Abstract
Recent theories of the development of lenticular grain face bubble populations and interconnected grain edge pores in UO2 during irradiation are described qualitatively. The stability of the tunnel-shaped pores is discussed and consideration is given to the importance of both kinds of porosity to the release of gaseous and volatile fission products, either stable or unstable. The influence of hydrostatic stress within the fuel, arising both from the mechanical interaction between fuel and cladding and from the plenum gas pressure is taken into account.