This paper describes a computer code, TOMBAS (Thermo-Mechanical Behaviors of Annular and Solid Type Fuel Using the Finite Element Method) for analysis of thermo-mechanical behavior of the fuel rod system under steady state operation by two-dimensional axisymmetric finite element method.
Isoparametric finite element formulation and Galerkin's technique are used in derivation of finite element equations for thermal behavior whereas the principle of virtual work is used in derivation of mechanical displacement and stress element equations.
Swelling, densification and relocation models are incorporated in calculation of thermal part parameters such as temperature and internal pressure.
Steady state temperature distribution and internal pressure are calculated by the Newton-Raphson iteration scheme and are used as thermal stresses and loads in mechanical analysis (elasticity, plasticity and creep).
For validation of the present code TOMBAS, results from thermal-part analysis are compared with the results obtained with use of the COMETHE code and results from mechanical part analysis are compared with analytical solutions. Thermo-mechanical results obtained with use of the present code are in good agreement with those obtained with use of the COMETHE code and with use of analytical solutions. As regards thermal behaviors of fuel rods of two different types, the center temperature and internal pressure of the annular type fuel are compared with those of solid type fuel. As regards their mechanical behaviors, the pellet-cladding interaction and pellet-pellet interaction are treated by use of nodal equilibrium equations and various boundary conditions. The pellet-cladding contact pressure and ridge height of the cladding are calculated under soft and hard contact conditions. All these thermo-mechanical evaluations show that the annular type fuel has improved fuel performance parameters than the solid type fuel.
