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The avgas tank(s) were built into the hull design of the ship. They were triple walled with a water jacket surrounding them on all sides. With all the other precautions built in, removing them or repurposing them would have entailed major shipyard time and disassembly of a big chunk of the hull to do. It was better just to leave them in place unused.
I cannot fathom where a third 'wall' would go (perhaps a double cofferdam?) but otherwise is pretty much as I described: water filled cofferdam surrounding a self-contained low-flashpoint tank. I believe there's also a requirement for the tank to be backfilled with water or an inert gas. The purpose of the water is to ensure that if you do get leaks, you at least just get water in your fuel. That makes whatever you were fuelling not work, but it's a lot better than explosive vapours making your ship not work in a very terminal fashion. It's seriously heavy, space consuming and a general pain in the backside unless you genuinely need the stuff.That sounds more extensive and space-consuming than the protection @Yellow Palace described above, is that the case? IIUC these big refits gave big increases in fuel capacity right when it was needed.
There'd be no disadvantage to cutting into the inerted pipework and using the tank for high-flashpoint fuel (probably F-44 in practice) but on a big ship like ENTERPRISE with plenty of fuel it's perfectly likely that it was deemed not worth the effort for a minimal increase in tankage.
I'm saying 'low flashpoint' because while it could be F-18 (AVGAS) , in the case where I worked with it was (IIRC) F-67. Petrol/gasoline, as you prefer. Likewise, high-flashpoint fuel would probably be F-44, but could conceivably be F-54 or F-76. Or some other weird things, but mostly those. If you want to use F-77 for some reason, you'll need special tanks again.