Observations from experience

In GE barrier fuel performance summaries [7.3, 7.4], the following damage distribution was given for 83 failed rods of 3.36 million rods in service:

— 25 axial splits >6 inch (30% average);

— 3 circumferential fractures (3.6% average);

— 49 minor damage;

— 6 uninspected.

GE states that circumferential fracture had been previously observed at a low frequency and is normal in the case of severe hydriding damage. Axial splits often correlate to control rod withdrawal, whereas a power increase is not required to initiate a circumferential fracture in a heavily hydrided cladding area.

In a combined paper with EPRI, Anatech and GE [7.5], hot cell examination results for seven rods were reported, highlighting three failed and three non-failed barrier rods, and one failed non-barrier rod. The failure distribution is shown in Fig. 7.1. Two of the failed barrier rods were degraded to axial splits with fuel washout. The

primary failure cause of the non-barrier rod and the non-degraded barrier rod was PCI. The other two rods had failed due to debris fretting.

All failed rods had suffered extensive secondary hydriding. The two degraded barrier rods showed evidence that most barrier oxidation had occurred after crack formation. The axial splits were initiated in a heavily hydrided region, but propagated in a brittle fashion through regions of moderately hydrided cladding. In the two non-degraded rods, the inner clad oxidation of the non-barrier rod was 30% to 40% heavier, but a comparison was difficult, since this rod operated in a failed condition significantly longer than the barrier rod. It was concluded that hydriding and hydrogen content in cladding assume a significant role, but the extent of cladding hydriding in itself is not sufficient to cause axial cracks. A severely hydrided rod, however, is degradable when there is a sufficient power rise. It was not possible to identify a simple direct correlation between one particular design feature and the propensity for degradation.

Since GE blamed cladding with small size precipitates for the splits, one of its remedies was ‘re-coarsening’

of the cladding, using only a solution quench on the outer portion of the cladding to maintain small size precipitates for nodular corrosion resistance at the outside surface, while the iron content in the liner was increased for better steam corrosion resistance. Two fuel failures were observed with this coarsened cladding, with primary debris failures just below the top spacer. The first rod had no visible secondary damage, but had fractured at the lowest spacer level during handling. The second rod had two circumferential fractures 64 cm and 74 cm in elevation, but no splits were observed.

ABB made the following observations regarding the failures [7.6, 7.7]:

(i) Seven axial splits >15 cm in liner and non-liner SVEA 64 fuel were observed, including five primary PCI failures. The largest split involved a sponge Zr liner rod. Splits were not observed in 10 x 10 fuels or in Zr–Sn liner fuel (a remedy for secondary degradation);

(ii) Sixteen circumferential fractures were reported, all in the lower part of the rod with primary debris failures mainly in the upper part of the rod. There was no correlation to local or average maximum LHGR, and power suppression did not decrease degradation. Circumferential fractures were less severe than axial splits in terms of fuel washout;

(iii) For five failures at high burnup >35 MWd/kg U, there was no or only minor degradation.

FIG. 7.1. GE failed fuel rods and symmetric sound rods examined in hot cells [7.5].

The frequency of both axial splits and circumferential fractures is judged to be high, which is supported by local TVO experience [7.8], with 14 failures in Olkiluoto-1, including one exception; a PCI failure in Siemens 9 ¥ 9 fuel, and 11 failures in ABB SVEA fuel in Olkiluoto-2, mainly through debris fretting, but including three PCI failures in SVEA 64. Whereas the 9 ¥ 9 fuel in Olkiluoto-1 did not show severe degradation, in Olkiluoto-2, the three PCI failures contained two rods with axial splits. The other eight failures (including two uninspected) revealed five rods with circumferential fractures.

A total of 18 failed rods were examined by ABB in hot cells [7.6, 7.7, 7.9]. Secondary hydrides and primary PCI cracks have functioned as incipience for crack propagation to axial splits. The very long split in the failed sponge Zr liner rod was attributed to large tensile hoop stresses in the cladding from rapid and extensive oxidation of the liner, thus promoting crack propagation. The circumferential fractures were a consequence of massive local pick-up of hydrogen and mainly occurred at low burnup, presumably due to the open pellet clad gap leading to good steam communication. The absence of severe degradation at high burnup was attributed to the closed pellet clad gap. Circumferential fractures occurred in the bottom part of the rod with primary fretting failures near the top, as is shown in Fig. 7.2.

The Krümmel plant (KKK) had a total of 10 failed barrier rods, as shown in Table 7.1 [7.10], with five degraded to axial splits >15 cm (ranging from 18 to 208 cm), two non-degraded, and four uninspected rods. Primary damage was believed to be caused by debris fretting, though this was not positively identified in PIE. Two failed rods with long axial splits were examined. One rod with a split had been in the reactor for nearly a year whilst the second had operated with an open crack for 29 days. The axial splits seen were considered to be secondary defects.

The rods operated within PCIOMR, so PCI was not considered to be the root cause of the initial failures, but they did experience power ramps which may have influenced secondary cracking. One rod showed significant failure following a power manouever to reduce power in the other, pre-existing failed rod. [7.10].

From the KKK, the two rods with a spiral crack and a 95 cm axial crack (No. 2 and 3 in Table 7.1) were examined in hot cell, beside one sound rod [7.10]. It was concluded that oxidation of the barrier supported hydrogen production in the fuel rod, but that alone was not the root cause of the long axial split. It instead supported the theory of gap closure and tangential cladding stresses leading to strong radial orientation of hydrides observed in both fuel rods. Thus, a major cause of the axial splits was seen in the evenly distributed, as opposed to localized, hydrogen pick-up and the radial hydride orientation. Minor power ramps are then sufficient to start crack propagation.

FIG. 7.2. Locations of primary failures and circumferential breaks on fuel rods for ABB SVEA fuel [7.7].

In reports from all three hot cell programmes, it was emphasized that hydrogen produced through oxidation of the pellet surface contributes significantly to clad hydriding.

7.2.2. PWRs

In PWR fuel rods, the types of defect are essentially the same as those in BWRs except for severe axial cracks or splits which are less frequent than in BWRs. One of the rare cases of axial cracking is shown in Fig. 7.3.

Typically, more secondary damage is located in the upper part of the rod in the region of high power and heat affected by the welding zone (upper end plug).

During the operation of Cattenom Unit 3 cycle 8 (15 Sept. 1999 to 27 Jan. 2001), a large number of fuel rods failed due to grid to rod fretting at the bottom grid level [7.11]. Coolant Xe-133 activity began to increase as early as October 1999 and continued to slowly increase in magnitude through June 2000. This coolant activity was consistent with the formation of primary defects caused by grid to rod fretting. From June through to the end of the cycle, coolant activity of short lived soluble fission products (I-134) and actinides increased considerably, indicating the development of secondary degradation. During on-site examinations, several large secondary defects such as circumferential breaks were observed on peripherals rods of leaking FAs. Two examples of circumferential break are shown in Fig. 7.4. The secondary break, perpendicular to the axis of the rod and on the edge of the broken area, indicated that the rod broke off at a pellet to pellet interface.

Since then, several similar defects have been observed on high burnup rods affected by fretting problems at lower grid levels in EDF reactors.

Similar secondary hydriding damage by circumferential cracks has also been observed in some United States of America PWRs over the last several years.

7.3. DEGRADATION CHARACTERISTICS