The sudden rupture of the main steam outlet pipe at one of the HRSGs providing steam to the steam turbine was reported to have seriously damaged piping upstream and downstream. Luckily no injuries were reported. The fracture occurred suddenly (no leak seen prior) at the weld between P91 pipe and Stainless Steel Flow Meter. The rupture was brittle and clean occurring at the interface between the Inconel weld and the P91 pipe. Dissimilar Metal Weld failures for these materials are not uncommon, especially in a cycling regime.