We present an assessment of the mechanical constitutive behavior after irradiation for a tempered martensitic steel, Eurofer97', and for Zircaloy by means of small-ball punch tests. For that purpose we use a finite-element model of the punch test device. We take advantage of the model to infer the alloy mechanical constitutive behavior after irradiation. We show that a necessary matching between the experimental and calculated force-deflection curves allows choosing consistently among the proposed constitutive behaviors. We discuss also the limitations of this method. For Zircaloy a complete description of the constitutive behavior in the unirradiated or irradiated condition was not possible in the framework of anisotropic plastic potentials and associated flow rules, since this demands to fully characterize experimentally the anisotropic yield surface and its evolution with plastic strain during punch deformation. However, an approximate description of the initial yielding anisotropy using Hill plastic potential is possible (although not unique) and necessary before an assessment of the irradiation- hardening can be done on a safe basis. Developing a conventional (non-axisymmetric) three-dimensional model was mandatory in order to deal with the problem of anisotropy in Zircaloy. (c) 2005 Elsevier B.V. All rights reserved.