When you buy a US government bond, there is no collateral. You trust the US Government to repay you when the bond comes due. If they don't, you are just shit out of luck, end of story. Bad on you for trusting them.

It is fundamentally different than a car loan or a home mortgage where you sign papers that if you default they can take your car or home from you.

So no repo at all. But there would be extreme repercussions for US residents if they actually stop paying interest, or devaluate the currency so the interest is worth nothing.