Impact of the relaxation of the tilt value on the tilt penalties of the safety studies

The phenomenon known as tilt represents an asymmetry in the power distribution across the quadrants of the core, where certain regions exhibit a certain imbalance in power compared to others. Although the average power remains unchanged, this imbalance leads to one quarter of the core producing significantly more power than its opposite. Due to its considerable impact on both normal and accidental operations, tilt must be carefully considered in nuclear reactor safety studies. To account for this phenomenon, penalties are applied to specific neutronic parameters, ensuring safety throughout the reactor’s operational lifecycle. Moreover, tilt must be actively limited during reactor operation to prevent significant deviations that could affect core stability and reactor behavior. In the 4th ten yearly safety review of the French fleet of 1300 MWE NPP, an imbalance of 3% power, at BLX and 100% nominal power, in one quadrant of the core is considered in order to calculate penalties for tilt. The objective of this intership is to evaluate the impact of the relaxation of the amplitude of the tilt for neutronic models exploited in safety studies. In the first part of this work the modelization of tilted models at 2% BLX at 100% NP and 2.5% BLX at 100% NP for different types of fuel cycles is performed. Then the impact of these perturbated models on tilt penalties related to factors that, according to previous studies at 3%, were most affected , will be evaluated. In particular studies regarding the shut down margin and hot spot factors. Subsequently, tilt penalties will be analyzed in the case of a rod ejection accident, which belongs to category 4, the most severe accident category. Once again, the impact on the neutronic parameters most affected in previous studies will be evaluated. The internship seeks to determine whether these penalties vary linearly with the percentage of tilt values and to see if tilt penalities at Y % of tilt could be estimated starting from tilt penalities at X % of tilt, with X<Y, by applying a factor Y/X. The results show that while linear approximation may work in some cases with negligible errors, it often underestimates penalties, particularly for hot spot factors and rod ejection accident studies. This underestimation is unacceptable for safety studies, highlighting the need for more rigorous recalculations to obtain accurate and conservative safety evaluations.