Graduate Thesis Or Dissertation
 

A model for the mechanical behavior of irradiated mixed carbide sphere pac fuel pins

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  • A model is developed that allows the prediction of the stress-strain distribution in a mixed carbide sphere pac fuel pin during irradiation. The model is coupled with a previously developed model for the thermal behavior of the pin. The one-dimensional (radial at various axial locations) solution is for quasi-steady pin operating conditions. Generalized plane strain is assumed for the fuel and cladding separately so that slip at the fuel-cladding interface can be accommodated. The elasto-plastic equations are solved including elastic-plastic transition and unloading. A finite element solution scheme is used with an incremental approach to linearize the equations. The nonlinear effects of elastic and plastic material property changes are included. Elastic, plastic, creep, swelling, and thermal strains are considered separately in the analysis. The elastic and plastic properties depend on the material properties of the spheres themselves and the geometry of the packed bed. The bed geometry changes during irradiation from a cohesionless random sphere array to a form resembling porous pellet fuel. Analytic expressions are derived for the fuel elastic constants and the yield strength parameters as functions of the bed geometry and the sphere material properties. Good agreement between available experimental data and the analytic prediction is observed. The models used for the fuel creep and swelling are described. The fuel creep strain includes the standard deviatoric component along with a volumetric component to model the fuel densification as the geometry changes. Swelling is based on available correlations. The numerical solution scheme as implemented in the SPECKLE-III code is discussed and results are presented for the simulation of a sphere pac pin irradiation experiment. The code predictions are in good agreement with post-irradiation examination results.
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