Thermal Analysis in Spent Nuclear Fuel Cask

Thermal Analysis in Spent Nuclear Fuel CaskCurrently, to prevent unacceptable degradation during storage, spent nuclear fuel temperaturecalculations are focused on maximum temperatures in compliance with the limits specified inthe Interim Staff Guidance of the NRC ISG-11 rev.3. These calculations can be extremelyconservative in some cases and non-realistic in others, especially when material ductility is animportant factor.Therefore, a long-term collaboration project between ENUSA Industrias Avanzadas S.A., S.M.Eand Universidad Politécnica de Madrid (UPM) has been launched in order to obtain morerealistic cladding temperature results during transport and storage, and its evolution with time.Thermal analysis have been developed for the TN-24P storage cask configured for PWR withhelium backfill gas. The main reason to choose the TN-24P cask is that experimental data areavailable and simulation results can be compared to measured test data.Two computer codes, COBRA-SFS and STAR-CCM+, have been used. This paper is focused inCOBRA-SFS, a thermal-hydraulic analysis code developed by PNNL specific for spent fuel storageand transportation casks.Considering the following parameters (velocity, enthalpy, temperatures, mass flux ?), goodagreement between the experimental thermocouple data and COBRA-SFS code predictions areobtained. Additionally, different sensitivity analysis for key parameters have been performed:(1) Gaps between the basket and the inside wall of the cask. Results show an importantimpact in the simulations.(2) Helium flow rate distribution. The analysis show a negligible effect by this factor dueto the low velocity and low pressure the helium has inside this cask.Additionally, several scripts that process COBRA-SFS input and output data have been set. As aconsequence, the results are shown as heat map or as a graphic. The user can choose theparameters and/or the assembly from which to show the results.Creating different simulation cask models and developing a methodology to obtain usefulunderstanding of thermal behaviour of spent fuel storage and transportation systems is one ofthe main objectives of this project, therefore in future works other storage casks designs aregoing to be analysed.