Manufacturing defects

The number of failures resulting from manufacturing defects has decreased over the years owing to improvements in manufacturing processes. However, such failures still occur and include mainly end plug and welding defects as well as, in some cases, tubing reduction flaws. An isolated event of PWR fuel failure by Ni

FIG. 5.39. Ramp test results for PWR Zy-4 and CONCERTO rods [5.87].

FIG. 5.40. Improvement of CANDU fuel elements PCI threshold due mainly to CANLUB.

contamination of the cladding surface due to improper loading machine operation during assembly has been described. In addition, fuel failure supported by insufficient free volume has taken place in CANDU fuel.

Incomplete welds and leaking end plugs have a common characteristic: the primary defect is a small hole near the end of the fuel rod or element. Because of the initial size of the hole, both types tend to limit the amount of coolant ingress and fission product release to the coolant. This means that these defects can remain undetected in the core until the primary hole size increases or secondary damage develops. The incubation period for secondary hydriding damage depends on the initial primary hole size, fuel temperature and other parameters as described in Section 7.

There has been a current tendency for fuel vendors to use automated process control to eliminate operator dependent faults in fabrication. Inspection methods have also improved. For these reasons, fuel failure due to manufacturing errors are at a low level.

5.6.2. Leaking end plugs

The fabrication of zirconium alloy end plugs can result in defects providing a leakage path for gases through the end plug. Chloride stringers present in some zirconium products sometimes form one such defect, which can be aligned longitudinally along the axis of the bar stock in the fabrication process [5.90]. Cold swaging of a barstock has also resulted in internal voids at the end of bars by deforming the outer surfaces more than the centre and eventually leaving central voids. Failures of this type have been rare. Clearly, the remedy is careful fabrication process control and inspection. Ultrasonic inspection of finished barstock is necessary.

Westinghouse reported that several rods identified as leaking may have problems as the result of an extrusion defect of end plug material [5.91]. An extrusion tail is introduced in end plug bar material during the manufacturing process and is subsequently removed as a specific step in the process. In a few isolated cases, the tail was not entirely removed. Most fuel rods believed to be leaking as a result of this mechanism have been identified through correlation with fabrication records rather than through direct visual confirmation. Two rods were confirmed to be leaking due to this mechanism during detailed destructive examinations in a hot cell. Several types of corrective action have been taken to eliminate this problem. The extrusion bar is cropped further away from the end to ensure that the actual end of the defect is removed. The end of the extrusion bar is then inspected metallographically.

Ultrasonic testing of the final bar material has also become increasingly sensitive and provides a final check that the defect has been removed.

5.6.3. Welding defects Contamination of the weld

The tungsten inert gas welding process for a zircaloy end plug is conducted either in a vacuum or in an inert gas atmosphere. Poor control of the gas atmosphere or leaky vacuum chambers have led to nitrogen contamination of the welds, and the resulting corrosion was sufficient in some cases to breach weld integrity. Clearly, the remedy is better atmosphere control and monitoring using impurity sensors [5.90].

Recently, several welding defects were observed on M5 cladding supplied by AREVA. According to information in [5.92], these failures were due to accidental pollution of the end plug welding during manufacturing.

In order to reinforce a return to good reliability of M5 rods, AREVA put a ‘cleanliness plan’ in place in its factories and decided to replace laser and tungsten inert gas (TIG) welding processes with the upset shape welding (USW) process, which is less sensitive to pollution.

Incomplete welds

Fuel failures due to incomplete end cap welds are extremely rare, and no particular incident has been reported regarding LWRs in recent years. An example of an incomplete end plug weld is given in Fig. 5.41. This defective product was inadvertently not rejected during controls [5.18].

For CANDUs, two events have been recorded. During the first year in service of Units 1 and 2 at Bruce

incomplete sheath to end cap closure welds. The majority of flaws were very small oxide filled stringers in the weld area which allowed ingress of the primary heat transfer fluid into the fuel element. Figure 5.42 shows a typical leak path.

A preferential distribution of defects was observed in 1981–1982 at the Douglas Point CANDU reactor during a defect excursion. At the time, the core contained many bundles with elements having incomplete closure welds.

The affected bundles were randomly distributed in the core, but the defects were preferentially located at high power positions towards the outlet end of the channel. The number of failures due to incomplete welds has diminished as a result of quality control programmes.

Grain boundary separation

In 1992, a weld defect type not previously recognized was experienced and identified by GE [5.94]. The weld defect type is known as grain boundary separation and is characterized by a microscopic crack-like defect initiated at the weld inner surface and extending radially along prior beta grain boundaries. This defect results from the application of tensile stresses in the weld during the cool down phase of weld operation. Figure 5.43 shows a representative grain boundary separation defect [5.94]. Corrective actions taken by GE include: (1) modification of weld station hardware and processes to minimize the extent and effect of applied tensile stresses, and (2) development and implementation of extended ultrasonic capability for 100% inspection of end plug welds for grain boundary separation.

Undercut in end cap welding (potential cause of failure)

BNFC [5.95] investigated the root cause of fuel failure and reported that approximately 2.8 per 100 000 fuel rods failed because of an undetermined failure mechanism, possibly a manufacturing defect. A potential cause of this failure mechanism was discovered: X ray examinations have identified an undercut in the cladding that occurs

FIG. 5.41. Incomplete end plug weld.

FIG. 5.42. Typical leak path due to incomplete cladding at the end cap weld [5.93].

very infrequently during end cap welding. This anomaly was not detectable using ultrasonic inspection. BWFC now X rays every end cap weld to ensure that all weld defects are eliminated. These defects, in most cases, are too small to result in fuel rod failure, although the occurrence of undercuts large enough to lead to failure have been discovered at a frequency similar to the rate at which fuel rods have been failing from an unknown mechanism.

5.6.4. External hydriding due to Ni contamination of the cladding surface

Two fuel assemblies failed at Ohi 1 in 1989 as a result of external local hydriding in the bottom area of the fuel rods.

An increase in outer rod diameter from external hydriding occurred in peripheral rods on one side. Only one rod in each assembly was leaking.

The root cause of failure was identified to be misloading of fuel rods because of a deformed lower finger guide in the fuel rod assembling machine. The deformed finger guide had led to a smaller than normal contact area between the rod and dimple or spring of the grid and to abnormally high loads, such that Ni from grid material coating deposited on the fuel rods, and residual strain was left on the cladding surface. Under these conditions, hydrogen from the coolant entered the cladding at the locations of Ni deposition and led to heavy local hydriding, as shown in Fig. 5.44 [5.96]. The problem was resolved through design improvements and better control of the

FIG. 5.43. Typical grain boundary separation (GBS) defect [5.94].

FIG. 5.44. External heavy local hydriding and crack due to Ni contamination of the cladding surface [5.96].

5.6.5. Cladding flaws

An increase in manufacturing defects affecting GE BWR fuel was observed at the end of the 1980s [5.97].

The cause was identified as an inability of standard tubing flaw inspection systems to reliably detect certain tubing reduction flaw configurations. Local undetected flaws can increase cladding stress during power transients and result in fuel failure. Such a failure mechanism is similar to PCI failure. To eliminate tube reducer generated flaws as a cause of in-reactor fuel failure, GE initiated corrective actions, including improvements to the ultrasonic (UT) tubing flaw inspection system and introduction of a supplemental, alternate technology (Eddy Current) tubing flow inspection system.

5.7. CROSS-FLOW/BAFFLE JETTING