To clarify, the first two explosions degraded the containment buildings only, their internal reactor vessels are still intact. This third blast was heard inside the reactor containment vessel and it is likely the vessel walls were degraded in some way. If so, melting uranium fuel rods would be exposed to atmosphere.
NHK reported 10,000 times more than acceptable radiation after the explosion...
Reactor #3 is the one to watch.
It has Plutonium as part of its fuel source.
Plutonium is far more deadly than Uranium and takes a very long time to decay substantially.
The report doesn’t share your analysis. This is all conjecture.
You’d expect a spike in radiation if the containment were compromised, which hasn’t occurred.
Two, the core is subcritical now, which means that even if it were exposed, it wouldn’t be able to go critical as what happened in chernobyl.