Nodular corrosion of zirconium tubes in channel-type reactors

Systematic visual observation of fuel channels in nuclear power plants with channel-type reactors showed that the surface of tubes made from the zirconium alloy E125 remains undamaged for some time, and then undergoes changes, which are signaled by nodular (pitted or ulcerous) corrosion. This corrosion starts roughly after 400 effective days of operation after reaching a neutron fluence of around 7E20/cm2 and appears as bright convex open-quotes pointsclose quotes with diameters over 0.1 mm to white or gray spots of diameter 1-2 μm. Formation of the first nodules is observed in the zone of maximum neutron flux, that is, in the center section of the fuel tubes at a length of 2-3 m. With time the damage spreads to annular regions. The destruction (opening up) of the nodules also starts in the center of the tubes after 700 effective days of operation. The birth of nodules in tube sections with a protective oxide film stops after 1500-1800 effective days, at which time 3 to 15% of the total surface is covered with nodules. Continuous surface whitening of specific tubes from the growth and sloughing off of corrosion centers is observed at a neutron fluence of 4.5 · 1021 cm-2. At locations where the lattice spacers of fuel elements bundles contact the fuel tubes, the rate of surface corrosion damage is much higher. This damage appears in the form of annular zones of continuous oxidation which correspond to the lattice dimensions. These zones are clearly observed on an unchanged dark background of the rest of the surface and are confirmed by eddy current monitoring. Accelerated damage development of the fuel tubes under the lattice is caused by fretting corrosion and results from local mechanical destruction of the surrounding protective oxide film