Application of the BEPU methodology to assess fuel performance in dry storage

Abstract

The stress at which fuel cladding is submitted in dry storage is the driving force of the main degrading mechanisms postulated (i.e., embrittlement due to hydrides radial reorientation and creep). Therefore, a sound assessment is mandatory to reliably predict fuel performance under the dry storage prevailing conditions. Through fuel rod thermo-mechanical codes, best estimate calculations can be conducted. Precision of predictions depends on uncertainties affecting the way of calculating the stress, so by using uncertainty analysis an upper bound of stress can be determined and compared to safety limits set.The present work shows the application of the BEPU (Best Estimate Plus Uncertainty) methodology in this field. Concretely, hydrides radial reorientation has been assessed based on stress predictions under challenging thermal conditions (400°C) and a stress limit of 90MPa. The computational tools used to do that are FRAPCON-3xt (best estimate) and Dakota (uncertainty analysis). The methodology has been applied to a typical PWR fuel rod highly irradiated (65GWd/tU) at different power histories. The study performed allows concluding that both the power history and the prediction uncertainty should not be disregarded when fuel rod integrity is evaluated in dry storage. On probabilistic bases, a burnup of 60GWd/tU is found out as an acceptable threshold even in the most challenging irradiation conditions considered.