Alternator stator hollow core conductor blocking mechanism and thermohydraulic analysis

The alternators in French nuclear power stations are water cooled. When they first entered service, the cooling circuits were kept under a reducing atmosphere (nitrogen or hydrogen). However, after several years of operation, several installations, for the most part in the 900 MW- CP2 series, have been subject to repeated blockages requiring a large number of chemical washes to restore the flow through the stator rods. This resulted in a change in the conditioning, with a change over to air conditioning up to 1992. In spite of this modification, these alternators continued to show abnormalities in temperature resulting in the multiplication of chemical an mechanical cleaning operations. Faced with the increasing damage to the windings, EDF decided to proceed with destructive examination of the stators rods. The in-depth examination concluded that there were compact plugs of copper oxides partially or totally blocking the hollow core conductors. These plugs resulted from an oxide tree structure formation, in the form or ripples, which grows against the current. The examination concluded that the blockages were located exclusively at the exit from the bar, in the hot part. A flow model of a 1300 MW water box revealed a small variation in throughput between the hollow core conductors in spite if the disturbed flow. It, also confirmed that this clogging-up phenomenon is independent of the sections of the hollow core conductors. Recent knowledge acquired in the blocking mechanism of hollow conductors made it possible to experience an innovative on-line method to flush stator windings and to find operating conditions which prevent CuO precipitation. (authors)