I've cut open a bunch of UST’s that had contained jet fuel, fuel oil, diesel, gasoline etc. Gasoline is definitely the most volatile and you almost always have a combustible atmosphere in tanks that have any gas at all remaining in them. Obviously you check the atmosphere with a CGI (combustible gas indicator) before opening up both ends of the tank. Usually that's done by pumping an inert gas into the tank or flushing the tank with compressed air while cutting and of course monitoring all the while.
The heavier fuel tanks very rarely have a combustible atmosphere. They may possibly if they're hot enough inside for the fuel to flash off creating a combustible atmosphere but in my experience that was rare. I'd usually go ahead and flush the tanks just to be sure, but I always liked cutting open fuel oil, jet fuel tanks etc as opposed to gasoline.
I never believed the nonsense about a spark inside the center tank on that 747. Even if somehow an ignition source was generated inside the tank the chances of a combustible atmosphere existing are close to nonexistent.
Yup.
Begin rant mode...
I look at it from practical redneck experience that mirrors your more technical. Sometimes simple works well enough to get the right answer ;)
In the not so distant past before OSHA regs, people used gasoline to degrease parts. The result was flaming people in too many cases. Because of exactly the combustable atmosphere created around the 55 gal drums used as degreaser tanks. So they switched to Diesel/Kero. Result? Parts washers in semi close proximity to open flame glowing hot wood stoves that NEVER exploded.
Now the inside of a fuel tank is a lot like a garage with a diesel (petrolium) based parts washer, Enclosed. But with lots of open flame (torches/grinders/wood stoves/OIL stoves). Those sparks do not ignite the washer or cause an explosion. I have seen SHOWERS of sparks DIRECTLY hit such. No fire. No boom.
Pour a cup of Diesel/Kero or jet fuel on a white hot coal bedded campfire. Boom? Nope. No boom. Burn, but no boom.
The conditions have to be perfect. Such as atmosphere to be compressed 22-1’ish in a Diesel, in the compressor stages of a turbine etc. before Diesel/Kero/Jet will ‘so easily’ ignite.
See diesel truck in winter for an example of how hard it can be to ignite.
One cannot look at the millions of aircraft flights and conclude that by miracle, this aircraft just suddenly defied decades of experience, reality and science.