As fission process heats up the fuel rods, UO2 pellets stacked on top of each other swell both radially and axially, while the surrounding Zircaloy cladding creeps down, so that the pellets eventually come into contact with the clad. This exacerbates chemical degradation of the protective cladding and high stress values may enable the formation and propagation of cracks, thus threatening the integrity of the clad. Along these lines, pellet-cladding interaction establishes itself as a major concern for fuel rod design and core operation in light water reactors. Accurately modeling fuel behavior is challenging because the mechanical contact problem strongly depends on temperature distribution and the pellet-clad coupled heat transfer problem is, in turn, affected by changes in geometry induced by body deformations and stresses generated at the contact interface. Our work focuses on active set strategies to determine the actual contact area in high-fidelity coupled physics fuel performance codes. The approach consists of two steps: in the first one, we determine the boundary region on standard finite element meshes where the contact conditions shall be enforced to prevent objects from occupying the same space. For this purpose, we developed and implemented an efficient parallel search algorithm for detecting mesh inter-penetration and vertex/mesh overlap. The second step deals with solving the mechanical equilibrium taking into account the contact conditions computed in the first step. To do so, we developed a modified version of the multi-point constraint strategy. While the original algorithm was restricted to the Jacobi preconditioned conjugate gradient method, our approach works with any Krylov solver and does not put any restriction on the type of preconditioner used. The multibody thermo-mechanical contact problem is tackled using modern numerics, with continuous finite elements and a Newton-based monolithic strategy to handle nonlinearities (the one stemming from the contact condition itself as well as the one due to the temperature-dependence of the fuel thermal conductivity, for instance) and coupling between the various physics components (gap conductance sensitive to the clad-pellet distance, thermal expansion coefficient or Young's modulus affected by temperature changes, etc.). We will provide different numerical examples for contact problems using one and multiple bodies in order to demonstrate the performance of the method. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/152540

An accurate implementation of glued mechanical contact was developed in MOOSE based on its Constraint system. This approach results in a superior convergence of elastic structure problems, in particular in BISON. Adaptation of this technique to frictionless and frictional contact models is under way. Additionally, the improved convergence of elastic problems results from the application of the split-based preconditioners to constraint-based systems. This yields a substantial increase in the robustness of elastic solvers when the number of nodes in contact is increased and/or the mesh is refined.

The transfer of heat and force from UO2 pellets to.

The implicit solution to contact problems in nonlinear solid mechanics poses many difficulties. Traditional node-to-segment methods may suffer from locking and experience contact force chatter in the presence of sliding. More recent developments include mortar based methods, which resolve local contact interactions over face-pairs and feature a kinematic constraint in integral form that smoothes contact behavior, especially in the presence of sliding. These methods have been shown to perform well in the presence of geometric nonlinearities and are demonstratively more robust than node-to-segment methods. These methods are typically biased, however, interpolating contact tractions and gap equations on a designated non-mortar face, which leads to an asymmetry in the formulation. Another challenge is constraint enforcement. The general selection of the active set of constraints is brought with difficulty, often leading to non-physical solutions and easily resulting in missed face-pair interactions. Details on reliable constraint enforcement methods are lacking in the greater contact literature. This work presents an unbiased contact formulation utilizing a median-plane methodology. Up to linear polynomials are used for the discrete pressure representation and integral gap constraints are enforced using a novel subcycling procedure. This procedure reliably determines the active set of contact constraints leading to physical and kinematically admissible solutions void of heuristics and user action. The contact method presented herein successfully solves difficult quasi-static contact problems in the implicit computational setting. These problems feature finite deformations, material nonlinearity, and complex interface geometries, all of which are challenging characteristics for contact implementations and constraint enforcement algorithms. The subcycling procedure is a key feature of this method, handling active constraint selection for complex interfaces and mesh geometries.

This updated version of Nuclear Energy Series NF-T-2.1 provides information on all aspects of fuel failures in current nuclear power plant operations.

This publication is the result of an IAEA technical meeting and reports on Member States' capabilities in modelling, predicting and improving their understanding of the behaviour of nuclear fuel under accident conditions. The main results and outcomes of a coordinated research project (CRP) on this topic are also presented.