Analysis of the vibration response of a safeguard piping system

Following the discovery of a longitudinal through-wall crack at the base of a pressure instrumentation tap on the system, Electricite de France started a huge campaign to check the safeguard pipes in its 1300-MW nuclear power plants. The analysis of several hundreds of taps of this type on-site revealed several tens of cases of cracking. These safeguard systems practically only operate during periodic test phases that generally correspond to partial now regimes for the pump and to hydraulic configurations that do not correspond to the main system function. These hydraulic configurations were clearly identified as open-quotes worst casesclose quotes for the system through vibration measurements during the startup tests, allowing the system to be tested under rated conditions. Experience feedback raises several difficulties in interpreting the damage; in particular, considerable differences were found in the vibration levels of the same tap on the same piping system from one plant unit to another. Also, the vibration measurements taken on the main pipe are not always correlated with the tap vibration level. To explain these phenomena, it was decided to model the discharge line of the 1300-MW low head safety injection system. To obtain the system response to pump excitation, a fluid-structure-coupled finite element model was built for which the boundary conditions are well controlled. Modal analysis revealed acoustic-mechanical coupling of the system in the environment of the discontinuous zone that cracked. Further analysis of this mechanical discontinuity showed that its first natural frequency is close to that of the coupled mode. Knowing the system transfer function, the force and displacement response are obtained