Degradation characteristics

The sequence of events prior to secondary degradation is fairly well understood and documented in a number of publications [7.13–7.17]. The process begins with a primary defect that allows coolant to enter the fuel rod. Any type of primary breach can, in theory, lead to a secondary failure. Primary failure modes include grid to rod fretting, TABLE 7.1. DEFECT BARRIER RODS AT KKK [7.10]

FA Number Type Burnup, MW◊d/kg Rod Position Failure Off-gas Activity

DA 039K 9 ¥ 9-5 18.700 c1 180 mm split moderate

JB 075K 9 ¥ 9 QA 7.522 g1 250 mm spiral split moderate to high

JB 039K 9 ¥ 9 QA 10.640 k9 950 mm split high

HA 144K 9 ¥ 9 Q 27.422 f3 230 mm split high

HA 057K 9 ¥ 9 Q 36.000 g7 crack moderate

HA 082K 9 ¥ 9 Q 33.000 d7 40 mm split moderate

KB 019K 9 ¥ 9 QA 18.300 f9 2080 mm split high

KB 077K 9 ¥ 9 QA 17.610 — not yet examined moderate to high

HA 067K 9 ¥ 9 Q 35.858 — not yet examined moderate to high

HA 087K 9 ¥ 9 Q 41.431 — not yet examined moderate

have been dramatically reduced in recent years. In addition, extensive application of barrier fuel (and operating restrictions on non-barrier fuel) has considerably reduced the frequency of PCI defects. Today, fretting is the most common initiator of primary failure. This is significant because, unlike other types of defects, frets are not sharp enough to initiate a longer crack. When degradation follows a fretting failure, it is always at a secondary site. . Steam starvation

When water enters a fuel rod through a primary defect, it flashes to steam and begins to react with the fuel and inside surface (IS) of the cladding. Steam continues to enter the rod until equilibrium with the system pressure is reached. Oxygen is stripped from the steam through the two simplified corrosion reactions below (a process known as steam starvation):

FIG. 7.3. Example of an axial split failure in a PWR fuel rod [7.12].

2H₂O + Zr → ZrO₂ + 2H₂ H₂O + UO₂ → UO_2+x + H₂

Steam is also subject to radiolytic decomposition, generating additional hydrogen and hydrogen peroxide.

The gas mixture becomes continuously enriched in hydrogen, with a maximum concentration some distance from the primary defect.

Hydrogen absorption

When the gas mixture becomes sufficiently enriched in hydrogen, absorption occurs by breaking down the once protective oxide on the inside of the cladding. The exact hydrogen to steam ratio at which rapid absorption occurs (the ‘critical ratio’) has been found to depend on a number of variables, including material type, thickness and integrity of the oxide and test temperature and pressure [7.18–7.20]. In a series of tests at 350°C and 7 MPa (the BWR system pressure), Kim et. al. [7.20] found the critical ratio H₂/H₂O for sponge zirconium to be between 1000 and 5000. Above this value, massive hydriding was inevitable, although it could be somewhat delayed with thicker pre-oxidized films.

Once conditions for absorption are reached, hydrogen is rapidly absorbed into the cladding. This gas phase absorption mode is much faster than the hydrogen diffusion rate in the cladding and blisters on the inside surface are formed. In the presence of a thermal gradient, the hydrogen is slowly transported to the outside surface (OS). Given enough time, all hydrogen above the solubility level will move to the OS and (potentially) form hydride blisters. At intermediate stages of the process, the hydrogen is distributed in a ‘sunburst’ pattern as shown in Fig. 7.5. The sunbursts are often found to align with cracks in the fuel pellet, which is likely a function of hydrogen (and fission gas) access to the cladding surface

FIG. 7.4. Example of circumferential (left) or ‘guillotine’ (right) break.

FIG. 7.5. Massively hydrided region with hydride sunburst [7.21].

Modes of secondary failure

Post-irradiation examinations of failed fuel rods have highlighted several stages in secondary degradation.

The different types of secondary damages affecting failed rods are classified in the following way:

a. ‘Sunburst’

This failure reflects localized hydriding radiating from the cladding. An illustration is given in Fig. 7.6.

b. ‘Blister’ or ‘bulges’

This is a local increase in the volume of cladding reflecting massive hydriding, typically with radial hydride precipitation under the ‘blister’. An illustration of this type of defect is presented in Fig. 7.7.

c. Perforation or holes

The final stage in the evolution of the ‘sunburst’ or ‘blister’ is perforation or holes; fuel is visible. Some examples of perforations are presented in Fig. 7.8.

d. Small cracks

These can be of different types, longitudinal, transverse or circumferential and often leave a ‘sunburst’. The different types of small cracks observed are presented in Fig. 7.9.

FIG. 7.6. Sunburst.

FIG. 7.7. Bulges.

FIG. 7.8. Holes.

e. Severe degradation

Deterioration of a fuel rod beyond the previous stages can lead to two specific forms of severe degradation.

One form is axial split, an example of which is shown in Fig. 7.10 in a BWR fuel rod. Axial splits can form either by extension of a primary defect (called a ‘propagating primary’) or by initiation and growth from a secondary hydride. The path chosen depends on the acuity of the defect, stress distribution along the rod and the local environment inside the fuel rod. Propagating primary splits are particularly interesting because the splits form (by definition) in an oxidizing environment away from any massive hydriding.

The particular significance of long axial cracks is that they tend to cause a high release of off-gas activity and can also result in large coolant contamination through greater fuel loss.

FIG. 7.9. Small cracks.

FIG. 7.10. Schematic and poolside visual of an axial split from a Hatch-2 reactor [7.22].

Although less frequent, a few cases of secondary axial splits were also reported in PWRs. An example of an open axial split observed on a PWR failed fuel rod is shown in Fig. 7.11. The hole, or cracked hydride blister, in the middle of the split suggests the crack propagated in both directions after nucleation at the location of the hydrides.

A second form of degradation is the circumferential break, in which cladding is massively hydrided around enough of the circumference to literally break into two sections.

For BWRs, these failures tend to occur in rods early in life, when the fuel–cladding gap is still open, and in the high power region of a rod [7.23]. Here the pellet–cladding gap is smallest and the hydrogen absorption rate is highest [7.14]. Hydride concentrations can be further localized in cladding at pellet–pellet interfaces that operate at slightly lower temperatures than the bulk of the cladding. The stress that produces the break is likely a combination of volumetric expansion (associated with the phase change to a zirconium hydride) and thermal expansion of the fuel column. The nominally open pellet–clad gap can be closed by oxidation, reduced conductivity and volume expansion of the hydrided region itself. Because of the susceptibility of zirconium to form hydrides in the presence of dry hydrogen gas, this type of failure is not easily mitigated.

For PWRs, secondary hydriding damage through a circumferential crack as shown in Fig. 7.4 has also been observed, especially in high burnup fuel rods.

In PWRs, circumferential breaks are frequently observed in the heat affected welding zone of the upper end plug. Figure 7.12 shows a fracture at the weld position of an upper end plug; no lower end plug fracture was found.

Since there is very little information published on circumferential breaks, the remainder of this review will focus on axial splits.

PIE characteristics

Over the years, a consistent description of degraded cladding has been developed from post-irradiation examination (PIE) at several hot cell facilities, including Studsvik [7.24, 7.25], GE Vallecitos [7.26, 7.27] and INER [7.28].

A low magnification SEM image of two fracture surfaces from a split in the Chinshan reactor is shown in Fig. 7.13.

The curved, periodic markings (wrongly called ‘striations’ by some) have been found on all splits reported to date and are a key identifying feature of the crack mechanism. More recent fractography by GE at Vallecitos on well preserved split fracture surfaces has indicated that curved features are always perpendicular to the crack front and are more accurately described as chevron patterns [7.27]. These findings have since been confirmed by an examination at Studsvik [7.25]. As shown in Fig. 7.14, chevron patterns (or half chevron patterns, when a crack leads close to the outer surface) define local crack advance through the varying micro-structural features encountered along the crack tip. A cross-section of the crack, shown schematically in Fig. 7-14c, shows that chevron patterns are roughly square channels with a brittle fracture along surfaces in the fracture plane and a ductile fracture along vertical surfaces connecting the channels [7.25, 7.27].

FIG. 7.11. Open long axial split from a PWR fuel rod [7.11].

FIG. 7.12. Fracture at an upper end plug welding [7.11].

Overall, the fractures are quite brittle and radial through the cladding. As shown in Fig. 7.15, the only significant ductility is at the inner surface. In this case, the barrier exhibits a knife edge failure. More recent work by Studsvik has shown that non-barrier cladding also exhibits some ductility (measurable as wall thinning) at the inner surface [7.25].

A metallography specimen from a split in Oskarshamn 3, shown in Fig. 7.16, revealed hydrides concentrated at and aligned with the crack tip. This turned out to be a common observation in degraded cladding [7.24, 7.28], and suggested a DHC-like mechanism was operating. Unfortunately, it could not be proven that hydrides precipitated at the crack tip were involved in the fracture, since they could have just as easily precipitated during cool down in the stress field of the crack tip.

The key characteristics of axial splits based on present knowledge are summarized below [7.13, 7.29]:

— Axial splits have mainly been observed in BWR fuel rods;

— Crack initiation occurs at sharp flaws (massive hydrides, PCI defects, etc.);

— Distinctive fractography is characterized by a chevron pattern;

— Crack advance through a combination of axial and radial propagation with crack fronts lead to near the outside surface;

— Macroscopically, brittle crack propagation is indicated by radially oriented fracture surfaces and little measurable plastic deformation;

— Fracture surfaces are brittle in appearance since they consist of quasi-cleavage facets (or at least of featureless micro-areas, which are similar to quasi-cleavage facets) and at a microscopic scale show a perfect fit of the two opposing surfaces;

— Hydrogen (or hydrides) is/are involved in the fracture process;

— Brittle crack propagation in cladding often occurs well away from massively hydrided regions, with hydrogen as low as ~150 ppm;

— Sections at the tip of axial clad splits show precipitation of hydride in front of the crack tip;

— Ductile separation is observed in the zirconium liner (slight ductility also observed at the inner surface of non-barrier cladding).

FIG. 7.13. SEM fractography of a split [7.28].

7.4. MECHANISMS