FABRICATION OF MECHANICAL MODULE Q601 FOR WEC AP1000 NUCLEAR POWER PLANTS

The mechanical modules for the WEC AP1000 design presented unique challenges for fabricators, but none more so than the Q601 Depressurization Module. Q601 is the biggest mechanical module, at 52 tons and 32 feet long, and is fabricated from box beams of A517 material, a high strength Q&T material with special welding requirements. The entire module is an ASME Section III NF support. The frame is supported on four A-frames made of box beams with complex angles. The flatness tolerance on the four base plates is +/- 0.030 inches. Stainless steel pipe spools up to 16 inches diameter and valves weighing up to 1-1/2 tons are then fitted and welded into the frame while lying horizontal. The entire module is mounted on a ring girder fitted to the top of the pressurizer, inside containment. The ring girder is fabricated in one piece to control distortion, but after machining and post weld heat treat for stability, is cut in two and flanged to allow it to fi t over the pressurizer nozzles. In all there are (3) post weld heat treatments and (3) machining stages. Most important, the machining of the ring girder bolt holes must match the base plates on the Q601 frame. Upon final paint touch up, the Q601 module is skidded out of the shop and loaded onto a low boy trailer. The Q601 presented First-of-a-Kinds for fitting, welding and machining. Mock-ups, welder training and a cautious, measured approach by an experienced fabrication team are key elements to success.