ABSTRACT
Fuel pellet material is the first and very important barrier in the system of barriers preventing fission product release outside the nuclear power plant. Under stationary conditions UO2 pellets hold more than 99 % of the activity of fission products, and under accident conditions, even after cladding burst and significant fuel heating, most of solid fission products are kept in the fuel until very high temperatures are reached. Moreover, the contribution of fission product release from intergranular bubbles and pores under accident conditions has been introduced into the calculation code independently from the volume diffusion processes. In the low temperature zone the process of fission product release occurs mostly due to kinetic energy of fission fragments. The experiments with VVER fuel elements irradiated in the MR reactor have shown similar effects. In order to provide a realistic evaluation of fission product releases during severe accidents it is necessary to be able to calculate the course of fuel heating.
