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SEPARATE EFFECT TESTS WITH ATF CLADDING MATERIALS WITHIN IAEA ATF-TS CRP
M. ŠEVEČEK1
1Czech Technical University in Prague, Prague, Czech Republic
Corresponding author: M. ŠEVEČEK, martin.sevecek@fjfi.cvut.cz
INTRODUCTION
This paper describes the separate effect tests performed on ATF cladding materials within the ATF-TS coordinated research project (CRP) [1]. Work Task 1 (WT1) of the ATF-TS CRP focused on experimental testing of ATF claddings with high technology readiness levels under both accident and normal operating. This programme included subtasks on ATF fabrication, bundle tests at QUENCH, DEGREE, and CODEX facilities, and separate effect tests (SETs). The plan for the WT1 SETs started with a selection of candidate materials. First, CRP members and partners proposed a range of different materials that have different levels of technological readiness and, as such, are available in different quantities and geometries. The limitations given by the geometry and quantity of testing materials were taken into account when designing the testing campaign. There were also test priorities defined by material providers that had to be taken into account, as well as political constraints resulting from the wide participation of member states [2, 3].
In the second step, CRP members proposed to conduct testing campaigns using their facilities. The original idea was to define a fixed set of testing parameters and perform round-robin exercises (RRT) with the available ATF materials. However, it was soon recognized that each institute has testing capabilities (e.g., temperature range, manpower, geometry limitations) which would substantially reduce the scope and number of participants in the WT. Therefore, a broader experimental scope was defined and tuned to allow wide participation but still produced valuable results. The objective was to coordinate sample providers and experimentalists to produce valuable data - i.e. not to repeat what was done in the ACTOF CRP or other projects/complementary testing, if feasible/RRTs, if feasible/support of future benchmarks (fuel performance ballooning/burst, DEGREE tests, QUENCH tests).). The constraints and strategy of WT1 are illustrated in Figure 1.
FIG. 1. Strategy and limitations in the WT1 SET experimental
The following priorities for the SETs were identified:
• Testing to support the following bundle tests, their interpretation, pre- and post-test calculations:
o QUENCH-19 with FeCrAl B136Y
o DEGREE DBA and DEC tests with Cr-based coated Zircaloy-4 (PWR geometry)
o CODEX-ATF with Cr-coated Opt. ZIRLO (VVER geometry)
• Derivation of correlations for fuel performance codes:
o LOCA-relevant ballooning/burst properties – creep and burst criteria (stress- and strain-based)
o Models for high-temperature oxidation (DBA and BDBA range) and corrosion kinetics (normal steady-state operation)
o Mechanical and Thermal Properties
• Generation of validation cases that can be used to validate models in single rod codes
o Separate effect test with Cr-coated Zr and FeCrAl
• Compilation of data for the IAEA Fuels and Materials database
o Derivation/fitting of experimental data produced by several laboratories.
o Sharing ATF properties and models
EXPERIMENTAL TESTING
In total, 14 institutes and laboratories participated in the experimental investigations within the ATF-TS CRP. All tests can be categorized into seven subtasks, as shown in Figure 2. The scope was first proposed by the members and later adapted based on the available materials and priorities presented in Chapter 1. It should also be noted that there was a different availability of recourses at various laboratories, and as a result, there is a big difference between the final scope. Some members performed hundreds of tests, and some were able to handle only a few.
FIG. 2. Classification of experiments performed within ATF-TS WT1
Table 1 shows the list of experimental measurements in participating laboratories. Most of the tests have been completed until summer 2024 and then documented in the TECDOC that is now in the publication process. Testing continued also in 2025 in different laboratories and will be presented in the IAEA Technical Meeting or different conferences. In total, more than 160 meters of reference and ATF tubes and several m2 of reference and ATF plates were shared and tested.[4, 5]
TABLE 1. LIST OF INSTITUTES/LABORATORIES INVOLVED IN THE EXPERIMENTAL TESTING AND THEIR FOCUS WITHIN THE ATF-TS CRP
Institute Country Experiments performed
Czech Technical University in Prague/UJP Praha Czech Republic Ballooning/burst tests, HT steam oxidation, long-term VVER corrosion
JRC Karlsruhe European Commission Thermophysical properties
JRC Petten European Commission Mechanical tests in the LWR environment
CRIEPI Japan Single rod and bundle tests
Karlsruhe Institute of Technology Germany Single rod and bundle tests
Institute of Nuclear Chemistry and Technology Poland Neutron activation analysis, long-term corrosion, HT annealing
China Nuclear Power Engineering China Long-term corrosion PWR, HT oxidation, ballooning / burst
Atomic Energy Organization of Iran Iran HT oxidation
China Nuclear Power Technology Research Institute China Axial Tensile Tests, Internal Burst Tests, Internal Climb Tests, Autoclave Corrosion Tests, HT Steam Oxidation
EK, Centre for Energy Research Hungary Ballooning/burst tests, Mandrel tests, HT steam oxidation, bundle test
Universidad Politécnica de Madrid Spain Cathodic charge, Hydriding, Ring compression tests, HT creep
Korea Atomic Energy Research Institute Korea Microstructural Investigations
Seoul National University Korea LOCA tests, DSC, and post-quench ductility evaluation
HIGH TEMPERATURE CREEP, BALLOONING AND BURST TESTS
Due to different limitations, the tests are not standard RRTs, but rather complementary tests of several cladding candidates. However, RRTs were possible in several instances, the laboratories and their test matrices are summarized in table 2.
TAB.2. CONTRIBUTORS TO THE BALLOONING/BURST TASK
Institute Country Experiments performed
CTU/UJP Praha Czech Republic Isobaric tests with temperature ramp (0.7K/sec and 7K/sec) + isothermal isobaric creep tests with coated Zr
EK, Centre for Energy Research Hungary Constant temperature – pressure ramps (100 kPa/s) with coated Zr and FeCeAl
CNPRI China Pressure ramp tests at 350ºC with reference and coated Zr
UPM Spain Pressure ramp tests at 600 ° C with reference and coated Zr
CNPE China Pressure ramp tests at 800-900 ° C with reference Zr coated and coated
SNU Korea Single rod ramp tests simulating first phase of LOCA
The conclusions of this test series can be summarized as [6, 7]:
- Coated Zr cladding
o Limited benefit of coatings in time-to-burst, generally within the uncertainty range
o Ballooning and opening size coated Zr provide some benefits, but
the extra margin depends on the specific test conditions and methodology.
o New creep correlation and burst criteria derived (both stress- and strain-based for coated cladding) and provided to WT2 for model implementation and validation. Correction coefficients proposed for fuel performance models.
o In most cases, the reference Zr materials behave similarly to the coated Zr given ±20% uncertainty.
- FeCrAl
o A limited volume of material was available, but a very different performance was discovered for FeCrAl.
o Fuel performance codes were unable to predict this behavior because the models were historically developed and tuned for Zr-based materials.
Dozens of experimental datasets were measured and prepared for the IAEA Fuels and Materials database. Further analysis is foreseen, as well as usage of the data for code validation, and the data are open to the wide fuel performance community.
HIGH-TEMPERATURE OXIDATION TESTS
A reduced high-temperature steam oxidation rate is a key advantage of ATFcladdings. This section presents an overview of the experimental results on the oxidation rates of various coated zirconium alloys, considering the types of base zirconium alloys, coating materials and methods, and coating thicknesses. The post-LOCA ductility of the Cr-coated cladding tubes tested is evaluated using Ring Compression Tests to assess the potential impact of Cr-coated zirconium alloys on the limits of the emergency core cooling system. Additionally, the formation of a Zr-Cr eutectic mixture, a critical safety challenge associated with Zr-Cr systems, and its influence on the structural integrity of Cr-coated cladding, was examined by tests conducted above 1330°C. The institutes contributing to this task are summarized in table 3.
TAB.2. CONTRIBUTORS TO THE BALLOONING/BURST TASK
Institute Country Experiments performed
CNPRI Czech Republic HT double-sided steam oxidation up to 1400 °C
EK, Center for Energy Research Hungary Small scale steam oxidation test
CTU/UJP China Pressure ramp tests at 350 ° C with reference and coated Zr
INCT Spain Ar annealing test
CNPE China Small scale steam oxidation test
SNU Korea LOCA testing; PQD evaluation
KIT Germany Single rod tests
CRIEPI Japan Single rod tests
The highlights of this test series can be summarized as follows.
- Coated Zr cladding
o Eutectic Cr/Zr – another degradation mechanism (e.g. similar to secondary hydriding or oxygen embrittlement), not hard limit (melting point). Effect of coating thickness observed.
o The ECR criteria for coated Zr do not have physical meaning and should be replaced. But it is conservative and can be used by industry if needed.
o CrN/Cr – benefits in BDBA conditions confirmed by CRIEPI, KIT. However, open questions remain, but it seems like a feasible way to further improve pure Cr coatings.
- FeCrAl
o Ramp rate sensitive oxidation kinetics, improved model derived.
o Transition of oxidation kinetics around 1375 ° C leading to rapid material degradation.
AUTOCLAVE CORROSION TESTS
Long-term corrosion tests were conducted on zirconium alloys with various coatings, focusing on their performance in simulated nuclear reactor environments. The research was carried out by multiple institutions, including UJP, CNPRI, INCT, and UPM.
The long-term corrosion behaviour of ATFs is a critical aspect of the safety and efficiency of nuclear reactors. The primary objective was to evaluate the performance of these coatings in simulated environments (PWR, WWER) over extended periods. Experiments are also included under extended conditions to accelerate corrosion processes (steam 400 °C). The tests were conducted in autoclaves, simulating the primary water chemistry of pressurized water reactors (PWRs) and WWER reactors.
FIG. 3. AUTOCLAVES, SAMPLE HOLDERS, AND SAMPLES TESTED WITHIN THIS ATF-TS
The results indicate that chromium-based coatings significantly improve the corrosion resistance of zirconium alloys. However, the long-term performance of these coatings is influenced by factors such as coating defects and the deposition process.
SUMMARY
WT 1 of the ATF-TS CRP focused on the experimental testing of various ATF cladding materials. This work task is fundamental for all other work tasks within ATF-TS CRP, namely WT2 – ATF fuel performance modelling and code validation; WT3 – LOCA methodology and uncertainty quantification with ATF materials; WT4 - open-source database similar to ATF MATPRO.
Eight institutes from IAEA member countries fabricated and shared different ATF cladding concepts, and in addition, three types of reference commercial Zr-based alloys were tested in the same campaigns to provide a baseline for performance comparison. 14 institutes proposed and completed experimental tests with the ATF and reference cladding materials provided. The experimental tests can be divided into ballooning/burst tests (7 institutes); HT oxidation tests or annealing (10 institutes); long-term autoclave corrosion tests (5 institutes); mechanical tests (4 institutes); other tests (4 institutes) and bundle tests (3 institutes). Despite significant challenges, namely the COVID pandemic, complicated political relationships between various members, and logistical/administrative complications with cladding materials, WT1 was able to generate several hundreds of unique datasets that have been uploaded to the IAEA Fuel Experimental Data Repository and opened to all interested partners from all over the world. In addition, WT1 produced new correlations and models for fuel performance codes (creep, strain/stress failure criteria, mechanical and corrosion models, and thermal properties) that were shared with WT2. Based on these new models, benchmark cases were defined to validate these new models in fuel rod or integral codes. The results of extensive experimental tests were also used to derive multiplication factors and uncertainty ranges for the LOCA methodology in WT3, and lastly, the results are available for WT4 to be processed and implemented in the new MATPRO-like ATF database.
In summary, the experimental programme within ATF-TS was the most extensive and successful CRP in the IAEA’s history in the fuel area. Some of the activities are still ongoing, and many new partnerships have been created. More details on individual experimental contributions from WT1 participants are presented in separate papers within the IAEA Technical Meeting. The complete results and their comparison will be presented in a dedicated TECDOC to be published at the beginning of 2026 by the IAEA.
ACKNOWLEDGEMENTS
The activities presented in the paper were conducted within the framework of the IAEA ATF-TS project, and I would like to acknowledge the contribution of many coworkers, students, and advisors who help to fabricate, optimize and share the materials, conduct the experiments, and evaluate the results.
REFERENCES
[1] ZHANG, J., XU, P., SEVECEK, M., SIM, K.S., KHAPERSKAIA, A., Contribution of IAEA coordinated research projects to light water reactors advanced technology fuel testing and simulation, Nuclear Engineering and Design 418 (2024) 112910.
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[3] NAKAMURA, K., INAGAKI, K., STUCKERT, J., ŠEVEČEK, M., TARUMI, N., Behavior of bundles with Cr coated claddings under BDBA conditions at the DEGREE facility, TopFuel 2024 Reactor Fuel Performance: 29 September-3 October 2024, Grenoble, France (2024) 261.
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[6] JOUNG, S., KIM, J., ŠEVEČEK, M., STUCKERT, J., LEE, Y., Post-quench ductility limits of coated ATF with various zirconium-based alloys and coating designs, Journal of Nuclear Materials 591 (2024) 154915.
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