Grid to rod fretting wear in EDF PWR from operating problems to new designs qualification method

For the last ten years, grid-to-rod fretting wear lead to a large number of leaking fuel rods in PWR Power Plants throughout the world. The potential consequences of experiencing leaking fuel rods in a core are: (1) anticipated plant shutdown in order to respect the mandatory fission gas threshold in the primary coolant, (2) changes in the loading pattern, (3) special operating personnel training and plant shutdown precautions to prevent contamination risks. For EDF, it is a safety and financial stake. Two main phenomena are promoting the grid-to-rod fretting wear: the fuel assembly (FA) flow induced vibration resonance and/or the rod vibration under turbulent flow excitation. EDF and its industrials partners have launched an important R and D program to provide EDF with analytic tools and methods to prevent new design from experiencing fretting wear problems. This paper presents how EDF qualifies each new design proposed by the fuel vendors. EDF qualification method is based on three step (1) Operating feedback, (2) Full size FA vibration and endurance tests, (3) Analytical simulation. (1) French PWR standardisation authorises to extend one plant operating feedback to the other plant with the same standard. Operating feedback gives relevant information regarding models and the qualification test methodology. (2) Full size FA mock-ups have been tested in an out-of-pile hydraulic loop to study the vibration response of the FA under different hydraulic loadings and the grid-to rod fretting wear after different period of time. (3) Rod vibration behaviour models have been developed based on the EDF Finite Element code CodeAster with an equivalent linear grid-to-rod contact modeling, or contact elements. Using both the experimental and the analytical approaches, this paper provides the criteria to avoid grid-to-rod fretting. Regarding this concern, the most important features to look at are the grid-to-rod contact conditions and the hydraulic loading. (author)