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As mentioned above, the WWER-440 collectors are made of the same Ti-stabilized stainless steel as the tubing. The WWER-1000 collectors are made of the same low alloy bainitic steel (Type 10GN2MFA) as the vessel, with stainless steel cladding on the inside surface. The tubes are embedded against the collector wall by explosion or hydraulic expansion and welded at the collector inside surface using argon-arc welding. Collector-tube crevices generally do not exist, however, some "under-rolling" of the heat exchanger tubes into the collector wall has been reported, resulting in crevices with depths up to 20 mm (explosive expansions) or 2 mm (hydraulic expansions).

PtinBiy Water Stress Cbnosion Cracking (PWSCQ Research has shown that stress corrosion cracking of austenitic stainless steels and nickel based alloys requires at least the following three conditions: susceptible tubing microstructure (alloy content or few intergranular carbides), high applied or residual tensile stress (near the yield strength), and a corrosive environment (high temperature water). The influence of the nickel content on the stress corrosion cracking processes in 18% chromium austenitic alloys when stressed slightly above the yield point of the material in demineralized water or water containing 1 g/L chloride ions is shown in Fig.

The tubes that actually leaked primary coolant are a small proportion of the tubes plugged or repaired. Steam generator tubes are sometimes plugged as a preventive action if they are judged to have a high probability of future failure. 28 100 73 74 75 76 77 78 79 80 B1 82 83 84 85 86 87 88 89 90 91 92 93 FIG. 13. Worldwide causes of steam generator plugging (EPRI 1995a). Copyright 1996 Electric Power Research Institute; reprinted with permission. The Alloy 800M tubing used in the Siemens/KWU steam generators has performed relatively well.

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Ageing of Steam Generators (IAEA TECDOC-981)

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