That explosion is clearly carrying concrete following the hydrogen gas and steam release.

The core is possibly exposed for that particular reactor at this time. Perhaps the steel containment vessel is intact but is obviously very hot, very brittle and contains melted U238 fuel and other isotopes.

4 more reactos could go at a total of well over 100 tons of bomb grade metal

A fission explosion would not have been captured by camera 2 miles away because the camera would have melted.

A picture would need to be taken many more miles away to record any unlikely chain reaction explosion.

Which is worse, fission or meltdown?

They are nearly equally bad but a meltdown would allow decades to try to cap the mess in a mountain of concrete. But being right at the sea's edgem a concrete masuoleum offers little containment.