I think it's easy to payback the $31 trillion dollars. Have the president direct the Secretary of the Treasury to print up a $31 trillion dollar note which is then handed to the Federal Reserve as completely payment.

The tricky question is what to do in the next few seconds/minutes. Personally, I would swallow the entire pill and have the Treasury Department immediately introduce a new asset-backed currency which has a maximum convesion payout from dollars to the new currency for each individual adult citizen of $50,000 (or $100,000 or $500,000 - some reasonable number) and at least for some time period (maybe forever) not be convertible to any foreign currency. That will get rid of all the billionaires and many multi-millionaires (no one needs billionaires anyway).

That's at least one way to handle it.

(edit - tillion to trillion)

Sometimes I wonder if nuclear war and mass extinction is the escape plan for this.

When the new electronic only dollar comes out be an early adopter or be screwed.

Do we really need to pay China and Japan for financing our debt? I think not! The rest will be on a case by case basis. I don't think Israel holds any bonds since they can't save and too busy sucking money out of our cash flow.