# Comparative Theoretical Analysis of Halden Reactor Creep Tests on M5 Cladding Material
## 1 Introduction  
The cladding material in nuclear reactors plays a critical role in ensuring the structural integrity of fuel rods, which enclose the nuclear fuel during both regular operation and transient conditions. One of the most significant challenges faced by cladding materials is creep, a time-dependent deformation process accelerated by high temperature, stress, and fast neutron flux. In particular, the M5 alloy, a zirconium-based material, has emerged as a promising candidate for cladding in pressurized water reactors (PWRs) due to its superior corrosion resistance and reduced hydrogen uptake compared to traditional Zircaloy-4 [1,2]. Nevertheless, understanding its long-term creep behavior under both tensile and compressive stress conditions, as well as the impact of neutron irradiation, remain crucial for optimizing reactor safety and performance [3]. Recent studies have focused on the complex interaction between creep, irradiation, and corrosion phenomena, which can significantly alter the mechanical properties of cladding materials over time. For example, [PERSON] et al. [4] demonstrated that high neutron flux can accelerate the creep process, particularly in the presence of tensile stress, leading to greater cladding diameter changes. Similarly, [PERSON] et al. [5] highlighted the exacerbating effects of localized corrosion, crud deposition, and surface oxidation, particularly under conditions of high surface temperatures, as experienced in nuclear reactors; in particular,Ref. [5] showed that crud deposition can induce a three-stage variation in cladding surface temperature, thus altering the corrosion mechanism and leading to a 9.5-fold increase in oxide layer thickness at the end of the fuel life. These findings underscore the necessity of considering both mechanical and chemical stressors in the analysis of in-reactor creep behavior. Despite the extensive research on cladding materials, there remain diverging opinions regarding the primary contributors to cladding failure under extreme operating conditions. While some researchers argue that creep-induced failure is primarily a mechanical issue [6], others emphasize the role of corrosion, suggesting that surface degradation due to crud deposition can significantly weaken the cladding, contributing to premature failure [7]. This discrepancy highlights the need for a more comprehensive approach to studying the combined effects of mechanical stress, neutron flux, and chemical factors like corrosion and crud formation, especially in the analysis of the test results and in view of new dedicated experimental activities. The present study aims to address these gaps by providing a detailed comparative analysis of the creep behavior of M5 cladding material under two distinct operational conditions: compressive stress in the IFA-617 test and tensile stress in the IFA-663 test, both conducted at the Halden reactor. By integrating experimental data with an analytical model, this work seeks to quantify the relative contributions of creep and corrosion in both environments. The findings of this study contribute to improving the predictive capabilities of codes such as TRANSURANUS, which are essential for simulating the long-term behavior of cladding materials in nuclear reactors. Additionally, this study provides insights that could inform future reactor designs and operational strategies aimed at mitigating cladding failure. The present paper specifically addresses two M5 cladding creep tests performed at the Halden reactor in the IFA-617 and IFA-663 test rigs, as reported in Halden reports HWR-658 [8] and HWR-755 [9]. These tests were conducted under different cladding stress conditions: the M5 cladding operated under compressive stress in the IFA-617 test, while, in IFA-663, the cladding was subjected to tensile stress. The primary objective of this analysis is to evaluate the creep behavior of the M5 cladding material in these contrasting environments, drawing conclusions from the test results.