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INTRODUCTIONPellet cladding interaction (PCI) in light water reactor (LWR) fuel is a coupled thermal-chemicalmechanical process that can lead to cladding breach and release of radioactive fission products into the coolant under certain conditions of operating history, power change, and fuel rod design characteristics [1][2][3][4]. Reactor operating restrictions, which limit power maneuvering, have been established to mitigate PCI, but they restrain operational flexibility and lead to loss of power generation. The Consortium for the Advanced Simulation of Light water reactors (CASL) has selected PCI as a key challenge problem and is developing an advanced, 3-dimensional fuel rod simulation capability (referred to as Bison-CASL) to evaluate fuel performance in general and provide PCI failure assessments in particular. With an advanced fuel rod modeling capability that considers the underlying mechanisms leading to cladding failure, fuel designers and engineers can investigate improved fuel concepts for PCI-resistance and better quantify margins to PCI for operating existing fuel rod designs.. PCI failures generally occur following an increase in the local power over a short period of time, and in fuel that has been previously exposed to irradiation. Classical PCI is driven by the localized strains in the vicinity of a pellet crack, as well as the presence of a chemical species, such as iodine, that drive stress corrosion-induced cracking of the cladding [5]. Fuel pellet cracks that form in brittle ceramic pellets by large temperature gradients, are believed important in the PCI failure mechanism [5]. During a local power increase, pellet expansion produces a high contact force between the fuel pellet and cladding material, when a reduced or eliminated residual pellet-clad gap is present because of previous irradiation. Furthermore, during the rapid thermal expansion of the pellet, the fuel cracks can further open, which transfers tangential shear forces onto the cladding. These tangential shear forces are a function of the equilibrium pellet-clad gap or residual contact pressure at the start of the power increase, the power level at gap closure, the interfacial friction, and the maximum local power.Non-classical PCI failure is associated with the presence of a missing pellet surface (MPS) defect [5]. These MPS defects form through mishandling or the manufacturing process, where the pellet is chipped leaving a flaw on the outer surface. The presence of an MPS defect during a localized power ramp can cause severe bending moments in the clad in the vicinity of the MPS when the fuel undergoes rapid thermal expansion due to this increase in local power. Furthermore, the localized region near the MPS also experiences a different temperature distribution compared to when the MPS is not present. The result is a localized hot spot in the fuel and cold spot in the clad.Both classical and non-classical PCI are significantly influenced by the geometry of fuel pellet flaws (i.e pellet cracks and MPS). The purpose of the current...

Potential mitigation strategies for preventing stress corrosion cracking (SCC) failures in CANDU fuel cladding that are based on lessons learned on both domestic and international fronts are discussed in this paper. Although SCC failures have not been a major concern in CANDU reactors in recent decades, they may resurface at higher burnup for conventional fuels or with nonconventional fuels that are currently being investigated, such as MOX or thoria-based fuels. The motivation of this work is to provide the foundation for considering possible remedies for SCC failures. Three candidate remedies are discussed, namely improved fabrication methods for fuel appendages, barrier-liner cladding, and fuel doping. In support of this effort, recent advances in experimental characterization methods are described—methods that have been successfully used in non-nuclear materials that can be used to further elucidate SCC behaviour in CANDU fuel. The overall objective is to outline a path forward for characterizing material behaviour as an essential part of investigating remedies to SCC failure. This will allow increased fuel discharge burnup, maximum linear power, and plant manoeuvrability, while maintaining a high degree of reliability.

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