In a hybrid slow positron beam, extraction of the positrons from the magnetic field (∼1 T for the PSI beam) to field-free space is a necessary operation. A theoretical and experimental demonstration is given which shows that the following proposed beam extraction method works as predicted: magnetic transport of the slow positrons down to field strengths of ∼100 G and passage through an aperture grid of ∼10 cm diameter in a field termination shield followed by some brightness enhancement stages. The simulation and measurement of the magnetic-field distributions along the beam axis show a steep drop down of the field from ∼100 G to a few gauss within 1 cm of shield thickness and a quasiuniform spreading of the transverse field strength across the grid opening. Measurement of transmission and divergency (transverse energy) of the beam exiting the extraction aperture confirmed theoretical estimations and ray tracing calculations for the aperture design used to be of the order of 75% and 20 eV, respectively. These data as a function of field strength and beam energy are used for optimization of the final extraction aperture design (≳85% transmission) to be used in the PSI high intensity beam facility.

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.

Trends in the properties of defects, reflecting the role of Ni and P in the enhanced precipitation of copper during irradiation, have been studied by small angle neutron scattering (SANS) in a set of 8 model RPV steels. The alloys in the set contain different, selected amounts of Cu, Ni and P, while the composition of all the 8 samples concerning the rest of the alloying elements, the ‘RPV steel background’, was invariant throughout the series. The irradiation was done in a test reactor at 290C, at controlled radiation conditions, for two neutron fluences of 5∙1018 and 5∙1019 n/cm2. The SANS data show an approximately quadratic relationship between the precipitated volume fraction fm of damage particles and the nickel content cNi of the sample for the medium-copper cCu = 0.16% alloys, with a nearly zero initial slope of fm; the coefficients in the equation vary substantially with the fluence. A nearly perfect correlation of fm with the increase of microhardness was found for all alloys. The size distribution, as deduced from magnetic scattering, differs systematically from that found by analysis of the nuclear scattering; this difference correlates with both cCu and cNi. For the two low-copper alloys, phosphorus is seen to enhance production of radiation damage centers, whereas for the medium-copper alloys a refinement of the precipitated phase occurs.

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It is demonstrated on a binary model alloy, Fe‐0.8% Cu, heat treated to different states of Cu‐distributions in the iron matrix that positron annihilation applied to such “simpler” systems can give hints for a detailed understanding of neutron irradiation induced defect structures in complex industrial reactor pressure vessel steels (RPV). This study is concerned primarily with a high Cu‐content weldment (0.3%) irradiated in a commercial nuclear power plant to a modest fluence of 5.4 × 1017 n/cm2 (E > 1 MeV) and a medium Cu‐content RPV forging (0.17%) irradiated in a materials test reactor up to 1.9 × 1019 n/cm2. Various positron lifetime and angular correlation‐peak height measurements, the latter as a function of annealing temperature up to 750 °C, reveal recovery stages which correlate quantitatively with the microhardness behaviour in the cases of the binary alloy and the forging. The interpretations of the peak height results lead to a detailed picture of precipitation, Ostwald‐ripening, and dissolution of Cu‐rich clusters in the steels. Lifetime measurements in these systems can be understood in terms of trapping effects at the misfit structure of the interfaces between the Cu‐precipitates and the iron matrix. They also show that under the irradiation conditions applied here voids are not produced in the weldment and probably also not in the forging. This finding is decisive also in the choice of possible model descriptions when interpreting small angle neutron scattering results.