"Fast reactors have always had this potential problem" - Not really. Check out the dramatic experiments at EBR-II (fast reactor) demonstrating unprotected los

"Fast reactors have always had this potential problem" - Not really. Check out the dramatic experiments at EBR-II (fast reactor) demonstrating unprotected loss of coolant at full power. It shut itself down. Without damage. And started back up later that day, and operated that year, as a research reactor, with better uptime (capacity factor) than the fleet at the time.