Year
2010
File Attachment
Abstract
TN International started recently a study of irradiated fuel claddings thermal creep in routine transport conditions to evaluate creep rupture risk of LWR fuel. Fuel cladding thermal creep consists in local swelling of cladding parts submitted to internal pressure and high temperature. Cladding rupture occurs when a critical strain is reached. Creep rate is influenced by a combination of several parameters: temperature profile, stress level, irradiation and oxidation profiles. The study is focused on the hottest fuel pin during transport. An axis-symmetrical finite elements modelling of a fuel pin is generally used with different temperature profiles, temperature evolutions, irradiation profiles and internal pressures. Thermal creep behaviour, described in creep laws, includes irradiation defects hardening recoveries with temperature. Claddings rupture criteria are based on a critical strain depending on irradiation damage and stress. The used creep laws and criteria have been developed by CEA R&D laboratories. Analytical calculations are not possible because of creep behaviour non-linearity and large number of input parameters. Finite Elements Analysis (FEA) is very time-consuming and not flexible in practice. Therefore, TN International has developed a simplified FORTRAN calculation model. The simplified modelling has the same input parameters as the FEA one. Cladding mesh has one element in thickness in order to calculate average stress. Axial mesh refinement is chosen to give good description of axial profiles of temperature and irradiation. Internal pressure is determined by thermodynamics balance. Creep laws and rupture criteria implementations permit to switch from one law to another one easily. Similarly to the FEA modelling, the rupture criterion consists in comparing at each integration point the local creep strain to the local limit strain. These calculations last only a few seconds versus several hours or days for FEA calculations.