The changes in the properties of fuel pellets are important since they affect the stress on the fuel rod cladding. The changes in the pellet are also more pronounced with a higher fuel burnup and hence there is increased impact on the cladding, which can cause fracturing of the cladding. Fuel pellet radioactivity provides also a source term for the evaluation of potential releases in accident analysis and public exposure. One of the parameters important for pellet change is grain size. Currently, PWR and BWR fuels taken to higher burnup are manufactured with grain sizes of 8-12 pm. During fuel irradiation there is some grain growth in the central hotter region of the fuel pellet. This grain growth depends on the operating temperature of the reactor but also on the irradiation time, which means with fuel burnup. During irradiation, fission gas is produced in the grains, which migrates to the grain boundaries where fission gas bubbles form. During prolonged irradiation of higher burnup fuel, the gas bubbles may become interlinked and may release gases and volatile fission products into the fuel cladding gap, which could assist the release of fission product gases in an accident or cladding breach. There is a tendency to increase the grain size in the course of fuel manufacturing in order to increase the diffusion length for fission gases within a grain.3 In conclusion, fission gas release into the cladding gap is accelerated with higher fuel burnup. In addition to gases, the release of fission product volatiles like iodine and caesium (Is) may also be important. The Cs release path follows the Xe path closely (besides being a direct fission product Cs-137 is also generated by the в decay of Xe-137, which has a short half-life) and it migrates from hotter grains in the pellet and may migrate to the pellet interface. At burnups above 40 GWd/tU a rim starts to form at the outer radius of fuel pellets. The pellet rim is characterized by much higher porosity with the formation of many smaller grains, which can retain noble gases.3 This rim generation related to burnup is also important for potential gas and particulate releases in a cladding breach.