ETD

• Accident Tolerant Fuel (ATF)
• Chromium-based coated Zirconium alloy
• Code Coupling
• Finite element modelling
• Fractography-SEM/TEM
• High-temperature oxidation
• Hydrogen uptake
• Pellet–Cladding Mechanical Interaction (PCMI)
• Profilometry
• Segmented Expanding Mandrel Test
• SNETP-OFFERR SPCMI-ATF project
• TRANSURANUS

Pellet–cladding mechanical interaction remains one of the key fuel performance concerns in light water reactors, particularly under aggressive power transients. While operational measures and design improvements have significantly reduced PCI–PCMI–related failures in boiling water reactors, the situation in pressurized water reactors still requires further investigation, especially with the introduction of accident-tolerant fuel concepts. Recent observations indicate that under hard PCMI conditions at normal operation, the radial relocation of wedged fuel pellets may lead to self-limiting cladding deformation and localized coating damage. In chromium-coated claddings, brittle fracture of the coating may result in loss of chromium protectiveness, potentially exposing the zirconium substrate to accelerated oxidation and hydrogen uptake.

To investigate these mechanisms, the SNETP-OFFERR Simulation of Pellet-Cladding Mechanical Interaction with Accident-Tolerant Fuel Claddings (SPCMI-ATF) project was conducted within the Euratom-funded framework. The project was carried out through a collaboration between EDF, Università di Pisa–GRNSPG (user team-lead), CIEMAT, CTU, EPFL, JRC, LEI, and NINE (user team), and HUN-REN EK-CER (infrastructure team). The primary objective was to evaluate the mechanical behaviour and failure mechanisms of chromium-based coated specimens and to compare the results with uncoated zirconium alloy claddings, as a verification of numerical values within the current fuel criteria implemented in the nuclear power plants by establishing basis of comparison of cladding response under NO-PCMI loading conditions through an integrated experimental and advanced numerical simulation approach.

The experimental campaign employed a modified segment expanding mechanical test, a displacement-controlled technique that reproduces plane-strain radial loading representative of PCMI conditions. A total of 25 unirradiated as-fabricated cladding samples were investigated, including chromium-coated, CrN-coated, innovative multilayer Cr/CrN coatings conducted for the first time by AEOI, and uncoated reference samples coated on Iranian Zr1%Nb and Opt-ZIRLO™ substrates. The tests were conducted at room temperature and 300 °C to represent conservative and realistic operating conditions. Post-test analyses included profilometry, Scanning Electron Microscopy (SEM), fractography using Transmission Electron Microscopy (TEM), metallography, and isothermal high-temperature oxidation tests in steam.

Force–displacement results indicated that the addition of thin chromium-based coatings does not significantly modify the bulk mechanical behaviour of the zirconium substrate, as similar diametral strains of approximately 5 % were reached under radial loading. Cyclic loading conditions resulted in approximately 30 % lower forces compared to monotonic loading, highlighting the importance of cyclic plasticity under effective plasticity and progressive fatigue effects. SEM observations revealed consistent cracking of the chromium coatings with crack surfaces normal to the tangential tensile stress, confirming brittle behaviour under the investigated conditions. However, no macroscopic delamination or substrate cracking was observed.

Preliminary and advanced fractographic analyses using scanning electron microscopy and transmission electron microscopy conducted at HUN-REN EK-CER and Università di Pisa, respectively, revealed complex fracture behaviour within the coating, including cleavage-dominant fracture along the radial–circumferential plane and intergranular fracture along the radial–axial plane. Phase mapping, STEM-EDS, and ACOM analyses identified impurities and interfacial phases, including zirconium oxide and calcium-carbon compounds, which may influence crack propagation paths, while crystallographic orientation was determined as the susceptible factor inducing intergranular fracture in coated claddings, thereby yielding more complex and severe oxidation (highlighted in the metallography section) compared with uncoated claddings.

Post-test high-temperature oxidation experiments further demonstrated that once the coating cracks, localized oxidation of the zirconium substrate occurs beneath the coating, forming α-Zr(O) intrusions that may act as mechanical weak points. Additional hydrogenation experiments showed accelerated hydrogen uptake in coated samples compared to uncoated cladding.

The HYCURATE project (HYdrogenated Coated versus Uncoated samples with heating RATE effects) was initiated under a separate framework, based on a proposal from the Università di Pisa to be carried out at HUN-REN EK-CER, which was later expanded by HUN-REN EK-CER to “Investigation of the reorientation of hydrides accumulating in the cladding”. This project focused on the hydrogenation of coated and uncoated, non-ruptured cladding samples to study the hydrogen absorption rate in coated claddings relative to uncoated reference samples and to assess the effects of slow cooling vs. fast cooling on hydride formation.

To complement the experimental work, advanced numerical simulations were performed using the HEXAGON Marc/Mentat 2022.2 3D-finite element analysis code for the mandrel test configuration. The models successfully captured contact interactions, plastic deformation, and coating stress evolution, and were validated against experimental force–displacement and profilometry data. The model was further improved to consider meshing and elastic bending of the universal tensile machine. A novel sequential code-coupling strategy was implemented for the first time to link the 3D finite element model results to the TRANSURANUS 1.5D fuel performance code, enabling a 1.5D TRANSURANUS code capable of simulating chromium coating behaviour under postulated PCMI conditions. The extended model predicted coating failure at approximately 2 % strain, consistent with experimental observations.

Overall, the integrated experimental and computational investigation provides new insights into the failure mechanisms of chromium-based coated ATF claddings under postulated PCMI mechanical loading. The results demonstrate that while chromium coatings do not significantly alter the bulk mechanical response of zirconium cladding, brittle coating fracture can compromise surface protectiveness and influence subsequent oxidation and hydrogen absorption processes, further impairing compliance with the current Emergency Core Cooling System (ECCS) criteria for coated cladding exhibiting brittle behaviour. The developed experimental methodology and coupled modelling framework provide a valuable basis for future evaluation of coated cladding performance under normal operating and accident conditions, presented at the International Atomic Energy Agency meeting on advanced fuel technology in Vienna.