Microstructurally informed modeling of high burnup structure formation in UO2 Fuel

Abstract

There is a push within the United States nuclear industry to extend the peak rod average burnup limits. However, fuel fragmentation, relocation and dispersal (FFRD) and transient fission gas release (tFGR) have been identified as impacting safety and therefore may limit the potential to increase burnup. These are interrelated to and affected by the formation of high burnup structure (HBS) in the fuel pellet. Thus, prediction of HBS is an important tool in developing a predictive understanding of FFRD and tFGR. Recent post-irradiation examination (PIE) of high burnup spent fuel using modern electron microscopy provides data to support the development of a methodology to create a microstructurally informed model of HBS formation. The work described here consists of a set of physically informed differential equations that track evolving porosity, grain boundary character, and grain diameter. These equations are physically informed but empirically shaped by recent PIE data. Additionally, these parameters are quantitatively related to a volumetric restructured fraction of fuel allowing for use in an engineering scale finite element fuel performance code. The model presented in this work is used to obtain predictions for fuel pellet microstructure and HBS as a function of radius using the H. B. Robinson power history.