meltdown is probably somewhere around 1000 times less likely than a nuclear attack from another country.
