Fed.gov will NOT allow FDIC to default, any more than they'd default on Treasury bills or savings bonds. Such a move would prompt a violent reaction - I mean, a pitchforks and torches at midnight type of a reaction - as it would completely delegitimize the government in the eyes of MOST people. (Including many government employees.)
They might rev up the printing presses and pay off everything with inflation-devalued currency, but they will not allow an actual default.