Analysis of tritium evolution in Al₂O₃-coated FeCrAl fuel rods for PWRs

FeCrAl is one of the ATF cladding solutions due to its excellent oxidation resistance. In this work, a 2D numerical simulation method was developed to study tritium release from the nuclear fuel pellets to the fuel-cladding gap, tritium adsorption on the inner surface of the cladding, and permeation through Al₂O₃-coated FeCrAl cladding into the coolant. For Al₂O₃-coated fuel rods, based on Sievert's law, tritium concentration at the pellet-cladding gap is much higher than that at the inner cladding surface for most of the time under normal conditions, which slows down the tritium release rate from the fuel rods. With Al₂O₃ coating, tritium release from the fuel rods remains rising throughout the lifetime. The release rate is calculated to be low at the end of the lifetime, indicating that the Al₂O₃ coating prevents tritium release. As the coating thickness decreases, the flux of tritium release is found to be increased. Locally uncoated drawbacks lead tritium in the gap to be preferentially adsorbed by the inner surface of FeCrAl, then diffused in FeCrAl, and finally released outside the fuel rod. Still, more actual irradiation operations and test data are needed.