Reactor Neutron Irradiation Research Articles

Effects of chemical composition and dose on microstructure evolution and hardening of neutron-irradiated reactor pressure vessel steels

The correlation of microstructure evolution and hardening was studied in two kinds of A533B-1 steel with high and low levels of Cu irradiated in a range of dose from 0.32 to 9.9 × 10 19 n cm −2 ( E > 1 MeV) under a high flux of about 1.7 × 10 13 n cm −2 s −1 using three-dimensional local electrode atom probe (3DAP), positron annihilation (PA) techniques, and Vickers microhardness. The early rapid hardening was found to be caused by mainly matrix defects such as mono- or di-vacancies ( V 1 − V 2) and/or dislocations indicated by the PA analysis. The 3DAP analysis showed that dense dispersion of dilute Cu rich clusters and lean distribution of Mn–Ni–Si rich clusters, which were identified to possess the same dislocation-pinning effect by applying a Russell and Brown model, were responsible for large and small hardening in high- and low-Cu steels irradiated above 0.59 × 10 19 n cm 2, respectively.

Thermal annealing study of F center clusters in LiF single crystals

The present work is devoted to study the thermal annealing process of F center clusters (F n ) induced in LiF single crystal under high-dose γ-rays and high reactor neutrons fluence irradiations. With heating under argon atmosphere, the F-type center aggregates and gives rise to a new absorption band at 500 nm attributed to Li colloids. The optical density associated with F 2 center observed in γ-ray irradiated LiF decreases with increasing annealing temperature and exhibits two distinct annealing processes with activation energies E 1=0.9±0.3 eV and E 2=1.6±0.5 eV. Also, it is clear from the results that the F 3 + and F 2 emission bands positions are affected by the irradiated dose. Reactor neutrons irradiation induces large Li colloids. These colloids persist even after annealing at 450 °C.

Damage structures in austenitic stainless steels during incubation period of void swelling

AbstractThe incubation period, a transient stage before the steady void swelling, in austenitic stainless steels is important, because it determines the duration of service under neutron irradiation in reactors. However, most of experimental studies have been performed after void formation by transmission electron microscopy, and point defects and their clusters under the resolution limits are impossible to detect. In this study, defect structures during the incubation period were studied by using positron annihilation lifetime measurement. The formations of small vacancy clusters below 363 K and precipitates above 563 K were detected by neutron irradiation. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of reactor neutrons irradiation on electrical, optical and structural properties of SnO 2 film prepared by sol–gel method

SnO 2 films elaborated by sol–gel method were irradiated with reactor neutron. The irradiations were made at a temperature of about 40 °C, with fast neutron fluences ( E n >1.2 MeV) up to 9.6×10 17 n cm −2. The induced defect has been studied by electrical, optical and structural measurements. The electrical measurements show that the resistivity rapidly increases with increasing fluences up to 3.2×10 17 n cm −2 and remains constant for higher fluence (>6.4×10 17 n cm −2). The optical measurements show a small decrease of the optical band gap with increasing fluences. From the X-ray diffraction patterns, it was found that the crystallinity and grain size are reduced.

Master Curve and Unified Curve applicability to highly neutron irradiated Western type reactor pressure vessel steels

While the Master Curve (MC) method is gradually entering brittle fracture safety assessment procedures world-wide, knowledge is still lacking about its limits of applicability to highly neutron irradiated material. In this paper two reactor pressure vessel (RPV) steels A533B Cl.1 (IAEA reference material code JRQ) and A508 Cl.3 (code JFL) were scrutinized for possible deviations of the postulated invariant MC shape and the MC validity for macroscopically inhomogeneous microstructure. Besides tensile and Charpy-V tests, MC tests were performed on Charpy-size three-point bend specimens in the unirradiated, neutron irradiated with fluences up to nearly 10 20 n/cm 2 ( E > 1 MeV) and recovery heat treated condition. Evaluation procedures include Master Curve reference temperature T 0 determination according to ASTM E1921-05 as well as additional analysis methods such as SINTAP, multi-modal MC method (MML) and the Unified Curve (UC). Integrity assessment according to ASME Code Cases N-629 and N-631 has been applied. It is shown that the standard MC concept provides a precise description of the fracture toughness for all conditions, even exceptionally well for the highly irradiated state. No MC shape change could be observed, whereas the UC concept indicates a significant influence of irradiation on the fracture toughness curves for the highly irradiated JRQ.

106 磁気特性に基づく照射損傷評価(OS7-(2) 材料損傷の機構・実験評価,オーガナイズドセッション)

We measured the magnetic flux desities and the magnetization curves on neutron irradiated fast reactor grade type 316 stainless steels by a flux gate sensor and a newly developed vibrating sample magnetometer, respectively. As the result, it was revealed that there is a good relationship between magnetic property and dose, one of representative irradiation damage parameters. This result shows the possibility of nondestructive evaluation of neutron irradiation damage based on magnetic properties.

J0406-1-4 中性子照射した圧力容器鋼およびモデル合金の弾性定数の超音波共鳴法による測定(超音波計測・解析法の新展開(1))(超音波計測・解析法の新展開(1))

Ultrasonic characteristics of neutron-irradiated reactor pressure vessel (RPV) steels and model alloys were investigated by resonant ultrasound spectroscopy (RUS). Specimens were neutron-irradiated at the Japan Materials Testing Reactor at 290℃ up to a fast neutron fluence of 5×10^<23> n/m^2 (E>1MeV), and isochronal annealed at temperature ranges from 360℃ to 570℃ at 70K interval for 30 min. Both Vickers hardness and RUS measurements were carried out after each post-irradiation annealing (PIA). The effects of irradiation on elastic constants are discussed.

Study of atomic clusters in neutron irradiated reactor pressure vessel surveillance samples by extended X-ray absorption fine structure spectroscopy

Copper and nickel impurities in nuclear reactor pressure vessel (RPV) steel can form nano-clusters, which have a strong impact on the ductile–brittle transition temperature of the material. Thus, for control purposes and simulation of long irradiation times, surveillance samples are submitted to enhanced neutron irradiation. In this work, surveillance samples from a Swiss nuclear power plant were investigated by extended X-ray absorption fine structure spectroscopy (EXAFS). The density of Cu and Ni atoms determined in the first and second shells around the absorber is affected by the irradiation and temperature. The comparison of the EXAFS data at Cu and Ni K-edges shows that these elements reside in arrangements similar to bcc Fe. However, the EXAFS analysis reveals local irradiation damage in the form of vacancy fractions, which can be determined with a precision of ∼5%. There are indications that the formation of Cu and Ni clusters differs significantly.

Magnetic properties of neutron irradiated RPV steel

The neutron irradiated reactor pressure vessel (RPV) steels at various dose of 0–1018 n/cm2 have been studied with Mossbauer, x-ray diffraction, and VSM. The Mossbauer data shows that the value of magnetic hyperfine field of Fe atom that exist at martensite is 330 kOe at site 1 and 305 kOe at site 2. At room temperature, the total absorption area of Mossbauer spectra with respect to irradiation of neutron is constant for the dose of 0–1016 n/cm2, while over the dose of 0–1017 n/cm2 the absorption area decreases rapidly. But the doublet area for the dose of 0–1016 n/cm2 is constant, while over the dose of 1017 n/cm2 it increases with increasing the fluence level of neutron. The coercivity and remanence of the neutron irradiated samples do not change significantly. But the maximum induction decreases by 5% at 1018 n/cm2, compared with that of the as-received sample.

Study of a neutron irradiated reactor pressure vessel steel by X-ray absorption spectroscopy

Reactor pressure vessel (RPV) reference steel samples submitted to neutron irradiations followed by thermal annealing were investigated by X-ray absorption fine structure (XAFS) spectroscopy. Several studies revealed that Cu and Ni impurities can form nanoclusters. In the unirradiated sample and in the only-irradiated sample no significant clustering is detected. In all irradiated and subsequently annealed samples increases of Cu and Ni atom densities are recorded around the absorber. Furthermore, the density of Cu and Ni atoms determined in the first and second shells around the absorber is found to be affected by the irradiation and annealing treatment. The comparison of the XAFS data at Cu and Ni K-edges shows that these elements reside in arrangements similar to bcc Fe. However, the local irradiation damage yields vacancy fractions which were determined from the analysis of XAFS data with a precision of ∼5%.

Effects of neutron-irradiation-induced intergranular phosphorus segregation and hardening on embrittlement in reactor pressure vessel steels

The effects of intergranular P segregation and hardening on the ductile-to-brittle transition temperature (DBTT) in several neutron-irradiated reactor pressure vessel steels with different bulk contents of P and Cu have been investigated using a scanning Auger microbe, a local electrode atom probe and positron annihilation spectroscopy. Increasing the neutron fluence at 563 K promotes intergranular P segregation, particularly in steels with high levels of P. The content of P (<570 ppm) more significantly affects irradiation-hardening than that of Cu (<0.17 wt.%) due to distinct formation of P-rich precipitates arising from the stabilization of vacancies. Analyzing the correlations between P segregation, hardening, fraction of intergranular fracture and DBTT, it is found neutron irradiation mitigates the embrittling effect of segregated P, and therefore the hardening more strongly affects the DBTT shift than the P segregation, with the exception of highly P-doped steel irradiated to high neutron fluence.

Li colloids formation study induced by reactor neutrons in LiF single crystals

Single crystal of lithium fluoride was irradiated with neutrons at about 40°C. The irradiation was performed with reactor neutrons at fast (E>1MeV) and thermal neutrons fluence of 1.8×1018cm-2 and 3.3×1018cm-2, respectively. The resulting damage was characterised by optical measurements, scanning electron microscopy and X-ray diffraction analysis. The obtained results show large Li colloids of about 0.513μm that are formed after intense reactor neutrons irradiation. Photoluminescence spectra obtained after annealing under argon atmosphere show the F centers aggregation process with increasing annealing temperature. Li colloids intensity increases with increasing annealing temperature and shifts towards higher wavelength (500nm).

The Clustering of Cu Atoms in Neutron Irradiated Reactor Pressure Vessel Steels Studied by Positron Annihilation

Defect studies of neutron-irradiated Cr-Mo-V (VVER-440) type reactor pressure vessel steels were performed in the present work. The steels were irradiated in the nuclear power plant reactor under the conditions of a regular operation. Characterization of the irradiation induced defects was performed by two complementary techniques of positron annihilation spectroscopy: (i) positron lifetime spectroscopy was used for identification of defects and determination of defect densities, (ii) coincidence Doppler broadening was employed for investigation of Cu atom aggregates. Long range diffusion of Cu atoms is assisted by the irradiation induced vacancies. The solute Cu atoms form small clusters in the irradiated steels. Subsequent isochronal annealing of the irradiated steel leads to vacancy assisted clustering of Cu atoms and formation of small precipitates. The Cu clusters exhibit maximum diameter at 400oC. Above this temperature the clusters dissolve again in the matrix.

Specificity of proton irradiation effects on polymers

Protons affect the polymers mainly through scission and cross-linking of macromolecule chains. Ionization processes at proton non-elastic scattering define these effects. One of the most important problems of radiation physics and chemistry of polymers is the dependence of radiation effects on polymers on the kind of ionizing radiation or the LET problem. Analyze of such dependence for protons and some other kinds of radiation is presented. A detailed examination of 100 MeV proton collisions in polyethyleneterephtalate (PETP, trade mark Mylar) with negligible quantity of additive, on the base of the sample heat conductivity behavior revealed an important role of atomic displacements resulting from elastic nuclear scattering of low energy ions. Such damage of polymer molecules was not obtained for other examined types of radiation, in particular, for recoil nuclei at reactor neutron irradiation and for 60Co γ-irradiation.

Phosphorus Segregation and Intergranular Embrittlement in Thermally Aged and Neutron Irradiated Reactor Pressure Vessel Steels

Abstract A study on phosphorus (P) segregation at grain boundaries and intergranular embrittlement of reactor pressure vessel steels due to thermal aging and neutron irradiation has been performed for A533B steel plates and weld metals doped with different bulk levels of P. The initial P concentration at grain boundaries of these materials determined by scanning Auger microscopy ranged from 9 to 33 %. The materials thermally aged at temperatures ranging from 450 to 550°C for up to 10000 h exhibited increases in P concentration described by the McLean’s equation. A linear correlation was obtained between P concentration and Charpy 41 J transition temperature (T41J). Fracture toughness tests in the ductile-to-brittle transition temperature region have been conducted by the Master Curve method using 1T-CT specimens. The material subject to intergranular embrittlement exhibited the dual fracture toughness distributions typically observed in inhomogeneous materials. This was probably due to two types of initiation of cleavage and intergranular fracture. The lower tail analysis based on the Master Curve method enabled conservative fracture toughness estimates by fitting the lower fracture toughness arising from intergranular fracture. The relation between median KJc values estimated from the lower tail analysis and temperature appeared to follow the Master Curve. For neutron irradiated materials up to a fluence of 6.9×1019 n/cm2 (E&amp;gt;1 MeV) at 290°C by the Japan Materials Testing Reactor, an increase in grain boundary P segregation associated with decrease in carbon segregation was observed with increasing fluence. Relative significant hardening was recognized for the material with high bulk P content. Shifts in the reference temperature T0 and T41J were comparable for the thermally aged and irradiated materials.

Precipitation of the FeCu system: A critical review of atomic kinetic Monte Carlo simulations

Cu is of primary importance in the embrittlement of the neutron-irradiated reactor pressure vessel (RPV) steels. It has been observed to segregate into copper-rich precipitates within the ferrite matrix under irradiation. Since its role was discovered more than 40 years ago, Cu precipitation in α-Fe has been studied extensively under irradiation as well as under thermal ageing using atom probe tomography, small angle neutron scattering, and high resolution transmission electron microscopy. Numerical simulation techniques such as Rate Theory or Monte Carlo methods have also been used to investigate this problem, however not always with a great success. In this paper, we provide a critical review of the different kinetic Monte Carlo models dedicated to the study of Cu precipitation, with the aim of determining which elements are key to an accurate description of precipitation.

Temper-bead repair welding of neutron-irradiated reactor (pressure) vessels by low-heat-input TIG and YAG laser welding

Weldability in neutron-irradiated low alloy steel for reactor (pressure) vessel has been studied by temper-bead repair-welding of low-heat-input TIG and YAG laser welding. A low alloy steel and its weld, and stainless steel clad and nickel (Ni)-based alloy clad were irradiated in a materials test reactor (LVR-15, Czech Republic) up to 1.4 X 1024 n/m2 (>1 MeV) at 290°C, which approximately corresponds to the maximum neutron fluence of 60-year-operation plants' vessels. The He concentration in the irradiated specimens was estimated to be up to 12.9 appm. The repair-welding was carried out by TIG and YAG laser welding at a heat input from 0.06 to 0.86 MJ/m. The mechanical tests of tensile, impact, side bend and hardness were carried out after the repair-welding.Cracks were not observed in the irradiated low alloy steel and its weld by temper-bead repair-welding. Small porosities were formed in the first and second layers of the repair-welds of low alloy steel (base metal). However, only a few porosities were found in the repair-welds of the weld of low alloy steel. From the results of mechanical tests, the repair-welding could be done in the irradiated weld of low alloy steel containing a He concentration up to 12.9 appm, although repair-welding could be done in base metal of low alloy steel containing up to only 1.7 appmHe. On the other hand, cracks occurred in the heat affected zones of stainless steel and Ni-based alloy clads by repair-welding, except by YAG laser repair-welding at a heat input of 0.06 MJ/m in stainless steel clad containing 1.7 appmHe. Based on these results, the determination processes were proposed for optimum parameters of repair-welding of low alloy steel and clad used for reactor (pressure) vessel.

Positron annihilation study and computational modeling of defect production in neutron-irradiated reactor pressure vessel steels

Positron annihilation spectroscopy (PAS) and a computer simulation were used to investigate a defect production in reactor pressure vessel (RPV) steels irradiated by neutrons. The RPV steels were irradiated at 250°C in a high-flux advanced neutron application reactor. The PAS results showed that mainly single vacancies were created to a great extent as a result of a neutron irradiation. Formation of vacancies in the irradiated materials was also confirmed by a coincidence Doppler broadening measurement. For estimating the concentration of the point defects in the RPV steels, we applied computer simulation methods, including molecular dynamics (MD) simulation and point defect kinetics model calculation. MD simulations of displacement cascades in pure Fe were performed with a 4.7keV primary knock-on atom to obtain the parameters related to displacement cascades. Then, we employed the point defect kinetics model to calculate the concentration of the point defects. By combining the positron trapping rate from the PAS measurement and the calculated vacancy concentrations, the trapping coefficient for the vacancies in the RPV steels was determined, which was about 0.97×1015s−1. The application of two techniques, PAS and computer simulation, provided complementary information on radiation-induced defect production.

Comparison of the radiation hardness of Magnetic Czochralski and Epitaxial silicon substrates after 26 MeV proton and reactor neutron irradiation

We report on the processing and characterization of microstrip sensors and pad detectors produced on n- and p-type Magnetic Czochralski (MCz), Epitaxial (EPI) and Float Zone (FZ) silicon within the SMART project to develop radiation-hard silicon position sensitive detectors for future colliders. Each wafer contains 10 microstrip sensors with different geometries, several diodes and test structures. The isolation in the strip detectors produced on p-type material has been achieved by means of a uniform p-spray implantation, with doping of 3×10 12 cm −2 (low-dose p-spray) and 5×10 12 cm −2 (high-dose p-spray). The samples have undergone irradiations with 26 MeV protons and reactor neutrons up to ∼10 16 cm −2 1 MeV equivalent neutrons (n eq/cm 2), and have been completely characterized before and after irradiation in terms of leakage current, depletion voltage and breakdown voltage. The current damage parameter α has been determined for all substrates. MCz diodes show less pronounced dependence of effective doping concentration N eff on the fluence when compared to standard FZ silicon, giving results comparable to diffusion oxygenated FZ devices for all irradiation sources. The observed increase of N eff with fluence can be interpreted in EPI material as a net donor introduction process, overcompensating the usual acceptor introduction process. This effect is stronger for 26 MeV proton irradiation than for neutron irradiation.

Embrittlement of RPV steels: An atom probe tomography perspective

Atom probe tomography has played a key role in the understanding of the embrittlement of neutron irradiated reactor pressure vessel steels through the atomic level characterization of the microstructure. Atom probe tomography has been used to demonstrate the importance of the post weld stress relief treatment in reducing the matrix copper content in high copper alloys, the formation of ∼2-nm-diameter copper-, nickel-, manganese- and silicon-enriched precipitates during neutron irradiation in copper containing RPV steels, and the coarsening of these precipitates during post irradiation heat treatments. Atom probe tomography has been used to detect ∼2-nm-diameter nickel-, silicon- and manganese-enriched clusters in neutron irradiated low copper and copper free alloys. Atom probe tomography has also been used to quantify solute segregation to, and precipitation on, dislocations and grain boundaries.