I have a book "how chernobyl happened" by some Russian guy, which is maybe the scariest thing I have ever read. It doesn't seem to be available any more.
He said that the lower tips of the control rods were some alloy that
*increased* the reaction rate as they were initially lowered, maybe an interpretation of what you said above.There were heroes. As I read the book, I wanted to scream back in time to some of those guys DONT GO IN THERE.
After the initial mess settled down, crews drove pickup trucks through nearby fields and collected shards of fuel rods with their bare hands.