What it sounds like you're saying is there's a larger window depending on certain variables for the rods to go fully hot? I don't think it's prudent to go into detail, but that's essentially correct. Nothing changes in the fuel rod from storage to installation in a reactor that is shut down with the control rods fully lowered or in some equivalent condition. The risk level for the fuel rods changes fairly rapidly after the control rods are raised and the reactor goes critical.
True, it wouldn't be prudent. Sorry for asking. I just spoke with a nuclear physicist I know, couldn't understand much but he elaborated a bit more, thanks for the answer nevertheless. Of course I'm the same way with my past hx with nuclear power...I don't ever tell a soul, just the basics, raw basics; sadly getting older I use a little pun I created or maybe heard it somewhere and just think it's mine:
"I've forgotten more than I ever remembered( goes with my tag line)."