We propose a mean-field lattice rate model for describing the kinetics of equilibrium as well as irradiationinduced surface segregation in concentrated alloys. The kinetics and thermodynamics are treated in a mutually consistent way. The model treats the composition field of the chemical constituents and of the point defects.The latter are nonconservative and are produced and destroyed either by the nuclear collisions in the bulk or by thermally activated exchange of the atoms with the free surface. The few adjustable parameters can be scaled to the thermodynamics and kinetics of the alloy. Published experimental segregation profiles in Ni-Cu alloy with two compositions and three temperatures are successfully reproduced with a single set of parameters: they exhibit inversion of segregation induced by irradiation.

In October 1991 the CHOOZ A PWR plant, operated by the French and Belgian company SENA, was shut down after 24 years. It was the first PWR shutdown in France. According to the EDF life duration project it was decided to perform an examination program on samples taken from the wall of the vessel in order to complete the feedback. The results obtained show the following: • The embrittlement measured by the pressure vessel surveillance program is representative of that of the Shell determined at different axial and angular locations for the 1/4 of the internal thickness. This observation is consistent with the relative homogeneity of the embrittling elements (Cu, P, Ni) throughout the entire Shell, • The mechanical properties evolution through the wall thickness of the Shell measured after irradiation is interpreted in relation with fluence variation and unirradiated mechanical properties profile through the thickness, • The comparison of direct toughness measurement with the French code RCCM evaluation confirms the safety nature of the conventional approach. For the weld, after an irradiation damage of 2 1019 n.cm-2 (E>1MeV), a margin of the order of the irradiation effect seems to exist.

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A solid under irradiation is a far-from equilibrium system, and therefore phase equilibria in such a system cannot be assessed from equilibrium thermodynamics. Starting from a kinetic description which incorporates the various processes responsible for atomic diffusion (e.g. thermally activated jumps, replacement sequences or displacement cascades), the various possible steady-states can be identified analytically or numerically, as well as their kinetic evolution on varying the control parameters of the system (e.g. temperature, average composition, irradiation flux, cascade density …). Furthermore, from stochastic versions of the kinetic model, potentials governing the stationary probability distribution of states can be derived, allowing to build dynamical equilibrium phase diagrams.Illustrating the above approach on the A2-B2 order-disorder transition, we have identified irradiation-induced two-phase state, cascade size and density effects on phase stability. By incorporating point defects, such description is well suited to study irradiation-induced segregation at sinks in concentrated alloys.