Cute theory. I assume that you are joking because that is not actually possible. UV light from arc welding cannot travel through water to ignite flammable gasses. And even if it were possible to boil a tiny amount of water at the bottom of a 1.5 million gallon tank. The tiny amount of chlorine vapors released would be dissolved before they go to the surface. And if this were a properly vented tank no gasses would have accumulated at the top in the first place.
Somehow this tank became over pressurized. I do not know how, but an explosion seems unlikely unless the "tie-in" was taking place at the top of the tank. The tank broke at the bottom and went flying through the air. There was no explosion as we would normally define one.
You have watched too many episodes of MacGyver.
“A chlorine bomb is a small explosive device which uses the pressure of chemically produced chlorine gas or other chlorine-containing gases such as hydrogen chloride to produce an explosion. The reaction produces an expansive increase in pressure, eventually rupturing the container.”