A mechanical model was constructed to probe into the mechanism of ductility loss. Fracture criterion based on critical localized deformation was undertaken. Two microstructure variables were considered in the model. Namely, the strength ratio of grain boundary affected area to the matrix, ..cap omega.., and the linear fraction, x, of grain boundary affected area. A parametrical study was carried out. The study shows that the ductility is very sensitive to those microstructure parameters. The functional dependence of ductility to temperature as well as strain-rate, suggested by the model, is demonstrated to be consistent with the observation.

Ferritic steels are currently being considered as fusion first wall candidates because of the relatively low thermal stresses induced during temperature cycles. However, fracture toughness is of concern in this alloy class. Limitations on irradiation space dictate that special techniques by developed for post-irradiation fracture toughness measurements. This study investigates the feasibility of electropotential techniques using single specimens to evalute fracture toughness. The electropotential technique was applied to obtain continuous crack extension measurements on miniaturized specimens and to trace out J versus ..delta..a curves. The J-integral results obtained from A286 small specimens compare favorably with those obtained from large specimens. Also, the experimental work shows that electropotential and multiple specimen methods produce consistent data in HT-9 in the transition region where crack extension occurs by mixed cleavage and dimpled rupture.

Two improved mirror systems, the tandem mirror (TM) and the field-reversed mirror (FRM) are being intensively studied. The twin practical aims of these studies: to improve the economic prospects for mirror fusion power plants and to reduce the size and/or complexity of such plants relative to earlier approaches to magnetic fusion. While at the present time the program emphasis is still strongly oriented toward answering scientific questions, the emphasis is shifting as the data accumulates and as larger facilities - ones with a heavy technological and engineering orientation - are being prepared. The experimental and theoretical progress that led to the new look in mirror fusion research is briefly reviewed, the new TM and the FRM ideas are outlined, and the projected future course of mirror fusion research is discussed.