Hydrogen burn INSIDE the primary?

There has been a theory floated that the explosion at Daiichi No. 2 which was heard to have occurred in or around the suppression pool may also have been a hydrogen explosion...

The writer is reminded that at Three Mile Island, after the hydrogen burn inside the containment building there was suspicion that there could actually be another hydrogen burn INSIDE the primary coolant system.

Considering the estimate that the entire core at No. 2 plant was exposed for about six hours, it's possible that there was a hydrogen burn inside the primary and that this was what caused the pressure pulse and explosive sound. This is all very speculative.

At present time (0711 AM Eastern 3/15) TEPCO still says only about half of the core at No. 2 plant is submerged.