Thanks to this page
I can tell you your friend was in at most a 2.4 PSI overpressure situation, which is more than adequate to break bricks (2-3 PSI), without causing eardrum rupture ( > 2.4PSI). So there's your "Hydrogen Explosion" overpressure.
RDX, which is a real explosive, detonates at 25 times the speed of sound, or about 8500 m/s. It devolves about 900 liters of gas per kilogram during detonation, creating overpressures close to the detonation center that can exceed 3000PSI, which will reduce concrete to dust and melt metal to slag through compression heating.
This versus the H2 + 2O2 --> 2H2O reaction where three liters of gas at STP become 2 liters of gas, which expand to about 200 liters because of heating to 2800C (100:1 expansion). The flame speed of hydrogen/oxygen is about 50 m/s
maximum. The expansion of the gas pressures the container until something breaks (a few bricks in the wall, some windows, or in the case of Fukushima Daishi, the failure of some corrugated metal siding and roofing materials.) Then the pressure escapes into the environment, and no further damage is done.
This is the difference between something burning, like a HydrOx reaction, and something exploding
. This is what you can't seem to grasp here. There was an "explosion" in the colloquial sense that a surrounding building was rapidly disassembled. But in the pure physics sense, there was a contained burn that ruptured its container. The H/O reaction may seem fast in our everyday experience, but in the realm of explosives and explosions, it's moving at a snail's pace.
The BWR3/4 reactors were designed for operation in excess of 1000PSI operating pressures - in fact, the primary coolant loop in the U.S. PWRs typically runs at 2250PSI. Meaning that your "EXPLOSIONS" (caps doesn't make it true) wouldn't have had the slightest effect on the reactor or the coolant loops.
Additionally, the corrugated steel outer walls would have failed at a 2 PSI overpressure, which they did. Again, because this is a burn (deflagration) and not an explosion (detonation) there would be no point in the process where the reactor would have been exposed to a higher pressure than 2-3PSI in the containment building.
Much like putting a garbage bag filled with hydrogen/oxygen on a bank vault inside a glass building, you blew out all the windows, but you're no closer to the money.
So, again, there might be someone out of their depth in their conversation, but it's not me.