# Interface Characterization within a Nuclear Fuel Plate
## 4 Results and Discussion
### Fuel-Cladding Interface in Alternative Zr Coating Processes Specimens  
The plates with the electroplated and plasma Zr coatings are shown to be weaker by 200% to 300% than the co-rolled foils. Figure 10 shows that the maximum back-surface velocity for the plated and plasma coatings ranges from 15 to 30 m/s, while the co-rolled plates have a range from 53 to 65 m/s as shown in Figure 7. The lowest maximum velocities near 20 m/s are near the lower limit of where the LST system is able to make reliable measurements. The approximate debond areas for most of the alternative Zr-coated plates are generally typical and are less than the 2-3 mm incident shock laser beam diameter, as shown in Figure 11. The atypical specimen is plate 102-E-2B1 that contains the foil \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Specimen** & **Maximum Velocity (m/s)** & **Standard Deviation (m/s)** & **Internal Stress (MPa)** & **Deviation (MPa)** \\ \hline
4E5 & 62.8 & 4.4 & 543 & 38 \\
4D5 & 54.0 & 3.4 & 467 & 29 \\
4A5 & 52.4 & 2.0 & 453 & 17 \\
3C5 & 65.0 & 3.3 & 562 & 29 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Maximum velocities are converted into estimates for the threshold interface debond strength for the variable bond strength fuel plate specimens produced by LANL. Figure 8: Results from bulge testing performed at LANL indicating that contaminated plates resulted in higher interfacial fracture energy compared to the plate made using the typical cleaning process [10]. Figure 9: Representative images of the colorized debond locations in LANL plates caused by laser shock testing are shown. Note: The images are not to scale. that had obvious Zr coating blisters and discoloration in the as-received condition. The debond area for 102-E-2B1 has a diameter close to 8 mm, which is significantly larger than the incident shock laser spot size. As expected, plate 102-E-2B1 also exhibited the weakest fuel-cladding interface. The plasma sprayed foil in plate 104-6 also exhibited a weak interface, showing a bond strength comparable to that observed for blistered Zr coated electroplated foil in plate 102-E2B1. The strongest fuel-cladding bond was observed for foil 21D-1. It is unknown what changes were made in the electroplating process for this foil if any were made. The foils did not come with documentation. Assuming that as-plated foil 21C-2B can be considered the reference condition for electroplated Zr, it would appear that annealing the foil after electroplating but prior to HIP had no significant influence on the interface strength, as shown by annealed foil 21D-2B in Figure 10. The conversion of maximum velocities to stress via Equation (1) is shown in Table 4. The debond stress for the electroplated foils are significantly lower than the co-rolled foils listed in Table 3.  
The debond images of the four electroplated plates and the plasma-spray plate after LST testing are shown in Figure 12. These LST plates are much more interesting to review. The dark regions indicate debonds in interfaces that reflect sound and keeps sound from traveling through the fuel plate to the other side. With the exception of plate 102-E-2B1, these debond images are typical of debonds caused by LST, being on the same order as the incident shock laser spot size. While the HIP process appears to have healed the coating blisters on foil 21E-2B1, the bond strength is low and the debond areas are large. Note that the exceptionally large debond areas for foil 21E-2B1 correspond to the blistered side of the foil (confirmed by reviewing the HIP can loading pictures); however, the interface strength appears to be similar on both halves (left and right) of the plate.