# An overview of cea LOCA separate effects testing applied to chromium-coated zirconium alloy claddings

Ali Charbal, Jean-Christophe Brachet, Jean-Mathieu Vanson

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## To cite this version:

Ali Charbal, Jean-Christophe Brachet, Jean-Mathieu Vanson. An overview of cea LOCA separate effects testing applied to chromium-coated zirconium alloy claddings. Technical Meeting on Advanced Technology Fuels: Progress on their Design, Manufacturing, Experimentation, Irradiation, and Case Studies for their Industrialization, Safety Evaluation, and Future Prospects, Oct 2025, Vienne, Austria. cea-05506220

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https://cea.hal.science/cea-05506220v1

Submitted on 11 Feb 2026

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# AN OVERVIEW OF CEA LOCA SEPARATE EFFECTS TESTING APPLIED TO CHROMIUM-COATED ZIRCONIUM ALLOY CLADDINGS

## A. CHARBAL

CEA, Service de Recherche en Matériaux et procédés Avancés, Université Paris-Saclay, 91191
Gif-sur-Yvette, France
Email: ali.charbal@cea.fr

## J.C. BRACHET

CEA, Service de Recherche en Matériaux et procédés Avancés, Université Paris-Saclay, 91191
Gif-sur-Yvette, France

## J.M. VANSON

CEA, DES, IRESNE, DEC, Cadarache F-13108
Saint-Paul-lès-Durance, France

During a hypothetical Loss of Coolant Accident (LOCA) scenario, zirconium alloy claddings may experience an accelerated heating and high-pressure gradient between the inside and the outside, potentially leading to ballooning and burst. Following this first step, the cladding tubes may also be exposed to high-temperature steam environments, which results in accelerated oxidation and potential embrittlement upon final quenching and in Post-Quenching (PQ) conditions. Additionally, the reaction between the zirconium alloy cladding and water vapor can release significant quantities of hydrogen gas. To reduce the high-temperature oxidation of the cladding tubes, numerous research institutes and fuel manufacturers are focusing on the development of Enhanced Accident-Tolerant Fuel (EATF) technologies. International efforts have been especially dedicated to the development and evaluation of chromium coated zirconium-based claddings as an “evolutive” EATF concept.

At CEA, these developments began more than 15 years ago [1]. Early studies identified Physical Vapor Deposition - High Power Impulse Magnetron Sputtering (PVD-HiPIMS) as a promising technique for applying chromium-based coatings on zirconium-based substrates. The coated cladding tubes were tested with separate effect tests facilities in both nominal and LOCA conditions. Additionally, ion and neutron irradiation experiments were initiated to have a first look on their post-irradiation behaviour [2-3].

Focussing here on the Cr-coated claddings LOCA behaviour, tests conducted in the EDGAR facility [4-6], for ballooning and burst, have shown a certain strengthening effect of the coating, particularly in the $α_{Zr}$ and $α_{Zr}+β_{Zr}$ temperature ranges (FIG. 1). Both isothermal and thermal ramped internal pressure tests have also revealed reductions in ballooning and burst opening sizes compared to uncoated claddings under identical LOCA conditions (FIG. 2 -a). The increase of chromium ductility with temperature results in clear improvements in cracking thresholds under LOCA-relevant high temperatures conditions [7]. Importantly, no spallation of the coating has been observed even under high circumferential strain, indicating that the coating remains adherent to the substrate and remain fully or partially (if cracking occurs) protective - including the highly deformed clad ballooning area - upon subsequent high temperature oxidation (FIG. 2 -b).
<figure><img src="image_3.png" /><figcaption>FIG. 1: Illustration of the Cr-coating strengthening effect on the Zr based claddings creep behaviour under inner pressure upon LOCA conditions for (a) isothermal and (b) thermal ramp tests [5-6]</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>FIG. 2: Illustrations of (a) Cr-coating effect on the reduction of the balloon size (i.e., maximum circumferential deformation at rupture location) following LOCA ramp testing under inner pressure [5] and an X-Ray tomography scan (b) showing a sub-volume with the cracked coating areas (dark grey) corresponding to areas where ZrO₂ formed during high temperature (double face) oxidation in air environment [7].</figcaption></figure>
The behavior of chromium-coated claddings during high temperature steam oxidation has been extensively studied [8] leading to the better understanding of the underlying Cr and O diffusion mechanisms (FIG. 3), and their impact on post-quench (PQ) mechanical properties. At temperatures above ~1300 °C, a Zr-Cr eutectic reaction causes rapid degradation of the coating and a significant increase in oxidation rate. However, as long as the Peak Cladding Temperature remains below this temperature threshold, optimized 10–20 µm thick HiPIMS-PVD Cr coatings have been shown to delay the onset of PQ embrittlement compared to uncoated claddings.

To quantify the effect of initial coating thickness on oxidation and PQ embrittlement threshold, a new metric — called “Equivalent Chromium Reacted” (ECrR) — has been proposed [9]. This parameter captures the relationship between coating thickness and PQ mechanical properties, such as residual strength and ductility, assessed by ring compression testing. For first-generation Cr-coated zirconium-based claddings, a critical ECrR value of approximately 70% has been identified, corresponding to the ductile-to-brittle transition of PQ Cr-coated claddings at 20–135°C (FIG. 4).

Additionally, different practical methodologies have been developed and used to evaluate the overall quantity of oxygen that has diffused into the sub-coating prior-β<sub>Zr</sub> substrate when the Cr-coating losses its protectiveness. Thus, it has been shown that, within the 1100-1300°C steam oxidation temperature range, the critical ECrR=70% corresponds more or less to the oxygen saturation of the prior-β<sub>Zr</sub> wall thickness achieving a “critical” 0.4-0.5 wt.% threshold value [10-11].

Finally, it should be mentioned that in addition to the oxygen effect, detailed investigations of the HT Cr diffusion effect on the resultant PQ microstructure and prior-β<sub>Zr</sub> hardening have been carried out using Cr enriched “model” Zr1Nb(O)-based alloys. Among other results, the PQ prior-β<sub>Zr</sub> hardening has been related to the final “two-steps cooling scenario” [12].
<figure><img src="image_5.png" /><figcaption>FIG. 3: Illustration of the consumption kinetics and underlying diffusion phenomena of Cr coatings on Zr-based substrate upon HT steam oxidation (1000-1300°C, LOCA and slightly beyond conditions) from [8] and some previous CEA publications</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>FIG. 4: Plot of PQ strength and ductility (from RCT) of Cr-coated claddings as a function of conventional "Baker-Just" ECR parameter (for uncoated claddings) and the new ECrR « Equivalent Chromium Reacted » parameter proposed, taking into account the initial Cr-coating thickness ($X_0$): ECrR (%) = 6.73.10⁵. exp(-15784/T).(t)¹/² / X₀ (X₀ in µm)</figcaption></figure>
REFERENCES

[1] J.-C. Brachet, et al., "Early studies on Cr-Coated Zircaloy-4 as enhanced accident tolerant nuclear fuel claddings for light water reactors", Journal of Nuclear Materials (JNM) 517 (2019) pp. 268-285, https://doi.org/10.1016/j.jnucmat.2019.02.018

[2] A. Wu, et al., "HRTEM and chemical study of an ion-irradiated chromium/zircaloy-4 Interface", Journal of Nuclear Materials 504 (2018) 289-299

[3] M. Bono, et al., "Mechanical testing of (Neutron) Irradiated ATF Chromium-Coated Zr-based Claddings", Poster presentation at NUMAT-2020, remote conference, (26-29 Oct. 2020)

[4] J.-C. Brachet, et al., "Behavior under LOCA conditions of Enhanced Accident Tolerant Chromium Coated Zircaloy-4 Claddings", Topfuel 2016-Light Water Reactor (LWR) Fuel Performance Meeting. 2016.

[5] J.-C. Brachet, et al., "Behavior of Cr coated M5™ claddings under LOCA conditions", Proceedings of 2017 WRFPM/Topfuel 2017, Sept. 10-14, 2017, Jeju Island, Korea

[6] M. Dumerval, et al., "Behavior of Chromium Coated M5 Claddings upon thermal ramp tests under internal pressure (LOss-of-Coolant Accident Conditions)", Proceedings of WRFPM/Topfuel 2018, (Sept. 3 – Oct. 04 2018), Prague, Czech Republic

[7] A. Charbal, et al., "X-ray Tomography Analysis of Chromium Coated Cladding Tubes After LOCA Testing", Proceedings of ANS/Topfuel 2025, (Oct. 5 – Oct. 09 2025), Nashville, United States of America

[8] J.-C. Brachet, et al., "High temperature steam oxidation of chromium-coated zirconium-based alloys: Kinetics and process", Corrosion Science 167 (2020) 108537, https://doi.org/10.1016/j.corsci.2020.108537

[9] J.-C. Brachet, et al., "A new « ECR » parameter to support high-temperature (one-sided) steam oxidation behavior of Cr-coated Zr-based E-ATF claddings, in relation with their post-quenching (PQ) mechanical strength and ductility (for LOCA and slightly beyond conditions)", Proceedings of WRFPM/Topfuel 2024, (Sept. 29 – Oct. 0, 2024), Grenoble, France

[10] J.-C. Brachet, et al., "Some practical methodologies to assess the overall high temperature (one-sided) steam oxidation protectiveness of chromium-based coatings on a zirconium-based substrate", JNM 606 (2025), 155620, https://doi.org/10.1016/j.jnucmat.2025.155620

[11] J.-C. Brachet, et al., "Evaluation of Equivalent Cladding Reacted parameters of Cr-coated claddings oxidized in steam at 1200°C in relation with oxygen diffusion/partitioning and post-quench ductility", JNM 533 (2020) 152106, https://doi.org/10.1016/j.jnucmat.2020.152106

[12] P. Gokelaere, et al., "Investigation of Cr diffusion hardening in Cr-coated Zr based cladding following various cooling scenarios from the β<sub>Zr</sub> temperature range", 21th. symposium on "Zirconium in the Nuclear Industry", (April 14-17, 2025), Aix-En-Provence, France, to be published in ASTM-STP (2026)
ACKNOWLEDGMENTS

The presented works have been supported by the CEA-EDF-FRAMATOME "Innovation
COMBustible" project. This work provides an overview of the research conducted at CEA,
reflecting the contributions of numerous individuals. The full list of contributors is extensive,
and the authors wish to express their sincere gratitude to all of them.