[hydrogen7800]

> It can be very difficult to weld some of the materials used in these systems.

Yes, lots of dissimilar metals welded together, using TIG, inertial/friction, electron beam, and probably other welding techniques.

>Cracks in welds can be hard to find, although I would expect every weld in a system like this to be X-ray inspected, and maybe also penetrant tested.

Indeed they are, and not just the welds. Any fracture critical component should have 100% nondestructive testing performed. This includes radiographic, liquid penetrant, helium leak test, etc., often all of the above. Like you, I would also be surprised if there were a single component or subsystem that was not subjected to these tests