Helium implanted FeCr alloys studied by positron annihilation lifetime technique

The influence of chromium on the radiation damage resistance of iron based alloys has been studied using conventional positron lifetime technique and a pulsed low energy positron beam. To simulate high neutron flux, the helium implantation has been used. Different levels of helium doses (6.24·10¹⁷–3.12·10¹⁸ cm⁻²) corresponding to a local damage of up to 90 dpa were accumulated in a thin 1 nm) and small vacancy clusters together with the initial dislocations and small point defects.Досліджувався вплив хрому на стійкість до радіаційного пошкодження сплавів на основі заліза за допомогою загальновизнаного методу вимірювання часу життя позитронів та імпульсного пучка позитронів низької енергії. Різні рівні доз гелію (6,24·10¹⁷– 3,12·101¹⁸см⁻²), що відповідають локальному пошкодженню до 90 зсувів/атом, накопичувались в області товщиною не менш 1 мкм. У даній роботі використовувались чотири бінарних сплави Fe-Cr (2,6; 4,6; 8,4; 11,6 ваг.% Cr). Отримані результати показують, що хром має значний вплив на розмір та розподіл створених дефектів. Характер цих дефектів визначався у вигляді великих пор (>1 нм) і малих вакансійних кластерів поряд з початковими дислокаціями і малими точковими дефектамиИсследовалось влияние хрома на стойкость к радиационному повреждению сплавов на основе железа с помощью общепринятого метода измерения времени жизни позитронов и импульсного пучка позитронов низкой энергии. Различные уровни доз гелия (6,24·10¹⁷– 3,12·101¹⁸см⁻²), соответствующие локальному повреждению до 90 смещ./атом, накапливались в области толщиной менее 1 мкм. В настоящей работе использовались четыре бинарных сплава Fe-Cr (2,6; 4,6; 8,4; 11,6 вес. % Cr ). Полученные результаты показывают, что хром оказывает значительное влияние на размер и распределение созданных дефектов. Характер этих дефектов определялся в виде больших пор (>1 нм) и малых вакансионных кластеров наряду с начальными дислокациями и малыми точечными дефектами

Optimization of Positron-Lifetime Measurement Geometry Based on Geant4 Simulation

Proper choice of measuring geometry and experimental setup of nuclear instrumentation modules and photomultipliers is a key element which affects substantial positron lifetime measurement properties: count rate and time resolution. An adequate compromise must be found, when it comes to geometry of measurement. The optimal geometry using three detector layout is inspected in this paper. During our work, we concentrated on the simulation of XP2020Q photomultipliers and the BaF₂ scintillator material. The Geant4 simulation allows to estimate an influence of the measuring geometry on detection efficiency and to choose the most appropriate crystals dimensions and positions. As mentioned in paper of Bečvaŕ et al., slight changes in geometry result in distortion or improvement of measured results. Experimental results already showed, changes of start crystals dimensions can result in significant increase in count rate of three-detector measurement

Optimization of Positron-Lifetime Measurement Geometry Based on Geant4 Simulation

Proper choice of measuring geometry and experimental setup of nuclear instrumentation modules and photomultipliers is a key element which affects substantial positron lifetime measurement properties: count rate and time resolution. An adequate compromise must be found, when it comes to geometry of measurement. The optimal geometry using three detector layout is inspected in this paper. During our work, we concentrated on the simulation of XP2020Q photomultipliers and the BaF₂ scintillator material. The Geant4 simulation allows to estimate an influence of the measuring geometry on detection efficiency and to choose the most appropriate crystals dimensions and positions. As mentioned in paper of Bečvaŕ et al., slight changes in geometry result in distortion or improvement of measured results. Experimental results already showed, changes of start crystals dimensions can result in significant increase in count rate of three-detector measurement

Effect of the Initial Microstructure on Defect Evolution in Neutron Irradiated Fe-Cr-C Alloys

The irradiation-induced evolution of vacancy type defects in various iron-chromium model alloys and high chromium ferritic/martensitic steels have been studied using coincidence Doppler broadening spectroscopy. Specimens were neutron irradiated to 0.11 dpa at two different temperatures, 290°C and 450°C. It has been found that the microstructure (ferrite vs. ferrite/martensite), more precisely distribution of dissolved carbon within the matrix, is one of the key factor that affect response of the materials to neutron irradiation. Presence of dissolved carbon within the matrix leads to formation of stable and immobile carbon-vacancy complexes which act as traps for irradiation induced vacancies and therefore, leading to increased formation of vacancy clusters. Impact of carbon-vacancy complexes on defects evolution during neutron irradiation is relevant only for certain irradiation temperatures

German and Russian Irradiated Reactor Pressure Vessel Steels from PAS Point of View

This paper presents a comparison of commercially used German and Russian reactor pressure vessel steels from the positron annihilation spectroscopy point of view, having in mind knowledge obtained also from other techniques from the last decades. The second generation of Russian reactor pressure vessel steels seems to be fully comparable with German steels and their quality allows prolongation of NPP operating lifetime over projected 40 years. The embrittlement of CrMoV steels is very low due to the dynamic recovery of radiation-induced defects at reactor operating temperatures. Positron annihilation spectroscopy techniques can be effectively applied for evaluation of microstructural changes caused by extreme external loads by proton implantation, with aim to simulate irradiation and for the evaluation of the effectiveness of post-irradiation thermal treatments. We used our actual and previous results, collected during last 20 years from measurements of different reactor pressure vessel steels in "as received", irradiated and post-irradiation annealed state and compare them with the aim to contribute to general knowledge based on experimental positron annihilation spectroscopy data

Thermal Annealing Influence on Ions Implanted Fe-Cr Model Alloys

Binary Fe-11.62wt%Cr alloys were investigated in as-received state as well as after a two step helium ion implantation at different energies (100 keV and 250 keV) with doses up to 3.12xl018 cm-2. In order to study changes in alloys in dependence on the temperature, thermal annealing was performed at temperatures of 400, 475, 525 and 600 °C and specimens were afterwards measured by a pulsed low energy positron system (PLEPS). Annealing out of defects at lower temperatures was not as significant as expected, and we also encountered difficulties with defect identification. However, an apparent decrease of defect size was observed in the specimen annealed at a temperature of 600 °C.JRC.F.4-Nuclear Reactor Integrity Assessment and Knowledge Managemen

Different chromium content and thermal annealing influence on ions implanted Fe-Cr model alloys

Reduced activation ferritic/martensitic steels (RAFM) represented by binary Fe-Cr alloys, with different chromium content, were studied in as-received state as well as after helium ions implantation. In order to study changes in dependence on the temperature, thermal annealing of He ions implanted Fe-11.62%Cr specimens was performed. Measurements by Pulsed Low Energy Positron System (PLEPS) in Garching, Germany were performed afterwards. Annealing out of defects at lower temperatures was not significant as was expected and some uncertainties are present. Extensive decrease of positron lifetime of defects was observed in specimens annealed at temperature 600 ºC.JRC.F.4-Nuclear Reactor Integrity Assessment and Knowledge Managemen