Fast breeder containment structures may be subjected to explosive accidents which deform the material at high strain rates, with load release and reloading by reflected waves, multiaxially. As a contribution to the study of such special loading conditions, the effect of the strain rate (from 10 −2 to 10 3 s −1) on the uniaxial flow curves of the steels AISI 304L, AISI 316L and AISI 321, virgin, welded and irradiated up to 2.2 dpa, at temperatures up to 950°C, is discussed here. The typical differences in dynamic material properties shown by virgin materials when compared to defective materials should also be investigated in a multiaxial state of stress. In fact two main questions demand an answer: 1. - Are the current yield and fracture criteria sufficient in order to transfer the uniaxial results to dynamic biaxial loading conditions? 2. - How may the results obtained for the small specimens used for dynamic testing be transferred to large structures? In order to provide an experimental basis for answering these questions and, therefore, for supporting the formulation of constitutive equations, the following biaxial testing devices have been developed: 3. - An electro-mechanical device and a hydropneumatic machine for strain rates up to 10 2 s −1, acting on small cruciform specimens (cross-section up to 20 mm 2). 4. - A high-load apparatus (5 MN, strain rate up to 10 2 s −1), acting on large steel specimens with a cross section of up to 5000 mm 2, where the statistical distribution of defects determines the strength of the real structure. Preliminary results obtained by developing such tests are reported. General remarks on the structure of some models of constitutive equations are presented.