Okay, let's make this stupid-simple.
Imagine a steel ball, sitting inside a cardboard box.
The ball is filled with nasty stuff, but the ball contains it -- because steel. (Actually, layers of steel, concrete, and steel and concrete in reality, but I digress)
The nasty stuff in the ball is making hydrogen, rather than break the ball, the hydrogen is released into the box. The box fills with an air/hydrogen mix.
A spark is introduced. The hydrogen burns with the air, and the term "deflagration" is not just a clever euphemism for explosion, there is a very real difference between the two. In deflagration, a flame front travels through the material, usually causing expansion through heating and burning byproducts, but the flame-front travels slowly through the medium,
far slower than the pressure wave. In an
explosion also known as
detonation the decomposition of the explosive occurs
at the pressure wave, amplifying it and creating a shock wave or
brisance. A detonation, therefore, is far more destructive than a deflagration. For example, you can deflagrate as much flammable material as you want on one side of armor plate, and the armor plate will not burn though (okay, if you did it for hours, or with a very focused deflagration, i.e. a cutting torch, you could get through it, but again, that's off topic.) On the other hand, even a small amount of an explosive can cut through, deform, or even shatter armor plate. I used the word
deflagration very, very intentionally.
Now, back to the example.
The cardboard box is torn apart by the increasing pressure. If it were a balloon, it would puff up, but cardboard, like the thin corrugated steel walls (think every cheap warehouse you've ever seen in the movies) does not stretch. Thus it tears apart. This is an "explosion" of the building.
The steel ball sits happily at the bottom of the now shredded box. No "nasty stuff" has been released.
That, in the simple-stupid version, is what happened at Fukushima. The "steel ball" is the reactor vessel, unbroken, and not leaking. The cardboard box was the surrounding building. The fact that everyone else gets this and not you, means you have no idea how a reactor is designed, how it works, or what actually happened. To answer your, "what matters" question -- The explosion scattered *ZERO* material, The area around it was not contaminated by *ANY* radiation from the explosion.
All of the leakage of radiation came from leakage from the cooling tanks in the primary loop coolant water. No one has ever said, or ever believed that the reactor vessels suffered any major breaches. (Yes, there were some minor cracks and seal breaks that leaked contaminated coolant, but no one has shown or believes that any of the primary fuel melt escaped the containment vessel.)
