looked like a fueled air explosive.
Of course you can make nuke mushroom clouds with Acetylene and plastic garbage bags.
The Confederate Air Force used to do simulated nuke detonations as part of their B-29 (FiFi) airshow routine. Not sure whether it involved the aceteline/garbage bag method that you mention, but I do remember it pretty well from an airshow in the early 80s when they were still cooking them off. Big flash, heat and concussion with a neat looking mushroom cloud.
I think they stopped that in the mid-80s when they triggered a diplomatic incident by setting one off at an airshow attended by a fairly high ranking delegation of Japanese military and civilian officials.