re: TMI and Reactor Accidents
They considered it, and they knew it would happen. They just weren't tasked to design a solution to the problem. They knew the problem had a technical solution, so they never gave too much thought as to how complex, or expensive, it might be.
A comparable issue might be the space junk that is accumulating and multipying by collisions in near-earth orbit and which is now being recognized as a growing hazard to spaceflight. Come to think about it, all the bad aspects of our technologies tend to sneak up on us this way. By the time we recognize it as a problem, it's a doozy.