They did do some survivability experiments at ground zero at atomic tests and found that some materials could stay intact. I recall reading that the biggest problem on paper for Orion was cooling the plate in space so it wouldn’t melt after a few pulses. I think they posited various ablative shielding materials plus some kind of active cooling. Of course also remember that once you are in space you don’t need to fire rapidly. One boom every hour or two or even longer would be fine. It could take quite a while to build up to cruising speed, and to decelerate.
Acceleration would require several times more charges since you have to accelerate all the deceleration fuel. With interstellar flight you have two massive problems: going fast enough, and then slowing down at the other end.
100 more years of R&D is not very long on geological or cosmic time scales. My point is that the math working for something as brute force as Orion makes the difficulty of interstellar flight a problematic answer to the Fermi paradox.
That hasn't been demonstrated. Project Orion was canceled before critical parts of technology were actually tested, and the mechanical viability of a pusher plate that has to absorb the impact of several nearby nuclear explosions in rapid succession is somewhat doubtful.