The answer to that question is ZERO!!!!!!
That does not mean there is no default. To the contrary, the way they default when still holding the scepter of power is by the swap out the old for the new currency. They simply come forward and say look, we have these new purple dollars and they are worth a lot more than those green ones. It is typically done in that manner. The odds of just outright defaulting are ZERO if the same government is in place. In a revolution, they always repudiate the bonds of the previous government. So without a revolution, the odds of the present government just defaulting on the debt and remaining in power is ZERO.