I remember from the HBO series that the final design error was the graphite-tipped control rods. The graphite accelerated the reaction, which jammed the rods a

I remember from the HBO series that the final design error was the graphite-tipped control rods. The graphite accelerated the reaction, which jammed the rods and prevented them from being fully inserted. Had the rods not been constructed with reaction accelerants, the disaster might not have been so severe.