Finite element pellet-clad interaction model

During the nuclear reactor operation, one of the most important limiting factors in the fuel element duty is the mechanical interaction of fuel pellets and cladding tubes. This interaction produces cyclic stresses and strains in the cladding, and these in turn consume clad fatigue life. Computer analyses of the behaviour of fuel pins have shown that the deformation of its components have significant effect on its temperature distribution, resulting mainly from variations with time of the gaps between the fuel pellet and cladding tubes. Changes in temperature distribution in turn influence the deformations of the pin and the stresses developed in its components. Therefore the heat conduction calculations (with convection heat transfer coefficient as a boundary condition) and the stress-strain calculations cannot be made separately, but must be made simultaneously. To this end, existing structural analysis body of revolution finite element code BREL and thermal analysis finite element code TEMPEL have been combined in order to yield a new TEMPEST computer programme. This code is using thoroidal finite elements, with simplex linear triangulation on the meridional cross-section of the fuel element. The simultaneous computation is performed by solving thermal and structural equations, H t = q and K u = f respectively, inside of each individual time step. With respect to the thermal conduction analysis, Dirichlet's, Neumann's and mixed type (of the third kind) boundary conditions are considered (while neglecting Stefan's law due to the nonlinearity), and within the structural analysis, elastic Hookes's generalised law is used with allowance being made for thermal expansion, irradiation induced shrinkage or swelling, creep, cracking etc. In due course, our code is to be implemented by circumferential Fourier's analysis for azimuthally dependent loading of geometrically axisymmetric bodies. (orig.)