The investigation is ongoing. I admit that the mechanism for the explosion is not clear. However, it appears that it was a pretty low level explosion, on the order of a propane or natural gas explosion. I say this because the (most likely) reinforced concrete cover for the safe room seems simply to have been lifted off, then fell back into the room. Here are some theories put forward thus far:
A room of stored ammunition and reloading supplies can hold enough explosive force to cause the damage seen in Burley, a local firearms expert said.
Safe rooms are often built very heavily, said Dan Hadley, reloading expert and employee at Red’s Trading Post in Twin Falls. If a spark occurs, he said, that ignites the stored gunpowder and the room can “become like a cartridge case with the pressure finding release at the weakest spot, which would be the roof.”
When gunpowder ignites, he said, it creates an expanding gas.
In rooms with a lot of stored ammunition and reloading supplies “it could easily blow a roof off,” he said.
Sparks or heat can come from several different sources. Hadley said if gun cleaning and oiling supplies are kept in a room in a closed container, it can lead to the possibility of spontaneous combustion.
“Something like that is more likely to happen in an auto repair shop where there are more rags,” he said.
He said although cleaning and oiling supplies are volatile, “they are safe if the directions are followed.” It’s always best to store them in a location separate from reloading supplies and ammunition, he advised.
Another source of ignition is some type of electrical short, Hadley said.
What you write is akin to a BLEVE — boiling liquid expanding vapor explosion. But in your write up the gas pressure is caused by the burning gunpowder which has its own oxidizer.
So essentially the ‘explosion’ was gas pressure — a burst if you will, of a pressure vessel— which introduced O2 to a combustible/explosive mixture (a back draft fire of sorts) and thus a ‘boom’ and only enough flame front to set her hair afire but not toast her.
Makes sense.
Plausible. AND interesting.