On Feb. 16, 2002, Davis-Besse (Oak Harbor, Ohio) nuclear plant personnel were repairing cracks in the vessel head penetration (VHP) nozzles. While being machined, the nozzles which were supposed to be embedded tipped over. Further inspection identified a large penetrated cavity of 20 to 30 square inches. The cavity penetrated completely through the 6.63 inches of carbon steel to the thin stainless steel cladding liner. The liner (0.38 inch) was all that was preventing a large loss of coolant accident with potential catastrophic consequences.