Stanislav Sojak

JRQ and JPA irradiated and annealed reactor pressure vessel steels studied by positron annihilation

ABSTRACT The paper is focused on a comprehensive study of JRQ and JPA reactor pressure vessel steels from the positron annihilation lifetime spectroscopy (PALS) point of view. Based on our more than 20 years’ experience with characterization of irradiated reactor steels, we confirmed that defects after irradiation start to grow and/or merge into bigger clusters. Experimental results shown that JPA steel is more sensitive to the creation of irradiation-induced defects than JRQ steel. It is most probably due to high copper content (0.29 wt.% in JPA) and copper precipitation has a major impact on neutron-induced defect creation at the beginning of the irradiation. Based on current PALS results, no large vacancy clusters were formed during irradiation, which could cause dangerous embrittlement concerning operation safety of nuclear power plant. The combined PALS, small angle neutron scattering and atomic probe tomography studies support the model for JRQ and JPA steels describing the structure of irradiation-induced clusters as agglomerations of vacancy clusters (consisting of 2–6 vacancies each) and are separated from each other by a distribution of atoms.

Investigation of materials for fusion power reactors

The possibility of application of nuclear-physical methods to observe radiation damage to structural materials of nuclear facilities is nowadays a very actual topic. The radiation damage to materials of advanced nuclear facilities, caused by extreme radiation stress, is a process, which significantly limits their operational life as well as their safety. In the centre of our interest is the study of the radiation degradation and activation of the metals and alloys for the new nuclear facilities (Generation IV fission reactors, fusion reactors ITER and DEMO). The observation of the microstructure changes in the reactor steels is based on experimental investigation using the method of positron annihilation spectroscopy (PAS). The experimental part of the work contains measurements focused on model reactor alloys and ODS steels. There were 12 model reactor steels and 3 ODS steels. We were investigating the influence of chemical composition on the production of defects in crystal lattice. With application of the LT 9 program, the spectra of specimen have been evaluated and the most convenient samples have been determined.

Investigation of laboratory produced ODS alloys

Irradiation, heat and mechanical stresses are factors which influence structural materials of nuclear power plants (NPP), e.g. reactor pressure vessel steels and may reduce lifetime of NPP operation [1-3]. High radiation and thermal loads are expected in the newest generation of nuclear power plants, such as Generation IV (GEN IV) and fusion reactors, which will be operated at temperatures between 550 - 1 000 °C and will be exposed to irradiation over 100 DPA during planned lifetime (more than 60 years) [4]. Consequently, the demands on the structural materials are very high and so the research and development needs to be focus on their improved characteristics. The advanced structural materials, as oxide-dispersion-strengthened (ODS) steels, are developed for application in cooling systems, reactor pressure vessel or fuel cladding of the GEN IV nuclear power plants. The ODS steels fulfill demands on radiation, thermal and mechanical resistance during operation of nuclear reactor. ODS steels have high the...

Positron and nanoindentation study of helium implanted high chromium ODS steels

Abstract Three oxide dispersion strengthened (ODS) steels with different chromium content (MA 956, MA 957 and ODM 751) were studied as candidate materials for new nuclear reactors in term of their radiation stability. The radiation damage was experimentally simulated by helium ion implantation with energy of ions up to 500 keV. The study was focused on surface and sub-surface structural change due to the ion implantation observed by mostly non-destructive techniques: positron annihilation lifetime spectroscopy and nanoindentation. The applied techniques demonstrated the best radiation stability of the steel ODM 751. Blistering effect occurred due to high implantation dose (mostly in MA 956) was studied in details.

PALS combined with Charpy-V tests at WWER reactor pressure vessel steels

This paper presents results from our long-term studies of irradiated, commercially used WWER reactor pressure vessel steels. Results from Charpy-V tests and positron annihilation spectroscopy techniques are compared and discussed in details, having in mind actual state of art and other microstructural studies in this area. The optimal region for annealing of irradiation induced defects was analyzed. It was shown that WWER steel with low impurity contents has good radiation stability and operation these reactor pressure vessels could be extended beyond a design lifetime.

QtCDB2 software for coincidence Doppler broadening measurement system

A digital system for the coincidence measurement of Doppler Broadening of positron annihilation (CDB) has been changed to one which is fully software based. Spectrometer is based on two HpGe detectors, HV sources and PC with PCI-9820D digitizer. Detector pulses are digitized directly at the HpGe detector pre-amplifier outputs. The previous external trigger chain was replaced by data processing system with a software trigger. All pulses from detectors are processed by a trapezoid filter. Spectrometer performance was tested and compared in various conditions.

Positron Annihilation Measurements Performed on Oxide-Dispersion Strengthened Steels

This paper is focused on investigation of oxide-dispersion strengthened steels in a basic state (before an experimental treatment) by two positron annihilation techniques – positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy. Three high chromium commercial steels – MA 956, MA 957 and PM 2000 were measured in order to identify and quantify defects in samples as well as to demonstrate defect depth profile in subsurface layers up to 1.6µm. Results indicate presence of di-vacancies in PM 2000, three-vacancies in MA 956 and six-vacancies together with dislocations in MA 957. Positron techniques were supplemented by scanning electron microscopy, which demonstrated also presence of larger defects as pores and different density of precipitates.

Software for Digital Coincidence Doppler Broadening Setup

An advanced digital measurement system for Coincidence Doppler broadening (CDB) technique is presented. Traditional CDB setups are based on shaping amplifiers and multi parameter multi-channel analyzers (MCA). We replaced them by 14-bit data acquisition card (DAQ). Replacing multi-parameter MCA by DAQ was realized also by several groups before, but in our setup the signal shaping tasks are performed digitally by software in a personal computer. Signals from high purity (HP) Ge detectors are sampled at preamplifier outputs. Pulse shaping is realized by a trapezoidal filter.

Thermal Annealing Influence on Ions Implanted Fe-Cr Model Alloys

Abstract. 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 3.12×1018 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.

Advanced Fe-Cr Alloys Studied by Pulsed Low Energy Positron System Before and After Helium Ions Implantation

Positron annihilation spectroscopy (PAS) is a non-destructive technique which provides information about microstructural damage of structural materials. In this paper, the Pulsed Low Energy Positron System (PLEPS) at the research reactor FRM-II at TU Munich was used to study depth profiling of binary Fe-Cr alloys. Fe-Cr model alloys with different chromium content were investigated in the as-received state as well as after helium ion implantation (dose up to 6.24×1017 ions/cm−2 ). Measured results show changes in the size of defects after implantation and also in non-implanted specimens depending on the Cr content.Copyright