That is absolutely ridiculous! Water molecules are not broken into hydrogen and oxygen using “intense heat”. They can be broken down through electrolysis. Intense heat turns water into steam it does not break water molecules down. Do a little research on the term electrochemistry and electrolysis.
I have... I was one of the few who stayed awake during chemistry class. And I have done years of experimentation with Hydroxy on vehicles. Boiling water does indeed produce byproduct gases of Hydrogen and oxygen. Especially at instant extreme temperatures.
But let me ask you this... How does an ELECTRIC arc welder work? Are positive and negative electrodes and a current involved? Could it also create electrolysis aside from the heat boiling it? Of course... Just as you yourself say here.
But since you pointed out it was not submerged welding. If they were not welding on that bottom seam and submerged on the inside of the tank my assumption does not apply. even though totally possible with real science to back it up. If they were not welding on that bottom seam and submerged on the inside of the tank my assumption does not apply.
Hydrogen + Chlorine gas + Oxygen + UV light = A big bang... I just gave you the video... Watch it...