Microstructural Study of High Irradiated Reactor Steels

Positron Annihilation Spectroscopy (PAS) techniques in combination with other techniques were effectively used in the testing and selection process of optimal reactor steels for use in Generation III and IV reactors or thermonuclear fusion facilities. Conventional PAS lifetime technique and pulsed low energy positron system were applied on wide spectrum of reactor steels together with other techniques viz., Transmission Electron Microscopy and Mossbauer Spectroscopy focused on the role of Nickel in the steel microstructure. Experimental experiences in this area collected over the last twenty years were very useful in the actual study by avoiding many mistakes in handling with specimens or in careful interpretation of the results.JRC.F.4-Nuclear Reactor Integrity Assessment and Knowledge Managemen

PALS Investigation of Chromium Effect in Ferritic/Martensitic Steels Implanted with Helium

Chromium is an element with significant effect on the elevated temperature corrosion resistance and the low radiation induced void swelling. There were several studies carried out with regard to effect of Cr on the defect creation and stability [1, 2], but this issue is not fully understood yet. This paper contributes to this research with the study of Fe-Cr binary alloys with different Cr content, implanted by helium by the use of cascade accelerator and investigated by conventional PALS set-up. Our measurements show that chromium plays a role in the formation of small vacancy clusters, affecting the size and distribution of these defects.JRC.F.4-Nuclear Reactor Integrity Assessment and Knowledge Managemen

On the Limitations of Positron Annihilation Spectroscopy in the Investigation of Ion-Implanted FeCr Samples

New materials for advanced fission/fusion nuclear facilities must inevitably demonstrate resistance to radiation embrittlement. Thermal and radiation ageing accompanied by stress corrosion cracking are dominant effects that limit the operational condition and safe lifetime of the newest nuclear facilities. To study these phenomena and improve the current understanding of various aspects of radiation embrittlement, ion bombardment experiments are widely used as a surrogate for neutron irradiation. While avoiding the induced activity, typical for neutron-irradiated samples, is a clear benefit of the ion implantation, the shallow near-surface region of the modified materials may be a complication to the post-irradiation examination (PIE). However, microstructural defects induced by ion implantation can be effectively investigated using various spectroscopic techniques, including slow-positron beam spectroscopy. This method, typically represented by techniques of positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy, enables a unique depth-profile characterisation of the near-surface region affected by ion bombardment or corrosion degradation. One of the best slow-positron beam facilities is available at the pulsed low-energy positron system (PLEPS), operated at FRM-II reactor in Munich (Germany). Bulk studies (such as high energy ion implantation or neutron irradiation experiments) can be, on the other hand, effectively performed using radioisotope positron sources. In this paper, we outline some basics of the two approaches and provide some recommendations to improve the validity of the positron annihilation spectroscopy (PAS) data obtained on ion-irradiated samples using a conventional 22Na positron source

Fe-Cr Alloys Behaviour after Helium Implantation

The paper discusses our recent experiments focused on the chromium influence on the microstructural changes of iron based alloys under radiation treatment. Our experimental method ¿ the positron annihilation lifetime spectroscopy (PALS) enables an observation of size and density changes of the vacancy type defects in the material microstructure. These defects have been created by implantation of charged particles (He2+). The cascade collisions in the crystal lattice and following Frenkel pair creation have been considered as possible approximation of the neutron flux damage up to 100 dpa in the region up to 1 micrometer from the surface.JRC.F.4-Safety of future nuclear reactor

Bubble Swelling in Ferritic/Martensitic Steels Exposed to Radiation Environment with High Production Rate of Helium

Reduced-activativon ferritic/martensitic (RAFM) steels are prospective structural materials for fission/fusion nuclear applications because their radiation and swelling resistance outperforms their austenitic counterparts. In radiation environments with a high production rate of helium, such as fusion or spallation applications, these materials suffer from non-negligible swelling due to the inhibited recombination between vacancy and interstitial-type defects. In this work, swelling in helium-implanted Eurofer 97 steel is investigated with a focus on helium production rates in a wide range of helium/dpa ratios. The results show virtually no swelling incubation period preceding a steady-state swelling of about 2 × 10−4%/He-appm/dpa. A saturation of swelling above 5000 He-appm/dpa was observed and attributed to helium bubbles becoming the dominant sinks for new vacancies and helium atoms. Despite a relatively low irradiation temperature (65 ± 5 °C) and a rather high concentration of helium, transmission electron microscope (TEM) results confirmed a microstructure typical of ferritic/martensitic steels exposed to radiation environments with high production rates of helium

Application of Fast-digitizer Card Acqiris DP-240 in Positron Lifetime Spectroscopy

Based on the available knowledge base in positron annihilation measurements, new software for digital positron lifetime spectrometry has been designed within the frame of joint activity between STU Bratislava and JRC Petten. The tests of the new positron digital-life time setup have been carried out. Detectors with BaF2 scintillators and fast-dynode outputs were chosen for such studies. As the digitiser unit, the card Acqiris DP240 was fully tested. Originally, start and stop timing signals were sampled to digital waveforms separately at a rate of 1 GS/s in 8 bit resolution. However, due to limited quality of collected waveforms which significantly affected post-processing of data, the joined delayed channel mode with 2 GS/s sampling rate was applied for further studies. The full setup was benchmarked by a series of calibration measurements of two high purity materials, specifically Si and Ni. The inter-comparison of conventional analogue- and digitalbased processing are discussed in detail. Further, the optimisation of main parameters for waveform-processing are analysed too. Such a new approach in the digital life-time spectroscopy will help to improve the resolution of existing methodology which might enhance the capabilities of this technique for further applications, especially in materials science.JRC.F.4-Nuclear Reactor Integrity Assessment and Knowledge Managemen

Evaluation of the Role of Cr, Ni, Mn and Si Si in Reactor Pressure Vessel Steel

In this paper we report initial results from a study focused on investigation of the role of elements as Cr, Ni, Mn and Si in the radiation stability of reactor pressure vessel steels. Twelve model ferritic steels with basic composition derivated from Russian WWER-1000 and Western PWR reactor pressure vessel materials were studied by Charpy-V impact, magnetic Barkhausen noise, Vickers hardness tests, Relative Seebeck Coefficient measurements and Positron annihilation spectroscopy. Higher Cr content in model steels was found generally to give increased root mean square values independent of Mn and Si contents. The ductile¿brittle transition temperatures and hardness values of the model steels were found to be independent of composition. The correlation between ductile¿brittle transition temperatures and hardness values has potential for prompt determination of the effect of composition and irradiation on the steel properties. The next stage of the assessment will study the effect of irradiation of the model steels to accumulated neutron fluences of 10e19 n.cm-2.JRC.DDG.F.4-Safety of future nuclear reactor

Application of Proton Irradiation in the Study of Accelerated Radiation Ageing in a GaAs Semiconductor

Proton irradiation experiments have been used as a surrogate for studying radiation effects in numerous materials for decades. The abundance and accessibility of proton accelerators make this approach convenient for conducting accelerated radiation ageing studies. However, developing new materials with improved radiation stability requires numerous model materials, test samples, and very effective utilization of the accelerator beam time. Therefore, the question of optimal beam current, or particle flux, is critical and needs to be adequately understood. In this work, we used 5 MeV protons to introduce displacement damage in gallium arsenide samples using a wide range of flux values. Positron annihilation lifetime spectroscopy was used to quantitatively assess the concentration of radiation-induced survived vacancies. The results show that proton fluxes in range between 1011 and 1012 cm−2.s−1 lead to a similar concentration of monovacancies generated in the GaAs semiconductor material, while a further increase in the flux leads to a sharp drop in this concentration

Application of Proton Irradiation in the Study of Accelerated Radiation Ageing in a GaAs Semiconductor

Proton irradiation experiments have been used as a surrogate for studying radiation effects in numerous materials for decades. The abundance and accessibility of proton accelerators make this approach convenient for conducting accelerated radiation ageing studies. However, developing new materials with improved radiation stability requires numerous model materials, test samples, and very effective utilization of the accelerator beam time. Therefore, the question of optimal beam current, or particle flux, is critical and needs to be adequately understood. In this work, we used 5 MeV protons to introduce displacement damage in gallium arsenide samples using a wide range of flux values. Positron annihilation lifetime spectroscopy was used to quantitatively assess the concentration of radiation-induced survived vacancies. The results show that proton fluxes in range between 1011 and 1012 cm−2 .s−1 lead to a similar concentration of monovacancies generated in the GaAs semiconductor material, while a further increase in the flux leads to a sharp drop in this concentration