MODELING OF CLADDING THERMAL EVOLUTION ALONG CASK STORAGE

摘要

One of the safety requirements in dry storage of spent fuel is to ensure the cladding integrity. In this regard, the understanding of the cladding mechanical performance along the storage period is indispensable, both to analyse the failure probability and to characterize the state of the cladding so that fuel management is conducted with accurate knowledge of the material conditions. The main interest is focused on cladding degrading mechanisms as creep and hydrogen related (e.g. hydrides embrittlement), which are strongly influenced by temperature. Therefore, cladding thermal characterization along dry storage is an important element to predict fuel rod mechanical performance. Cladding temperature decay models found in the literature are fuel burnup independent and they cannot be applied to storage periods longer than some decades. The goal of this work is to develop a simplified model of cladding temperature as a function of burnup that spans up to 300 years of cask storage. To do so, a methodology is established based on FLUENT steady state calculations fed by heat decay data found in the literature for different burnups (33-63 MWd/kgU). From the results, a temperature correlation as a function of burnup and out-of-reactor time has been derived. It shows an average relative error less than 2% with respect FLUENT calculations. Finally, significance of having an accurate thermal characterization of the fuel rod has been highlighted by comparing fuel rod thermo-mechanics based on the derived correlation and the one resulting from using a correlation developed by EPRI.