In the case of ceramic fuel pellets, gas atoms and vacancies in the as-manufactured pores can diffuse into the surrounding fuel matrix or be ejected into the matrix by the disruptive action of fission fragments. Once in the matrix, the vacancies diffuse to grain boundaries where they are absorbed. The result is a densification (i. e. a reduction in volume) of the fuel pellets over the initial few months of irradiation. The increase in the pellet-cladding gap size tends to increase fuel temperatures, while the increase in fuel thermal conductivity due to the reduction in the porosity volume fraction tends to decrease fuel temperatures. Since the

former is a stronger effect, the net outcome is an increase in maximum fuel temperature at any given power. The extent of densification is dependent upon the fuel type, the initial porosity volume fraction, the porosity size distribution and the evolution of the fuel temperature and fission density distributions with time.