fissiongas

Fission gas refers to gaseous fission products produced when nuclear fuel such as uranium dioxide undergoes fission. The major constituent gases are the noble gases krypton and xenon, formed directly as fission fragments; smaller amounts of helium arise mainly from alpha decay of actinides within the fuel. These gases are initially dissolved in the fuel matrix or contained in microscopic bubbles inside the fuel.

Formation and distribution: Fission gases form continuously during irradiation as fission fragments are created. They tend to accumulate as gas bubbles within the fuel grains, at grain boundaries, and in pores. Temperature, burnup, irradiation rate, and the microstructure of the fuel influence diffusion and bubble growth. At higher burnup or temperatures, gases can migrate toward the grain boundaries and be released from the fuel to the plenum.

Release mechanisms: The release of fission gas involves intragranular diffusion, grain boundary diffusion, and bubble growth, and is affected by the fuel’s microstructure, such as the development of a high-burnup structure with interconnected porosity. The fraction released to the rod plenum increases with temperature and burnup, and with fuel-cladding interaction.

Impact: The buildup of fission gas contributes to internal pressure in the fuel rod, causes swelling of the fuel, and can influence cladding integrity and fission gas management requirements. Accurate modeling of fission gas release is essential for fuel performance analyses and safety assessments.

Measurement and modeling: Fission gas release is evaluated through post-irradiation examination and in-core monitoring, and is represented in fuel-performance codes by release fractions that depend on burnup, temperature, and microstructure.