First I suggest to google 'self bunded fuel storage tanks'. You'll be busy for a while...
Secondly, the Australian standards could be your answer;- AS 1940 and AS 1692. Thousands of self bunded fuel storage tanks are used in Down Under, including other flammable liquids like methanol/ethanol, etc. Industrial, tank farms, small and large storage facilities.
The gap between shells is minimal, down to fabrication tolerances required to slide the inner strakes inside the external shell. Use some welded 'sliding strips' (3mm thick strips) in the lower half of external shell to support the inner shell and allow 'volume' for the fluid (fuel) to flood the interstitial space in case of containment loss of inner shell. This space will enable detection of liquid between the shells with a dip stick smeared in the end with water detection paste, diesel detection grease or alcohol detection paste. You will have to use the venting and overpressure regulations strictly, because that's the main protection for these tanks.
Cheers,
gr2vessels