Evaluation of variability in the thermo-mechanical response of nitride nuclear fuels through microstructurally explicit models

Techniques were developed to formulate simplified explicit finite element models (FEMs) of a (U,Pu,Zr)N fuel pellet using the microstructure of ZrN. ZrN has a microstructure similar to that of (U,Pu)N, and is planned to be a major component of proposed fuels that will not form more actinides (inert matrix fuels); hence, it can be considered an appropriate "surrogate" for actual nitride fuels. Three different 2-D Representative Volume Element (RVE) FEMs that include pore and grain geometry as well as grain orientation, were obtained from microstructure characterization in ZrN. Appropriate conditions were applied separately to these RVEs to quantify the variability of stresses due to microstructure heterogeneity and to obtain effective elastic properties. It was found that the four central moments of the statistical distribution of grain size had a strong correlation with stress variability within the different RVE realizations, and that the highest stresses were found to be located at inter-granular pores sites and at the grain boundaries that connect clusters of pores.;Finally, these RVEs were employed in a two-scale method in which both microstructure and bulk material were considered (i.e. the "embedded cell" approach) to evaluate the thermo-mechanical response of an inert-matrix nuclear fuel. Two-dimensional thermo-mechanical FEMs of a longitudinal cross-section of a cylindrical fuel pellet, with the RVEs positioned at different locations, were assessed to investigate variability in the thermo-mechanical response (e.g., temperature distribution, stress field and strain field) due to microstructure heterogeneity of the RVE, which was surrounded by homogeneous material with the effective properties obtained from the RVE and literature values. Results showed that the variability of stresses inside the RVEs was also affected by changes in the third and fourth central moments of statistics, and anisotropic creep incorporated to the grains led to increased variability of stresses inside the RVE, which were high enough to lead to fracture within a few hours at steady state temperature. Comparisons with UO2 clearly indicated that stresses in the oxide fuel were higher and showed more scatter, due to differences in thermo-mechanical properties between nitride and oxide fuels. All of these observations indicate that nitride fuels would have a better overall thermo-mechanical behavior in pile than oxides and would be a good substitute to them.