7. SECONDARY FUEL FAILURES
7.5. Circumferential fractures
There is broad agreement in literature that circumferential fractures are the consequence of heavy local hydriding leading to cracks and eventually to rod fractures when pellet cladding contact has been established in an area [7.1].
GE discussed the mechanism in more detail [7.4]. Early life failures should be more susceptible to circumferential fracture due to the still open pellet cladding gap and good communication conditions along the rod, especially when the primary defect is far away from the peak power location, i.e. when the location of heavy secondary hydriding by oxygen starvation coincides with the peak power location where the pellet cladding gap FIG. 7.20. Net increase in tubing diameter for barrier cladding with 100 ppm Fe (“B100”), 400 ppm Fe (“B400”) and non-barrier (NB) cladding [7.49].
FIG. 7.21. Summary of liner oxide thickness data from PIE results published by ABB [7.50], GE [7.51–7.54], INER [7.28] and Siemens [7.55].
closes earliest. This implies that a primary failure at the top more likely leads to fracture, since peak power is usually near the bottom. In fact, circumferential fractures are found in the bottom range, and GE has observed — with a primary failure near the bottom — heavily hydrided cladding in the top range of a rod without fracture, as no pellet clad has contact in this range.
This fully agrees with ABB observations and a similar description in [7.9], as reported circumferential cracks were found in the bottom area in conjunction with debris fretting at a higher axial level. According to Fig. 7.2, the average distance between primary and secondary failure was around two metres, which is quite compatible with oxygen starvation conditions. Of course, one cannot exclude an additional material effect on susceptibility to circumferential cracks, e.g. by the degree of hydrogen pickup and mode of hydriding. However, there is currently no direct evidence for a material effect.
Some years ago, EDF sent a broken fuel rod to hot cell for further examination in order to determine the cause of failure and to explain the fracture mechanism of the fuel rod [7.56]. The rod burnup was about 51 GW◊d/t U.
Metallographic investigations were performed on the axial sample and a radial cut was located close to the rupture.
It was shown that the high burnup rod had initially failed due to grid to rod fretting in the lower part of the rod.
Examinations confirmed that secondary hydriding of the cladding was the cause of these fractures. Severe secondary hydriding of the cladding caused the formation of hydride rings at pellet to pellet interfaces, as shown in Fig. 7.22. At the pellet to pellet interface, cladding areas are typically cooler than in adjacent areas and hydride precipitates preferentially, thus forming hydride rings. The hydride causes severe embrittlement of the cladding, which when then submitted to a stress, has a tendency to break at a pellet to pellet interface.
REFERENCES TO SECTION 7
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