In case of US it is not a big problem..It is the European countries which make this topic interesting.
National debt is not a big issue, if there is a high possibility of decent long run growth rate. With growing economy, the % of debt will eventually go down. When this does not seem very feasible, countries use inflation to melt the debt as in most cases debt is not tied to any kind of real asset. Also, since $ acts as a reserve currency, exchange rate depreciation itself will erode some debt if needed.
Also, what matters in such cases is who holds the debt. In case of US,
foreigners hold approximately 32% of its public debt. So, reneging on debt will essentially mean reneging on its own public, because they hold a high % of the US debt.
Who holds the debt always matters because Japan has the
highest public debt as % of its GDP, but most of it is domestically held. So, again, reneging on debt does not make much sense for Japan because it is their own citizens who will lose the money. So, foreigners can still buy their government bonds or invest in Japan because they know, Japan would not default.
But it is definitely an interesting topic!!