TL;DR they have several genes that are very good at repairing damage from radiation and they are expressed at high rates after damage.<p>HOwever I think the growing consensus in the tardigrade field is that the radiation resistance is mostly an unexpected byproduct of evolving to survive lower-moisture conditions, which are much more common than huge radiation bursts.<p>If there is nuclear war, if anything survives, it will be bacteria, algae and tardigrades, and then the cycle will begin again, but with tardigrade overlords.