All reasonably modern reactors are built so that the reactor containment vessel completely contains the nuclear fuel, even during a period of full meltdown.
Even some basic cursory research on your part will help you understand the whys and hows.
GE/NRC documents also describe all these multiple layers of containment as an impenetrable defense-in-depth strategy.
Well, it turns out that the outer concrete building is mostly just environmental. The roof blows off and panels blow out. The pressure vessel has an assortment of through-holes for pipes and can't actually be trusted to hold in the temperatures and pressures seen by an actual meltdown. The concrete containment has an achilles' heel in this flimsy toroidial suppression pool.
When it came down to it, none of it was able to hold up to the temperatures and pressures and hydrogen explosions seen in this actual meltdown. Many of us were led to believe that inserting the control rods would immediately "stop the reaction" and prevent a meltdown from occurring.
These reactors melted down, all layers of containment were breached, radioactive materials were released into the environment, rendering a 20km radius uninhabitable for the forseeable future. And this is with the control rods in.
So sorry if people don't simply accept your word that such-and-such "completely contains the nuclear fuel, even during a period of full meltdown". I don't think you really know.
I'm not sure how much you were "led" to believe that, given that the control rods became fully inserted immediately after the earthquake, where they did in fact "stop the reaction". The entire crisis revolved around the removal of decay heat, not halting the reactors.
These reactors were presented as fail-safe systems. It turns out that they do not, in fact, fail into a safe state. They are more like airplanes over the ocean, literally months away from the nearest safe landing spot.
The entire crisis revolved around the removal of decay heat, not halting the reactors.
Perhaps you find that to be a meaningful distinction, but most people do not. Especially when the result is fundamentally similar: a meltdown, containment breach, evacuation, contaminated residential area, etc.
I mean this in the :-) nicest possible way :-), but I don't think you 'get it'.
[1] The term is not officially defined by the International Atomic Energy Agency or by the U.S. Nuclear Regulatory Commission. However, it has been defined to mean the accidental melting of the core of a nuclear reactor, and is in common usage a reference to the core's either complete or partial collapse. "Core melt accident" and "partial core melt" are the analogous technical terms.