Urgh! I read Chapter One before reading the prologue.

“Superman, I hate to have to say this, but I’m afraid that if you continue, well, being you . . .” Dr. Klein rotated in his chair to face the superhero. His hands fiddled with the buttons of his lab coat, and he was unable to meet Superman’s eyes. “Next time you’re exposed to kryptonite, you may not recover.”
As my fellow commenters have mentioned, this is an intriguing concept! Repeated exposure to Kryptonite which leads to a weakening of his molecular structure makes a great deal of sense. As with ordinary humans prolonged contact with any kind of radioactive material will have negative effects. Hmmm, he might have to consider retiring...
Oh boy, he has to tell Lois all about this! Now is not the time to be a Lunkhead!
Thanks so much for posting!