The present research program on irradiation embrittlement and annealing of reactor pressure vessel (RPV) steels in Switzerland is carried out at the Paul Scherrer Institute for Reactor Research. The particular program structure and its relation to the concept for an improved surveillance program is described. Based on past experience with surveillance programs, especially for older plants, the main goal of the research program is determined by an effort to overcome identified shortcomings.Therefore the following topics are discussed: 1. Obtaining additional data to supplement those from specific surveillance capsules. 2. Understanding the mechanisms of irradiation embrittlement by microstructural investigations of the materials. 3. Investigations of the annealing-recovery behavior of irradiated RPV steels. 4. Validation efforts concerned with the transferability of test reactor and surveillance program results to the real RPV. Within the framework of bilateral research agreements, participation in research programs coordinated by the International Atomic Energy Agency (IAEA), and collaborative research with other Swiss and foreign institutions, a review of past and future activities is presented; the technical facilities (irradiations, mechanical testing, physical methods) and the advanced and new tools used in the investigations, especially those for the microstructural studies, are outlined and put into perspective. Finally, first results obtained in the research program are summarized.

Recent trends in coping with embrittlement problems in reactor pressure vessels (RPVs) show two main directions of development: (1) improvement of the vessel materials and (2) limitations of fluence over the design life of the RPV. For several reasons, however, adequate irradiation surveillance programs are still considered to be necessary in the future, despite possible improvements resulting from such research activities. Since the introduction of linear elastic fracture mechanics (LEFM) and elastic-plastic fracture mechanics, (EPFM), irradiation surveillance programs show a trend towards direct measurement of fracture toughness, in addition to relying on the conventional nil-ductility transition temperature (NDTT) shift as a relative measure of embrittlement. Some basic considerations concerning the selection of specimen types for irradiation surveillance programs and some technical aspects of currently used specimen types are discussed.

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Brittle-to-ductile transition behavior is one important aspect in the material behavior of ferritic steels. Because of the many influencing parameters involved, the transition behavior is known to cause great difficulties in its theoretical treatment and modeling as well as in the development of a sufficiently reliable empirical correlation. In spite of this, some means have to be provided to treat or characterize brittle-to-ductile transition behavior in an appropriate or at least in an acceptable way, especially for integrity assessments of components. In this paper, some basic considerations and treatments of brittle-to-ductile transition, as well as relevant influencing parameters and their effects, will be discussed. Also, experimental results will be presented and conclusions drawn and new aspects discussed that can have significant consequences in the application of structural integrity assessment methods.