CANDU reactors have experienced two types of feeder degradation:

• Pipe wall thinning due to flow accelerated corrosion (FAC) and

• Cracking.

FAC wall thinning has been seen at most stations while cracking has only been observed in a few situations. The thinning rate of the feeder pipes has been shown to be dependent on water chemistry, particularly the pH and the electrochemical potential. The mechanistic understanding of feeder cracking is still limited. To date, inter-granular cracks have been observed on both the inner and outer surface of the first and second bends on the outlet feeder and on one repaired weld. Inner surface cracks are postulated to be caused by stress corrosion cracking. Figure 10 shows the schematic diagram of feeder pipe.

Header ^

Weld

Channel

Fig. 10. Schematic diagram offeed pipe.

Outer surface cracks are currently believed to be caused by low temperature creep cracking, assisted by hydrogen ingress due to feeder thinning. The single crack on the repaired weld is currently believed to be an interrelation of both mechanisms. It is believed that all cracks are caused by unrelieved residual stresses induced during manufacturing or welding, plus other factors such as material susceptibility and chemical environment.

Due to the currently insufficient understanding of feeder cracking mechanisms, feeder pipes with known cracks cannot continue operation and must therefore be repaired or replaced. Current practice is such that, if an inspection identifies a crack, the cracked piping section is removed and replaced with new pipe. Upon crack discovery, the inspection scope is expanded to the sites deemed to have a similar risk of cracking. Substantial inspection for cracking has been performed at most plants. In some cases, 100% of bends considered susceptible were inspected. Since the discovery of the crack in a repaired weld in 2003, the risk and number of repaired welds, the safety case for continuing operation with any such potentially affected feeders, and surveillance methods and their implementation have been assessed.

Feeder Ageing Management Programmes have been developed and are updated periodically to account for inspection findings and subsequent assessments. This programme includes inspection plans at each planned outage and R&D plans. In addition, improved feeder inspection tools have been developed to inspect inaccessible sites. More reliable leak detection systems are also being considered.