Failure mechanism

Nuclear fuel assemblies are exposed to severe thermal, mechanical and radiation conditions during operation.

Global core and local fuel assembly flow fields typically result in fuel rod vibration. Under certain conditions, this vibration, when coupled with other factors, can result in excessive cladding fretting wear. This phenomenon is of significant concern for nuclear fuel designers, especially in light of a growing need for higher burnup, longer cycle lengths, and operational safety margins. Understanding fretting wear margins, the probability of a fuel assembly being at risk of excessive wear, and the factors affecting fretting wear resistance are important in order to better guide design, testing, and operational flexibility. Thus, an important effort has been made in this area during the last several years and is described below.

Fretting wear is governed by a set of complex physical phenomena, which do not remain constant during nuclear fuel operation in reactor cores. Based on various field data for grid to rod fretting wear, we believe that the amount of grid to rod fretting wear depends on:

— The extent of flow induced vibration caused by fuel design and/or plant specific operating conditions;

— Grid to rod support conditions;

— The initial grid to rod contact area;

— Grid materials.

A larger flow induced vibration, larger grid to rod gap, smaller initial grid to rod contact area and softer cladding tube contribute to a higher probability of grid to rod fretting wear.

Unexpected excessive flow induced vibration may occur at certain locations with severe flow conditions in the reactor core. Calculations of flow distribution at the bottom of the reactor vessel and within assemblies (axial and cross-flow), performed in the case of Cattenom 3, showed that higher cross-flows (red and yellow areas in Fig. 5.2) were distributed in an intermediate ring around the core. This correlated strongly with failed fuel assembly locations [5.14].

Grid design may sometimes lead to self-excited fuel assembly vibration and/or self-excited spacer grid strap vibration. A good example is the VANTAGE 5H fuel assembly with low pressure drop zircaloy grids, which were susceptible to self-excited fuel vibration at specific flow conditions.

Also, some events demonstrate that failure risk from fluid–elastic instability can increase in mixed cores when flow mismatches in neighbouring assemblies lead to added cross-flow and turbulence. Many existing spacer designs seem to be insufficiently robust to resist turbulence caused by added cross-flow. During cycle 9 of D.C.

Cook unit 2, four Siemens fuel assemblies failed in the burnup range 45–50 GW^.d/t. Poolside examinations revealed that the cause of failure was grid to rod fretting [5.15]. The four failed Siemens assemblies were all surrounded by one cycle Westinghouse Vantage 5 assemblies and the core baffle. The fretting failures were located at spacers close to the intermediate flow mixers (IFMs) in the Vantage 5 assemblies.

Finally, in some reactors, more severe flow conditions can exist, mainly at fuel assemblies located on the core periphery.

The second, more important parameter is grid to rod gap support condition, which is determined by initial elastic spring deflection, cladding creep-down, irradiation induced spring force relaxation and fuel assembly location in the reactor core. Knowledge of fuel rod support in the grid cell is an important parameter for comprehension of global behaviour in relation to local behaviour of a rod (fretting wear). Generally, rods are supported by a spring dimples system. Under irradiation, grid spring force is decreasing and a rod to grid gap opening may appear, impacting the vibration of rods. An example of grid to rod support condition evolution calculated with the model presented in the reference [5.16] is illustrated in Fig. 5.3. This model takes into account the evolution of cladding diameter, grid growth, and spring and dimple creep under irradiation. As a result, the model provides the evolution of fuel rod support force (or corresponding cell size) versus burnup. The evolution calculated is in good agreement with operating feedback results.

It is obvious that larger initial elastic spring deflection, less cladding creep-down and lower spring force relaxation will produce a smaller grid to rod gap.

Also, assuming the same previous operating history, fuel assemblies located at the core periphery may generate a larger grid to rod gap than those inside the core because the former show relatively lower fuel rod temperatures and thus smaller fuel rod diameter than the latter.

(a) (b) Transverse flow

velocity

Leaking fuel rods (red points)

FIG. 5.2. Comparison between cross-flow computation (a) and leaking rod locations due to grid to rod fretting (b) on EDF 1300 MWe core [5.14].

A larger grid to rod contact area generates less grid to rod contact stress and thus reduces the fretting wear rate when the same vibration induced force is present for different failed fuel rods. Therefore, it is recommended that spacer grid spring and dimple design with a larger grid to rod contact area be developed to eliminate fretting wear.

Some other specific causes of fretting failure are worth mentioning. An unexpected root cause of grid to rod fretting due to spacer breakage in the bottom grid occurred in 1994–1995 in two German plants [5.17]. The cause of failure was traced back to a combination of high stress and high stress corrosion cracking susceptibility in spacer springs, caused by improperly heated Inconel springs and the use of these spacers at the lowermost spacer position, which is below the active length in the affected plant. This problem was solved by using proven Inconel spacers outside the active region.

Other cases of fretting failure due to spacer fabrication were also related. Fretting defects were observed in two Fragema fuel assemblies [5.18]. A defect is shown in Fig. 5.4 [5.18]. A very extensive programme of investigation was performed on the fuel assemblies containing the two affected rods, yielding detailed grid cell dimension information. The results of dimension measurements on the bottom grid cells show very little difference between cells in which fuel rods were securely held and the two cells where fretting corrosion took place. In both fuel assemblies, the defect occurrence is estimated to take place either at the end of the second irradiation cycle or during the third cycle. The conditions for such a defect occurrence might plausibly be a combination of: (a) specific transverse flow at the bottom end grid; and (b) initial grid cell characteristics in excess of specification values, leading to excessive relaxation of the restraining force.

Grid to rod fretting failures also occur occasionally as a result of a damaged spacer, which can stem from interference between adjacent PWR fuel assemblies during fuel shuffling.

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Burnup (GWd/t)

Relaxation

Experimental measurements model

End of life (EOL)

FIG. 5.3. Evolution of fuel rod support condition versus burnup [5.16].

FIG. 5.4. Fretting defect induced by grid to cladding interaction [5.18].