Failure analysis: Vapor overheads discharge line for high level radioactive waste evaporator

A 2 in. schedule 40 steel (60.3 mm diameter, 3.91 mm wall) core pipe in an evaporator overheads discharge line broke at several locations downstream from a section of the line that bridges a road. The failure occurred during pump restart. Fish mouth openings that developed along the pipe seams were initiated at lack of fusion defects in the pipe welds. Full drainage of the water from the pipe did not occur when the pump was shut down because such drainage created an upstream vacuum in the piping system. Freezing of water in the pipe (the line was not heat traced) and water hammer during pump restart each contributed to the extensive deformation and tearing observed at the breaks. The weld flaws served as crack initiation sites and the water hammer provided the overpressure that led to the fish mouth fracture. The pipe was replaced and a vacuum break was installed to eliminate the drainage problem.