[en] This progress report summarizes work performed by Argonne National Laboratory on long-term embrittlement of cast duplex stainless steels in LWR systems during the six months from October 1987 to March 1988. A mechanistic understanding of the activation energy of aging is described on the basis of the results of microstructural characterization of various heats of Grades CF-3, CF-8, and CF-8M stainless steel that were used in aging studies at different laboratories. The kinetics of the spinodal decomposition of ferrite (i.e., the primary mechanism of aging embrittlement) appear to be strongly influenced by a synergistic effect of G-phase nucleation and growth. When the activation energies (ranging from 18 to 50 kcal/mole) were plotted as a function of the volume fraction of G phase produced during accelerated aging, a good correlation was obtained regardless of variations in grade, bulk chemical composition, and fabrication process. Spinodal-like decomposition of austenite in heats containing a relatively high level of Ni has also been investigated. Charpy-impact data for thermally aged cast stainless steel were analyzed to determine the kinetics and extent of embrittlement. The ferrite morphology had a strong effect on the extent of embrittlement, whereas the material composition influenced the kinetics of embrittlement. Results obtained from the present study of mechanical properties, and data of other investigators were analyzed to develop the procedure and correlations for predicting the kinetics and extent of embrittlement, under reactor operating conditions, from the material parameters. 31 refs., 30 figs., 1 tab