Fuel rod design and modeling of fuel rods. Pt. 1

The design of the fuel rods is essential for the safe and economic operation of nuclear power plants. Extensive operational experience shows the safety standard of the fuel rods to be very high, meeting internationally acknowledged safety requirements. Over the course of time, economic considerations have led to an increase of the average fuel assembly burn-up from 35 MWd/kgHM to around 55 MWd/kgHM today. Besides being more efficient, this also contributes to improved environmental control by reducing the required amount of nuclear fuel and the number of fuel assemblies to be disposed of. However, every further increase of the burn-up also means additional requirements for the fuel rods, particularly for the cladding tube material, and necessitates re-verification of the fuel rod integrity. The change from deterministic to statistical design methods has been discussed for some time, with the aim of arriving at a more realistic description of the processes in the reactor core by considering their inherent variability. As a result, more reliable and more accurate predictions could be made about the behavior of the fuel rods, forming the basis of good fuel rod design and proof of fuel rod integrity. This paper discusses the extent to which the existing models consider the variability, from a stochastic point of view, and shows that a stochastic model provides new possibilities for showing the integrity. The proposed stochastic model rectifies certain weaknesses of the traditional proceedings, directly applying all existing knowledge and revealing any remaining ignorance. In addition, several questions which recently arose in Germany regarding fuel rod specifications are critically discussed in the paper. The paper consists of two parts. The first part deals with the necessary basic principles, while the second part deals with more technical questions. (orig.)