That is false, at least 2 of the reactors were or are melting down. Happily the containment vessels are holding. We know the reactors were or are melting down because of the hydrogen explosions. Hydrogen is produced inside the reactor during a melt down.
Water breaks apart into Oxygen and Hydrogen at aprox 2,000C the same temperature that melts the fuel rods. Notice the word "melt".
The reactors are not melting down. The fuel rods have overheated due to loss of coolant but the reactor is still intact and inside the containment.