What is the worst case szenario of the national debt?

The economy will collapse as will all the economy’s of the world.Than the money  will be devalued.

Prices in the US will increase significantly when the USD loses world currency status. The US credit rating has already been downgraded. That's a shot across the bow. It's the kind of thing you don't want to learn the consequences when it's too late. Somewhere else in the world they will be rejoicing and the US will drop significantly across the board.

The world stops using the dollar as the international "reserve" currency... and will likely be replaced by the Euro or the Chinese Yuan...this makes the holders and printers of those currencies more influential than the holders and printers of dollars...