One ton of Zr can make about 88 lbs of H2, which is about 15,780 ft^3 of H2. That would probably be enough to take the roof off the buildings, especially as this would be uniformly distributed over a big area - like a fuel-air bomb. It would be ugly.

Ionizing water, or catalytically decomposing water at cherry red temps would make both H2 and O2 which could also recombine, especially with all that ionizing radiation. Maybe some ionizing radiation from a "hot" isotope of Xe or I2 kicked off the explosion that blew off the roof...or maybe just a motor or a switch that was not rated" explosion-proof".

Nb41