Cryogenic, or some hybrid. If insulation involves some actual gap I'd imagine that reliable methods to detect any cookoff failures would be simple and having ac

Cryogenic, or some hybrid. If insulation involves some actual gap I'd imagine that reliable methods to detect any cookoff failures would be simple and having access to a near sonic airsteam should give plenty of emergency venting opportunity. You'd still have a serious problem, but more of the ETOPS kind than of the fireball kind.