Changes in micro-structure and xenon distribution in VVER fuel at high burnup

Modern design of the VVER fuel rods should provide a burnup exceeding 70 MW·day/kgU. The growth of fuel burnup initiates physical processes that define not only quantitative change of fuel characteristics, but also its qualitatively new state not observed earlier with a design burnup. Therefore, definition of allowable limits of fuel rod serviceability with an increase of burnup is based on a detailed study into the processes occurring in fuel and their influence on the state of fuel rods. Two key factors define a state of the VVER high burnup fuel: restructurization in the low- temperature external area of the fuel meat and accumulation of gaseous fission products. This paper reveals and analyses experimental laws of xenon accumulation in various micro-structural components of the UO2 fuel pellets with an average burnup of up to 70 MW·day/kgU. Mechanism of change in the UO2 microstructure on a radius of fuel pellets at various burnups is also revealed. (authors)