Unit mechanisms of fission gas release: Current understanding and future needs

Abstract

Gaseous fission product transport and release has a large impact on fuel performance, degrading fuel and gap properties. While gaseous fission product behavior has been investigated with bulk reactor experiments and simplified analytical models, recent improvements in experimental and modeling approaches at the atomistic and mesoscales are beginning to reveal new understanding of the unit mechanisms that define fission product behavior. Here, existing research on the basic mechanisms of fission gas release during normal reactor operation are summarized and critical areas where work is needed are identified. This basic understanding of the fission gas behavior mechanisms has the potential to revolutionize our ability to predict fission product behavior and to design fuels with improved performance. In addition, this work can serve as a model on how a coupled experimental and modeling approach can be applied to understand the unit mechanisms behind other critical behaviors in reactor materials.

An equation of state for xenon/krypton mixtures confined in the nuclear fuels

“…The noble gas density in the intragranular bubbles ranges from about 10 nm-to slightly more than 30 nm- [3,7,8] while for the intergranular bubbles, the estimated densities are less than 10 nm- [6]. The molar fraction of Kr in the Xe/Kr noble gas mixture in the bubble, of 0.12-0.14 [2,6], mirrors the production rates of the two noble gases.…”

“…These noble gases have a very low solubility in the fuel matrix and, consequently, they precipitate together into the fuel under the form of intragranular and intergranular bubbles [1]. In low burnup fuels, the intragranular bubbles are typically less than 10 nm in diameter [2][3][4] while in high burnup fuels, some of these small bubbles can coalesce to lead to a bimodal bubbles population, with the largest bubbles of tens [2,3] or even hundreds [5] of nanometers in diameter. The intergranular bubbles are generally larger in size than the intragranular ones, with a diameter of several hundreds of nanometers [5,6].…”

“…This is the reason why modeling the evolution of the fission gas bubbles in nuclear fuels under irradiation is a crucial step in any kind of simulation of nuclear fuel rod behavior in accident conditions. 2 The growth rate équations allowing to model the fission gas bubble évolution in nuclear fuels [9] make use of equations of state (EOS) in order to characterize the behavior of the noble gases confined in bubbles. To our knowledge, none of these EOSs is designed to describe mixtures of Xe and Kr or to take into account a possible bubble confinement effect.…”

Radiation-Induced Effects on Material Properties of Ceramics: Mechanical and Dimensional Properties

“…This mechanism is affected by other parameters, as departure from stoichiometry, local chemistry, and particular microstructural features (as for instance microporosity) that can interact with the fission gases. [146][147][148] In some cases, for particular burnup and temperature conditions, the concentration of gas bubbles seems sufficient to release a burst of fission gases, eventually producing cracks in the fuel pellet.…”

Modelling of fine fragmentation and fission gas release of UO2 fuel in accident conditions

“…This is not an easy task from a modelling point of view: the gas pressure calculation requires an equation of state that is applicable to high pressures and that the input to this equation, namely the cavity volume and gas inventory (molecular density and composition of gaseous fission products) are accurately calculated together with the gas temperature. Prevalent computational tools for thermal-mechanical analyses of nuclear fuel do not generally model the fuel microstructure evolution and the fission gas distribution at this level of detail [39]. This is particularly true for the evolution of the high burnup structure.…”