Search

This paper presents the results of atom probe tomography studies on radiation-induced phase formation in light-water reactor pressure vessel steels after neutron irradiation under various conditions in comparison with the literature data. The given irradiation conditions are fluence (10–100) × 1022 m−2, flux (5–2700) × 1014 m−2s−1 and irradiation temperature (50–400) °C. The composition of the studied steels varies in a wide range for the elements significantly affecting radiation and thermal resistance of steels: Ni in the range of 0.2–6.0 wt.%, Mn–0.03–1.1 wt.%, Cu–0.01–0.16 wt.%, and P–0.01–0.03 wt.%. The number density, volume fraction, size, composition, and nucleation sites of precipitates are determined. The regularities of the effect of various operational factors on the phase formation in these steels have been analyzed and revealed. The study shows that in materials with high copper content, Cu-rich precipitates are formed by a radiation-enhanced mechanism. In materials with low copper content, their formation upon irradiation at 300 °C occurs by a radiation-induced mechanism since the main nucleation sites are point defect clusters formed in cascades. At the same time, the density, volume fraction, and composition of the precipitates depend on the steel composition (Ni and Mn content). In the steel with increased Ni content up to 5 wt.% but with ultra-low Mn content ≤ 0.03 wt.%, it is possible to suppress the formation of Ni-Si-Mn precipitates under irradiation. [ABSTRACT FROM AUTHOR]

This paper presents the results of atom probe tomography studies on radiation-induced phase formation in light-water reactor pressure vessel steels after neutron irradiation under various conditions in comparison with the literature data. The given irradiation conditions are fluence (10–100) × 1022 m−2, flux (5–2700) × 1014 m−2s−1 and irradiation temperature (50–400) °C. The composition of the studied steels varies in a wide range for the elements significantly affecting radiation and thermal resistance of steels: Ni in the range of 0.2–6.0 wt.%, Mn–0.03–1.1 wt.%, Cu–0.01–0.16 wt.%, and P–0.01–0.03 wt.%. The number density, volume fraction, size, composition, and nucleation sites of precipitates are determined. The regularities of the effect of various operational factors on the phase formation in these steels have been analyzed and revealed. The study shows that in materials with high copper content, Cu-rich precipitates are formed by a radiation-enhanced mechanism. In materials with low copper content, their formation upon irradiation at 300 °C occurs by a radiation-induced mechanism since the main nucleation sites are point defect clusters formed in cascades. At the same time, the density, volume fraction, and composition of the precipitates depend on the steel composition (Ni and Mn content). In the steel with increased Ni content up to 5 wt.% but with ultra-low Mn content ≤ 0.03 wt.%, it is possible to suppress the formation of Ni-Si-Mn precipitates under irradiation.

The existing knowledge about the effect of neutron irradiation on the mechanical properties of reactor pressure vessel steels under reactor service conditions relies to a large extent on accelerated irradiations realized by exposing steel samples to a higher neutron flux. A deep understanding of flux effects is, therefore, vital for gaining service-relevant insight into the mechanical property degradation. The existing studies on flux effects often suffer from incomplete descriptions of the irradiation-induced microstructure. Our study aims to give a detailed picture of irradiation-induced nanofeatures by applying complementary methods using atom probe tomography, positron annihilation, small-angle neutron scattering and transmission electron microscopy. The characteristics of the irradiation-induced nanofeatures and the dominant factors responsible for the observed increase of Vickers hardness are identified. Microstructural changes due to high flux conditions are smaller nm-sized solute atom clusters with almost the same volume fraction and a higher concentration of vacancies and sub-nm vacancy clusters compared to low flux conditions. The results rationalize why pronounced flux effects on the nanofeatures, in particular on solute atom clusters, only give rise to small or moderate flux effects on hardening. [ABSTRACT FROM AUTHOR]

The existing knowledge about the effect of neutron irradiation on the mechanical properties of reactor pressure vessel steels under reactor service conditions relies to a large extent on accelerated irradiations realized by exposing steel samples to a higher neutron flux. A deep understanding of flux effects is, therefore, vital for gaining service-relevant insight into the mechanical property degradation. The existing studies on flux effects often suffer from incomplete descriptions of the irradiation-induced microstructure. Our study aims to give a detailed picture of irradiation-induced nanofeatures by applying complementary methods using atom probe tomography, positron annihilation, small-angle neutron scattering and transmission electron microscopy. The characteristics of the irradiation-induced nanofeatures and the dominant factors responsible for the observed increase of Vickers hardness are identified. Microstructural changes due to high flux conditions are smaller nm-sized solute atom clusters with almost the same volume fraction and a higher concentration of vacancies and sub-nm vacancy clusters compared to low flux conditions. The results rationalize why pronounced flux effects on the nanofeatures, in particular on solute atom clusters, only give rise to small or moderate flux effects on hardening.

By solid-phase synthesis, macrocrystalline Cu2 − δZn2 − xSnxS4 − ySey monograin powders have been prepared, the chemical composition of which was shown by XRD and Raman spectroscopy to be different in powder fractions of different sizes formed during the synthesis. The influence of synthesis conditions on the decay kinetics of photogenerated charge carriers in different fractions has been studied using the frequency–time-resolved microwave photoconductivity method. The characteristic photoresponse half-life, τ1/2, increased with increasing grain size. The smallest values of the lifetime were observed for the fraction with sizes of 50–70 μm (τ1/2 <5 ns), and the largest values were for the fraction with a grain size of 70–90 μm (τ1/2 ~ 12 ns). [ABSTRACT FROM AUTHOR]

Abstract Li+ co-doping is used as a prevalent way to improve the luminescent intensity of CeO 2 :Eu3+ [25]. Here, we investigate the structural and luminescent properties of CeO 2 :Eu3+ by the different co-doping ways of Li+ ions, including the direct co-doping of Li+ by hydro-thermal co-precipitation and the indirect co-doping of Li+ by a post calcining of Li 2 CO 3. The different co-doping ways of Li+ ions was found to cause different effect on structure and luminescence of CeO 2 :Eu3+ phosphor. The result showed opposite trend with conventional explanations [25] where the substitution of Ce4+ with doped Li+ ions are the prime reason for luminescence intensity enhancement. Oxygen vacancies should be formed due to the charge imbalance. It is found that the Li+ direct co-doped CeO 2 :Eu3+ didn't improve the crystallization of the samples due to the increase of oxygen vacancies, as is supported by X-ray diffraction(XRD) patterns and Raman spectra. Meanwhile, the spectroscopic investigations reveal that the Li+ direct co-doped CeO 2 :Eu3+ lead to less asymmetry around Eu3+ and lower luminescence. However, results show that the indirect co-doping of Li+ lead to a increase of crystallinity and decrease in oxygen vacancies concentration. The luminescent intensity of CeO 2 :Eu3+ can be significantly enhanced by indirect co-doping of Li+. Consequently, we reason that the enhanced luminescence mainly results from a increase of crystallinity and decrease in oxygen vacancies concentration due to the flux effect of Li+ ions, instead of the substitution of Ce4+ with doped Li+ ions. [ABSTRACT FROM AUTHOR]

The effect of flux in a wide range from 1.39×10−10 to 1.39 ×10−3 dpa/s has been studied by the cluster dynamics (CD) simulation for pure iron neutron irradiated at a temperature of 300°C for the two fluences. The critical flux values which mark the regimes of flux dependence of average size, number density and irradiation induced hardening for vacancy clusters (VC) and self-interstitial atoms clusters (SIAC) are found. It was found that the effect of neutron irradiation on average size of SIAC was similar for low and high fluxes in the case of the same fluence. [ABSTRACT FROM AUTHOR]

In-service exposure to high-energy neutrons embrittles reactor pressure vessel (RPV) steels. An increase in the yield stress (Δσ y) results in a corresponding increase in the brittle to ductile transition temperature (ΔT c). Most existing models underpredict ΔT c at higher fluence following accelerated irradiations in test reactors. High fluence, up to 1020 n/cm2 in some cases, will be reached over extended RPV vessel operation of 80 years, or more, at low service flux. Embrittlement has been extensively studied in accelerated, higher flux test reaction irradiations. However, the use of test reactor data naturally raises the question of flux effects. This study used a machine learning approach trained on a set of hardening data, covering a wide range of flux, fluence, and steel compositions to determine the interactive effects of both irradiation and material variables on Δσ y. The analysis included machine learning-based cross-plots of the variable dependence of Δσ y for six core steels (i.e., CM6, LC, LD, LG, LH, and LI), with controlled differences in their Cu and Ni contents. A primary objective is to evaluate an effective to actual fluence (ɸt e /ɸt) ratio, as a function of flux, fluence, and steel composition. This is information critical to properly use intermediate flux-high fluence data in calibrating a low flux-high fluence embrittlement model. The predicted ɸt e /ɸt is reasonably consistent with estimates previously derived from a physics-based solute recombination trap model. [Display omitted] [ABSTRACT FROM AUTHOR]

Systematic differences in the radiation embrittlement kinetics of steels irradiated with different fluxes requires a clear understanding and assessment of the mechanisms responsible for the flux effect. This paper presents results of research of hardening mechanism contribution to flux effect of VVER-1000 reactor pressure vessel (RPV) welds. Transmission electron microscopy (TEM) and atom probe tomography (APT) investigations were carried out. Studies of hardening phases of RPV-steel (VVER-1000) after accelerated irradiation allowed to estimate the contribution of the hardening mechanism to flux effect. [ABSTRACT FROM AUTHOR]

Irradiation flux is an important parameter to the material irradiation damage investigation, for space material irradiation damage evaluation and the accelerated ground-based test, the irradiation flux effects cannot be ignored. In this paper, the polyimide was set as the research object, the irradiation flux effect and mechanism are investigated by the means of electron paramagnetic resonance test, and based on the free radical dynamic analysis, the dynamics mode of free radical population in vacumm can be established as a function of irradiation time (or proton irradiation flux). The results show that the free radical anneal process in vacuum follows the exponential mode, and the characteristic time constant τ radical-vacuum is about 1.9 h. Furthermore, a simplified method is proposed with the investigation of the free radical behavior of the irradiated surface modification polyimide (TiO 2 /PI), and the characteristic time constant τ bulk is 1.9 h, which is almost the same to the value of τ radical-vacuum . [ABSTRACT FROM AUTHOR]

The study of one-dimensional (1D) migration of dislocation loops is essential for understanding the evolution of material microstructure in the irradiation environment. The 1D migration of dislocation loops under thermal activation and electron irradiation has been widely studied, but the effect of ion flux on the 1D migration of dislocation loops has not been concerned. In this study, the 1D migration and evolution of dislocation loops in the Fe9Cr1.5W0.4Si ferrite/martensite steel during 400 keV Fe+ irradiation was investigated by in-situ transmission electron microscopy at room temperature. Compared with 1/2<111> dislocation loops, 〈100〉 dislocation loops had weak migration ability and low migration frequency. The effect of ion flux on the 1D migration of dislocation loops were explored, which showed that increasing the ion flux was not only beneficial to the migration of large dislocation loops, but also increased the migration frequency. In addition, the higher the ion flux, the higher the density of mobile dislocation loops per unit irradiation dose. The knowledge gained here provides an important reference for understanding and predicting the degradation process of irradiated materials. [ABSTRACT FROM AUTHOR]

This work extends the research of the basic regularities of segregation processes in the grain boundaries (GB) of VVER-1000 reactor pressure vessel (RPV) steels. The paper considers the influence of irradiation with different fast neutron fluxes on the structure, yield strength and ductile-to-brittle transition temperature (T K ) changes as well as on changes of the share of brittle intergranular fracture and development of segregation processes in the VVER-1000 RPV weld metal (WM). The obtained experimental results allow to separate the contribution of the hardening and non-hardening mechanisms to mechanical properties degradation of material irradiated at the operating temperature. It is shown that the difference in T K shift in WM irradiated to the same fluence with different fast neutron fluxes is mainly due to the difference in the GB accumulation kinetics of impurities and only to a small extent due to the material hardening. Phosphorus bulk diffusion coefficients were evaluated for the temperature exposure, accelerated irradiation and irradiation within surveillance specimens (SS) using a kinetic model of phosphorus GB accumulation in low-alloyed low-carbon steels under the influence of operational factors. The correlation between the GB segregation level of phosphorus and nickel, and the T K shift - in WM SS was obtained experimentally and indicates the non-hardening mechanism contribution to the total radiation embrittlement of VVER-1000 RPV steels throughout its extended lifetime. [ABSTRACT FROM AUTHOR]

An abnormal phenomenon may occur during gas-well testing: the wellhead pressure initially rises and then drops when shutting-in a well; the wellhead pressure initially drops and then rises when opening a well. To determine why and how this phenomenon occurs, a transient nonisothermal wellbore flow model for gas-well testing is developed. Governing equations are based on depth- and time-dependent mass, momentum equations, and the gas state equation. Temperature is predicted using the unsteady-state heat transfer model of Hasan. Boundary conditions include the restriction of formation inflow and wellhead throttling to the flow. The difference equations are established based on the implicit central finite difference method. The model can simulate the influences of temperature and flux (mass velocity). The model also considers the effects of formation inflow and surface throttling on the system. The results indicate wellhead pressure under flowing temperature is higher than that under static temperature, thus causing the abnormal phenomenon. A larger pressure difference makes the abnormal phenomenon more significant. Without considering temperature variation, simulated wellhead pressure would not exhibit the abnormity. Without considering flux variation, simulated pressure curve is not smooth. A new model has thus been validated using a gas field example. [ABSTRACT FROM AUTHOR]

This study explores the degradation of silicon-based bipolar junction transistors (BJTs) exposed to a californium-252 source. Both n–p–n and p–n–p BJTs received neutron irradiation of different fluxes (6.3 × 104 cm −2 s−1 to 1.5 × 106 cm −2 s−1). The proportion of the change of reciprocal current gain to neutron fluence increases with the decrease of neutron flux, implying that devices exposed to irradiation of lower neutron flux will suffer severer radiation damage if the total neutron fluence is kept constant. Results obtained by eliminating the impact of accompanying gamma irradiation and the impact of different neutron energy spectra from the degradation of BJTs confirmed this conclusion for both types of BJTs. A kinetic model based on the theory of diffusion-limited reactions was used to analyze the microscopic mechanism, and its results agreed with the experimental results. The simulation results also indicate that lower neutron flux favors the formation of stable defects and reduces the recombination of vacancies and interstitials. This work demonstrates that neutron flux effect also exists in silicon-based BJTs exposed to a californium-252 source and it is related to the defect evolution process in irradiated materials. [ABSTRACT FROM AUTHOR]

Studies of WWER-1000 reactor vessel weld materials with different Ni and Mn content after thermal exposure at operating temperature for up to 10,000 h have been performed. It is shown the hold time interval under study is not sufficient for development of thermal ageing processes. [ABSTRACT FROM AUTHOR]