4. METHOD OF SOLUTION
At each time step, the energy equation is integrated numerically along the channel to obtain the coolant axial temperature distribution(s). In the case of a tubular fuel pin, the radius of the adiabatic surface, which determines the heat fluxes to the inner and outer channels, is found by iteration. Radiation and convection heat transfer mechanisms are assumed, using appropriate empirical convective heat transfer correlations. The one-dimensional Fourier equation is used to determine the fuel pin internal temperature distribution. Heat transfer across internal (helium filled) gaps is assumed to be by conduction and radiation. The neutron-irradiation-induced dimensional changes (Wigner strains) are determined from sets of polynominals fitted to experimental data.
Incremental creep strains are calculated iteratively assuming a Maxwell model with dose and temperature dependent parameters. Total creep strains are obtained by summation of the incremental creep strains and the stresses are calculated using closed-form expressions. Radial displacements are calculated and checked for interaction between components. In the case of interaction, the interface pressure is found by iteration.