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This paper focuses on the PLEIADES fuel performance software environment and its application to the modeling of pellet-cladding interaction (PCI). The PLEIADES platform has been under development for 10 yr; a unified software environment, including the multidimensional finite element solver CAST3M, has been used to develop eight computation schemes now under operation. Among the latter, the ALCYONE application is devoted to pressurized water reactor fuel rod behavior. This application provides a three-dimensional (3-D) model for a detailed analysis of fuel element behavior and enables validation through comparing simulation and postirradiation examination results (cladding residual diameter and ridges, dishing filling, pellet cracking, etc.). These last years the 3-D computation scheme of the ALCYONE application has been enriched with a complete set of physical models to take into account thermomechanical and chemical-physical behavior of the fuel element under irradiation. These models have been validated through the ALCYONE application on a large experimental database composed of approximately 400 study cases. The strong point of the ALCYONE application concerns the local approach of stress-corrosion-cracking rupture under PCI, which can be computed with the 3-D finite element solver. Further developments for PCI modeling in the PLEIADES platform are devoted to a new mesh refinement method for assessing stress-and-strain concentration (multigrid technique) and a new component for assessing fission product chemical recombination. [ABSTRACT FROM AUTHOR]

• Development of a phase-field model for a two-phase multicomponent system • Coupling of the phase-field model with the TAF-ID • Derivation of a partitioned fixed-point numerical solving • Simulation of incipient fuel melting and oxygen transport in nuclear fuel In this paper, a phase-field model is developed for a two phase compositional (multicomponent) system and combined with the thermodynamic description provided by a CALPHAD database. The model is applied to a uranium-oxygen binary system within a solid/liquid mixture to simulate incipient melting and oxygen transport (oxygen-uranium inter-diffusion and Soret effect) in the fuel with thermodynamic properties coming from the Thermodynamics for Advanced Fuels-International Database (TAF-ID). The interface with the TAF-ID required the coupling of the phase-field model with the OpenCalphad thermochemical solver. This coupling led to the development of a partitioned scheme that is solved at each time step with a fixed-point algorithm. To analyze the response of and to parameterize the phase-field model, two 1D demonstration problems from the open literature have been considered and solved using a finite-difference scheme. The first problem deals with oxygen transport in UO 2.005 fuel under an imposed parabolic temperature profile below liquidus. The second one describes the incipient melting and oxygen transport in oxidized fuel under an increasing temperature profile. Oxygen thermal diffusion is considered both in the solid and liquid phases. All the results presented in this paper are in good agreement with those already published but are here performed considering a direct coupling with the TAF-ID database. They demonstrate the thermodynamic consistency of the phase-field model as well as its capability to simulate incipient melting and oxygen transport in the fuel with advanced thermodynamics databases. [ABSTRACT FROM AUTHOR]

• Coupling between ALCYONE fuel performance code and OpenCalphad thermochemical code. • 3.5D challenging simulation of a complete rodlet during a power transient. • Reduction of the equilibrium calculation time by defining a good initial estimate of stable phases. In this paper, a robust, accurate and efficient coupling between the PLEIADES/ALCYONE 2.1 fuel performance code and the OpenCalphad thermo-chemical solver is presented. A challenging 3.5D simulation of a power ramp, consisting of the 3D simulations of all the fuel pellets within a complete rodlet, serves as a base case to illustrate the difficulty inherent to extensive thermochemical equilibrium calculations (more than 1.5 million in this example) within fuel performance simulations, related both to the wide temperature and pressure range encountered in irradiated nuclear fuels. In the first part of this paper, a detailed analysis of the main quantities of interest (i.e. temperature, hydrostatic pressure, fission products concentration) is provided to illustrate the couplings at hand during a power ramp and, above all, to demonstrate the good agreement of the results with those already published in the literature and with available measurements of xenon, cæsium and iodine releases. In the core of this paper, several numerical strategies are proposed to reduce the overall time spent in the thermochemical solver by defining at each node and for each time step a good initial estimate of the set of stable phases that are likely to form. The first one, referred to as the spatial strategy, relies on an intelligent reorganization of the mesh nodes as a function of temperature or hydrostatic pressure with an appropriate initialization of the thermochemical equilibrium calculation at a node by the phase composition calculated at the node with the closest thermo-mechanical conditions. The second strategy, referred to as the hybrid strategy, combines the spatial strategy during the power transient (where large power variations take place at each time step) with an initialization of the nodal equilibria from the solution obtained at the previous time step in case of small power variations. The 3.5D simulation has been run several times in order to assess and compare the spatial and hybrid strategies. The simulations show that the spatial strategy is the most efficient. It leads to an overall increase of the calculation time related to the incorporation of thermochemistry in the 3.5D fuel performance simulation of less than 2.25%, showing the feasibility of large thermal-chemical–mechanical simulations within the PLEIADES/ALCYONE 2.1 fuel performance code. Finally, as a first step towards more complicated simulations (i.e. including a greater number of phases and/or performed with a finer spatial discretization), a primitive parallelization algorithm based on a multiprocess approach is proposed. Combined with the most efficient spatial strategy, it leads to a substantial reduction of the extra (real) calculation time related to thermochemistry which become less than 0.65% of the total computation time when the thermochemical equilibrium calculations are distributed over 4 cores. [ABSTRACT FROM AUTHOR]

The aim of this contribution is to deeply introduce the ALCYONE Fuel Performance Code. ALCYONE is being implemented within the PLEIADES numerical framework and is dedicated to the multiphysics behavior of fuel rods in pressurized water reactors (PWRs). It has been a joint development between CEA, EDF and Framatome since 2005 and has been used since then for numerous studies covering normal, off-normal and accidental conditions. However, a detailed description of ALCYONE's physical and numerical foundations has never been published. In this sense, this paper is intended to be the reference presentation paper for ALCYONE. On the first hand, it provides a comprehensive description of the modeling features and associated multiphysics and multiscale computational schemes. On the other hand, focuses are made on advanced modeling features showing enhanced capabilities of simulation of ALCYONE. • Detailed description of ALCYONE, the PLEIADES PWR Fuel Performance Code. • ALCYONE's physical models and numerical features are presented. • ALCYONE's simulations cover normal, off-normal, and accidental conditions. • Verification, validation and uncertainty quantification process is discussed. • Insights on some advanced modeling capabilities and numerical coupling schemes. [ABSTRACT FROM AUTHOR]