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In order to validate the long-term behavior of fast breeder reactor steam generator tubes, a creep test monitoring program has been undertaken. A series of over one hundred tests was performed on small specimen tubes which were cut from the as-received tube material of lncoloy 800, similar to those used in generator applications. In contrast to uniaxial conditions in which constant load conditions usually apply, multiaxial experiments were performed assuming constant stress conditions. Thin-walled tube approximation was used to estimate the stresses across the tube wall thickness required for given internal tube pressures. This paper deals with a refined numerical modeling of stresses and strains in the pressurized test specimens, taking into account: internal pressure changes, nonlinear geometrical effects, and creep deformation during all the test procedure cycles. Comparison of the numerical results with the experimental measurements shows a good agreement which validates the multiaxial generalization of the RCC-MR strain creep laws which were used in the calculations. A multiaxial creep rupture criterion is discussed.