Ageing of Steam Generators (IAEA TECDOC-981)

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3. 4. 5. 6. Cracks in U-bends typically are axial in orientation, though occasional off-axial cracks have been detected. Cracks in standard roll transitions are mostly axial, though occasional short circumferential cracks occur between axial cracks. Rarely are isolated circumferential cracks detected. When the tube expansion is kiss rolled the crack is always axial and the growth rate decreases as crack length increases. Some large circumferential cracks have been detected in the sludge pile area of kiss-rolled plants in France.

Wastage Phosphate wastage was the major cause of tube failures in PWR steam generators until about 1976. However it is no longer an active degradation mechanism in most of the PWRs because phosphate water chemistry is no longer used. 2. PWR ONCE-THROUGH STEAM GENERATOR TUBES Once-through steam generators in the USA use the same Alloy 600 tubing materials as RSGs, yet these steam generators have experienced substantially fewer tube failures. The lower failure rate is attributed to the differences in the steam generator design, manufacturing processes, and operation.

Freespan IGA/IGSCC can occur if there are deposits on the tube, which concentrate impurities. The sensitivity of the eddy-current signal is poor and a special analysis in absolute mode is needed to detect a freespan flaw before the flaw achieves a critical size. Tube ruptures have occurred due to freespan ODSCC. 13. Fretting, Wear and Thinning These steam generator degradation types are broadly characterized as mechanicallyinduced or -aided degradation mechanisms. Degradation from small amplitude, oscillatory motion, between continuously rubbing surfaces, is generally termed fretting.

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