Investigation of coated cladding ballooning behavior using bison

Numerous fuel vendors throughout the world have proposed new fuel rod designs that include a coating on the exterior surface of the clad for improved oxidation performance. These designs are proposed as a potential enhanced accident tolerant alternative to conventional fuel rods currently used in commercial light water reactors (LWRs). The current front runner in the majority of coated clad designs is the use of chromium (Cr) for the coating. While the oxidation benefits have been widely studied from both experimental and modeling point of views; a more recent speculation is that the inclusion of the coating reduces the observed balloon size when compared to uncoated clads during a loss of coolant accident (LOCA). In this paper, the Bison fuel performance code is used to qualitatively confirm the observation of reduced balloon sizes between Cr-coated and uncoated zirconium-based clads. Bison is coupled to the Dakota software to perform a parametric study that varies the coating thickness (0, 10, 20, 50, and 100 μm), the cladding substrate (Zr-4, M5^®, and ZIRLO^TM), the maximum cladding temperature (900, 1000, 1100, and 1200 K), the internal pressure ramping rate (5, 10, and 20 kPa/s), and the fast neutron fluence (0, 2.5, and 5 × 10²⁵ neutrons/m²) to investigate these parameters influence the ballooning. behavior. It is observed that the thermal expansion coefficient mismatch between the chromium and the Zircaloy substrate causes the coating to undergo permanent plastic deformation. This permanent deformation reduces the balloon size observed in the coated clads. Of the parameters varied in this study the peak cladding temperature and pressure ramping rates had the largest impact on the balloon size whereas the fast neutron fluence and cladding substrate had limited to no influence. Further work is required to validate these observations once the experimental data becomes publicly available.