The US governemnt is not going to fail to pay and principal on it's obligations as it comes due.
I believe the risk, if any, is to the value of the dollar in terms of goods and services, and in terms other currencies, as well as interest rate risk.
If one is going to hold US dollars, and most of us US residents will tend to do so, US debt is as safe as any and safer than most. So we come down to maturity or duration risk, and IMO one must stay short which means horrible current real and nominal returns.
Ha