Technical and licensing challenges and their solutions for high burnup

Significant progress has been achieved in the past by Siemens in meeting the demands of utilities for increased fuel burnup. The technical challenges posed by the increased burnup are mainly related to the corrosion and hydrogen pickup of the clad, the high burnup properties of the fuel and the dimensional changes of the fuel assembly structure. Clad materials with increased corrosion resistance have been developed which are appropriate for high burnup. The high burnup behaviour of the fuel has been extensively investigated and the decrease of thermal conductivity with burnup, the rim effect of the pellet and the increase of fission gas release with burnup can be described, with good accuracy, in fuel rod computer codes. Advanced statistical design methods have also been developed. Materials with increased corrosion resistance are helpful controlling the dimensional changes of the fuel assembly structure. In summary, most of the technical questions for the fuel operational behaviour and reliability in the high burnup range have been solved or the solutions are visible. This is also acknowledge by regulators. The main licensing challenges for high burnup fuel are currently seen for accident condition analyses, especially for RIA and LOCA. (authors)