In life, when you talk about nuclear energy, there always happens to be a guy who says that famous Einstein's equation. "Yeah, they just convert mass to energy, $E = mc^2$ ya know?"

When I think about that, all I learned about nuclear power resembles dominoes arrangements. You tonk a block and it falls. On its way, it tonks other dominoes and when it falls it releases energy (sound waves).
Quite same in the nuclear physics. You send slow neutron to a core. The core absorbs it, breaks and sends another neutrons and energy (electromagnetic waves).

So in the end, I see no domino blocks disappearing in this game. All we do is, that we tonk domino arrangements that has been built by old stars long time ago.

So why is this equation related to nuclear power? What mass disappears in nuclear power plants?