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INTRODUCTION: In the framework of the IAEA Coordinated Research Project Testing and Simulation for Advanced Technology and Accident Tolerant Fuels (ATF-TS), a comprehensive experimental program was conducted to investigate the performance of advanced cladding materials. The goal was to characterize their behaviour under various conditions and to compare the newly obtained data with that of traditional cladding alloys used in water-cooled reactors.
1. OVERVIEW
Comparative tests were carried out using cladding tubes supplied by multiple ATF-TS project partners. The tested cladding types included traditional reference materials—such as Zircaloy-4, ZIRLO™, Optimized ZIRLO™, and Zr1%Nb alloys—as well as advanced materials featuring coatings or alternative compositions. These included claddings with external surface coatings of chromium (Cr), chromium nitride (CrN), multilayer CrN/Cr, titanium-aluminium (TiAl), and a high-performance FeCrAl (B136Y3) alloy.
The HUN-REN Centre for Energy Research was responsible for performing a substantial number of separate effect tests and one integral test using an electrically heated bundle (CODEX-ATF). Data from the CODEX test were stored in a separate database. This report summarizes the main characteristics and results from the following separate effect tests: Ballooning and burst tests, Oxidation in high-temperature steam, Tensile tests using ring samples, Ring compression tests, Mandrel tests and Scanning electron microscopy (SEM) examinations.
2. MATERIALS AND COMPLETED TESTS
Thirteen different cladding types were used in the HUN-REN EK test program, provided by several ATF-TS partners: Karlsruhe Institute of Technology (KIT), Atomic Energy Organization of Iran (AEOI), Canadian Nuclear Laboratories (CNL), Czech Technical University in Prague (CTU), Belarusian State University (BSU). Although parallel tests were planned for all materials, the available tube lengths varied between suppliers. Consequently, the number of tests performed for certain cladding types was limited. Detailed test matrices, the separate reports and a full list of completed measurements are provided in Appendix I of the TECDOC.
3. DATABASE DIRECTORY STRUCTURE
The experimental database created for the ATF-TS program builds on the structure of the earlier E110 cladding database developed by the predecessor of HUN-REN EK. The previous database is publicly available in the OECD NEA databank under the code NEA-1799 IFPE/AEKI-EDB-E110. The new database for the ATF-TS tests mirrors this directory structure to facilitate future integration [1].
Each test series is documented in a PDF report, and raw data are provided in both Excel spreadsheets and ASCII files. Each individual measurement has a dedicated spreadsheet and corresponding ASCII file, ensuring consistency and traceability across formats.
4. BURST TESTS
Several types of potential ATF candidate cladding materials were subjected to ballooning and burst tests at elevated temperatures using various internal pressurization rates. In total, 119 burst tests were conducted. While the majority of tests took place in an inert atmosphere, a subset was carried out in a steam-rich oxidizing environment following pre-oxidation. The primary outcomes included the measured burst pressures, non-destructive evaluations of the deformed geometry, and visual inspections of the coatings to assess damage incurred during ballooning and rupture.
The results indicated that, under isothermal burst test conditions, there was no statistically significant difference in burst pressure between coated and uncoated reference claddings. Similarly, the analysis of maximum strain at the burst site revealed no substantial variation between the two groups; however, this was largely attributed to high data scatter, which resulted in broad confidence intervals. Notably, all tested coatings remained adherent to the underlying cladding with no signs of spalling. Nevertheless, the observed cracking patterns varied significantly depending on the coating type.
5. OXIDATION EXPERIMENTS
The oxidation resistance of various cladding materials was evaluated using samples with different surface treatments, including Cr, CrN, CrN/Cr, and TiAl coatings, as well as uncoated zirconium alloys and the FeCrAl alloy. Oxidation was carried out in a high-temperature tube furnace equipped with a quartz tube. The setup consisted of a steam generator, a three-zone resistance furnace with precise temperature control, and a condenser. The cladding tubes were cut into 8 mm long segments, suitable for subsequent ring compression testing. Each segment was oxidized on both sides while placed in a quartz boat. The tests were conducted under isothermal conditions at two temperatures 1000 °C for 1 hour and 1200 °C for 30 minutes. In total, 92 oxidation tests were conducted. The mass gain and Equivalent Cladding Reacted (ECR) were calculated to quantify the extent of oxidation. For coated samples, it was necessary to determine the specific mass gain on the coated surface separately, as total surface-based calculations did not yield accurate results.
The majority of coated samples exhibited better oxidation resistance than uncoated ones, with two notable exceptions: the titanium-aluminium coated and plasma-treated chromium-coated zirconium alloys. These coatings were not effective in preventing oxidation beneath the surface. In contrast, the Cr-coated Optimized ZIRLO™ and the FeCrAl alloy demonstrated excellent resistance to steam oxidation at both test temperatures.
6. RING TENSILE TESTS
Tensile strength was assessed using ring segments from both coated and uncoated zirconium alloy claddings in as-received and oxidized conditions. The tubes were supplied by KIT, AEOI, CNL, and CTU. In total, 60 ring samples were tested. Each cladding tube was sectioned into 2 mm rings. For each material, six rings were tested: two in as-received condition, two after oxidation at 1000 °C for 3600 seconds, and two after oxidation at 1200 °C for 1800 seconds. Geometrical measurements were taken at three locations per ring. Tensile tests were conducted using an INSTRON 1195 machine at room temperature with a crosshead speed of 0.5 mm/min. Load-displacement data were recorded every 0.4 seconds until the samples fractured.
Coatings had little influence on the tensile strength of as-received zirconium alloy samples. However, the oxidation treatment led to significant changes. Zirconium-based samples became brittle after oxidation, due to the two-sided exposure during steam treatment, which led to heavy internal oxidation. For samples oxidized at 1000 °C, ultimate tensile strength (UTS) ranged from 3.5 to 188 MPa. After oxidation at 1200 °C, UTS values dropped to between 0 and 24 MPa. In contrast, FeCrAl samples retained much of their mechanical strength even after oxidation. Their UTS values were over 600 MPa in as-received state, ~500 MPa after oxidation at 1000 °C, ~440 MPa after oxidation at 1200 °C. These results suggest that thermal exposure played a more significant role than oxidation in altering the mechanical properties of FeCrAl. Notably, all oxidized FeCrAl samples remained ductile.
7. RING COMPRESSION TESTS
Ring compression tests were performed on both coated and uncoated zirconium alloy claddings, as well as on FeCrAl samples, following steam oxidation. A total of 62 samples were tested. The cladding tubes were sectioned into 8 mm long rings and oxidized in high-temperature steam. For each cladding type, three samples were tested after oxidation at 1000 °C for 3600 seconds, and three others after oxidation at 1200 °C for 1800 seconds. Compression tests were conducted using an INSTRON 1195 universal test machine at room temperature (~20 °C), with a crosshead speed of 0.5 mm/min.
The results showed that most uncoated zirconium alloys became brittle after oxidation at 1000 °C. An exception was the Zircaloy-4 provided by KIT, which exhibited a ductile plateau in its load-displacement curve. All other uncoated alloys fractured quickly following elastic deformation. Differences in maximum load values were strongly influenced by cladding wall thickness. For instance, Zircaloy-4 samples from CNL had thinner walls, which explained their lower maximum loads.
Among the tested materials, Cr-coated and uncoated Optimized ZIRLO™ samples demonstrated distinct mechanical behaviours. The uncoated sample exhibited brittle failure after reaching a maximum load of 321 N, whereas the Cr-coated sample maintained ductile deformation and reached a maximum load of 435 N. Thus, the coating imparted a measure of ductility under the given oxidation conditions.
FeCrAl samples remained fully ductile, even after oxidation at 1200 °C for 1800 seconds. However, under these extreme oxidation conditions, Cr coatings failed to prevent embrittlement in either Optimized ZIRLO™ or Zr1%Nb claddings. The titanium-aluminum (TiAl) coating also did not improve high-temperature oxidation resistance or mechanical performance.
8. MANDREL TESTS
Drawing on available literature, a mandrel test setup was designed and constructed to study the mechanical interaction between fuel pellets and cladding under PCMI-like radial loading conditions. Mandrel ductility tests were carried out to evaluate the ductility of both coated and uncoated ring specimens sectioned from fuel cladding tubes. All tests were conducted at room temperature. Similar to previous mechanical tests, the coatings on zirconium alloy claddings did not significantly alter the deformation behaviour of as-received samples. The results showed that the presence of coatings had a negligible impact on the overall strength of the cladding materials. For the FeCrAl cladding, the measured maximum force was lower than that of the zirconium-based samples, primarily due to its reduced wall thickness. Additionally, the maximum diameter increase observed in FeCrAl was significantly smaller compared to the other reference materials. All tested materials met the target deformation without failure, though the coatings developed surface cracks, which were further analysed in the SEM investigations.
9. SEM EXAMINATIONS
Morphological analysis of the cladding samples was conducted using two scanning electron microscopes. Secondary electron (SEI) and backscattered electron (BEI) images were taken at 5 keV. Cross-sectional and fracture surfaces were also examined. A total of 23 cladding samples in both as-received and oxidized states were analysed. Investigations focused on the outer surfaces, cross-sections, and fracture surfaces after mechanical testing. The SEM analyses corroborated mechanical and oxidation test results, illustrating the limits of coating effectiveness at elevated temperatures and identifying failure modes associated with each cladding/coating combination.
In selected cases, focused ion beam (FIB) analysis was used to ablate micro-trenches (~a few micrometres deep) into the surface to reveal subsurface structures. Samples were tilted at 52° during FIB preparation, and measured layer thicknesses were corrected by a factor of 1.27 to account for this tilt. Elemental analysis was performed using an Oxford X-MAX 20 energy-dispersive X-ray (EDX) spectrometer with a silicon drift detector. Measurements were conducted on selected areas of interest.
Cr-coated Optimized ZIRLO™ samples: The outer surfaces were dominated by plate-like Cr-oxide crystallites. At lower oxidation temperatures, these plates were numerous and randomly oriented, with sizes ranging from a few tenths of a micrometre to about 1 µm. At higher temperatures, the number of crystallites decreased while their size increased. Smoother surface regions contained detectable amounts of Cr and O alongside Zr.
Zr1%Nb samples coated with Cr, CrN, or Cr+CrN: These displayed heterogeneous surface structures with a combination of plate-like crystallites, smooth patches, cracks, cavities, and particle agglomerates. At 1200 °C, the crystallites became more prominent in both size and contrast. Cr-oxides were confirmed as the primary phase. Some smoother regions and grains also revealed the presence of Zr and impurities.
TiAl-coated Zircaloy-4 samples: The TiAl layer failed to prevent oxidation of the underlying zirconium. Oxidation led to spalling of the coating in several locations. The spalled layers contained higher concentrations of Ti compared to the remaining cladding material, indicating separation of the Ti-rich coating during the exposure to steam.
10. SUMMARY
The separate effect tests conducted at the HUN-REN Centre for Energy Research under the IAEA ATF-TS program provided a comprehensive dataset on the behaviour of advanced cladding materials under thermal and mechanical stresses. Key conclusions from the test series include:
— Cr coatings significantly improved oxidation resistance of zirconium alloys at high temperatures, with Optimized ZIRLO™ and FeCrAl performing best. However, certain coatings such as TiAl and plasma-treated Cr were ineffective under the tested conditions.
— FeCrAl retained ductility and strength after oxidation, whereas Zr-based claddings became brittle, particularly after two-sided oxidation. Cr coatings improved ductility in ring compression tests, but could not prevent embrittlement at 1200 °C.
— Surface cracks and morphological degradation were observed in coated claddings after testing. Crystallite growth and surface inhomogeneities were strongly temperature-dependent. SEM imaging confirmed that TiAl coating is unstable and prone to delamination during oxidation.
All test data are available in a structured digital format in the IAEA database. This supports future access for fuel modelling and safety analysis purposes. The findings contribute to the broader international effort to qualify accident-tolerant fuels for use in current and next-generation water-cooled nuclear reactors. Further studies, will be required to fully validate these materials for in-core application.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the Czech Technical University, the Atomic Energy Organization of Iran, the Karlsruhe Institute of Technology, the Canadian Nuclear Laboratories and the Belarusian State University for providing the materials used in the tests.
REFERENCES
[1] Zoltán Hózer, Márton Király, Márta Horváth, Péter Szabó, Dávid Cinger, Tamás Novotny, Erzsébet Perez-Feró, Anna Pintér, Barbara Somfai, Levente Illés, Zoltán Kovács: Database of separate effect tests carried out at HUN-REN EK with cladding samples provided by IAEA ATF-TS partners. Research report, EK-FRL-2024-120-1-1-M0, 2024
