Post-irradiation Annealing Research Articles

IRRADIATION EMBRITTLEMENT MECHANISMS AND RELEVANT INFLUENCE FACTORS OF NUCLEAR REACTOR PRESSURE VESSEL STEELS

Nuclear reactor pressure vessel is the irreplaceable component of the nuclear power plant and its integrity is one of the key issues of any nuclear power plant for long term operations. Various nanofeatures, includ- ing solute clusters, matrix damage and grain boundary segregation formed in reactor pressure vessel steels in the face of neutron irradiation. These ultrafine microstructural features lead to an increase in the ductile brittle transi- tion temperature as is the measure used to describe the irradiation embrittlement. The balance of features depends on the composition of the reactor pressure vessel steels and the irradiation conditions. This paper reviews the cur- rent phenomenological knowledge and understanding of the basic mechanisms and relevant influence factors for ir- radiation embrittlement of nuclear reactor pressure vessel steels. To be specific, the formation and evolution pro- cesses of the embrittling features are presented. Also, the influences of material variables, such as copper, nickel and manganese contents on irradiation embrittlement and those of irradiation variables, such as neutron flux and post irradiation annealing are summarized. In addition, fundamental research issues that remain to be addressed are briefly pointed out.

In situE-SEM and TEM observations of the thermal annealing effects on ion-amorphized 6H-SiC single crystals and nanophased SiC fibers

Hi Nicalon type S (HNS) SiC fiber and 6H-SiC single crystals have been irradiated with 4 MeV Au3+ at room temperature (RT) and to a fluence of 2 × 1015 cm−2. These irradiation conditions lead to the complete amorphization of the irradiated layer in both samples. Post-irradiation thermal annealing effect on the amorphized samples has been characterized in situ with both Environmental Scanning (E-SEM) and Transmission (TEM) Electron Microscopes. E-SEM observations reveal cracking and exfoliation of the amorphous layer in both samples for temperatures between 850 and 1000 °C. In parallel, TEM observations reveal recrystallization of the amorphous layer in both samples. Even though TEM thin foil specimens did not suffer mechanical failure during the tests, the good agreement between the recrystallization temperatures – 870 °C for the single crystal and 900 °C for the HNS fiber – place the recrystallization process as the stress source for the cracking and exfoliation phenomena. Crack detail of a Hi Nicalon type S SiC fiber triggered by the post-irradiation thermal annealing.

Effect of phosphorus on vacancy-type defect behaviour in electron-irradiated Ni studied by positron annihilation

Very dilute Ni–P system (containing 50–240appm phosphorus) irradiated by 5MeV electrons at various temperatures (270–543K) was studied by positron annihilation spectroscopy (PAS) and the electrical resistivity measurements. Under irradiation at 270K (below stage III in Ni), the accumulation of the monovacancies in the Ni–P system is 1.5–2.0 times greater than that in pure Ni irradiated in the same conditions. This fact attests to the strong interaction between P atoms and self-interstitial atoms (SIAs). As a result of the non-mobile SIA–P complexes formation, the mutual recombination of point defects is suppressed and the vacancy accumulation is, respectively, enhanced. During post-irradiation annealing, the vacancy migration induces the transport process of the phosphorus atoms and leads to the formation of the vacancy clusters decorated with P atoms. The annealing behaviour of the defect structures in Ni–P systems after irradiation at enhanced temperatures was also studied. The influence of phosphorus on the formation and further evolution of the vacancy aggregates decrease with increasing of the irradiation temperature.

Study of radiation-induced processes in a layered Be-Fe-Be system subjected to ion irradiation and subsequent isochronous annealings

The method of Mossbauer spectroscopy at 57Fe nuclei was used to study the structure and phase state of a three-layered Be-Fe-Be system after irradiation with helium and krypton ions and subsequent isochronous annealings. Before the irradiation, the samples represented a layered system (stable to a temperature of 650°C), the middle layer of which was predominantly an α-Fe(Be) solid solution and, in the near-surface layers, the intermetallic compound FeBe2 + δ was predominant on both sides. Upon the irradiation of the system, in the zone of the action of the beam of particles, a local enrichment of the material in beryllium atoms was observed irrespective of the type of ions used for irradiation, which led to the passage of an essential part of iron atoms in the near-surface layer from the magnetically ordered into the paramagnetic state. The extent of passage depended on the irradiation dose. The post-irradiation isochronous annealings almost completely restored the local concentration of components and structure of the irradiated near-surface layer that existed prior to the implantation. The data presented in this work can be used to prognosticate the effect of gaseous precipitates on the irradiation effects in the structural materials of atomic and thermonuclear plants.

Post-irradiation annealing behavior of neutron-irradiated FeCu, FeMnNi and FeMnNiCu model alloys investigated by means of small-angle neutron scattering

Neutron irradiation of reactor pressure vessel steels gives rise to the formation of thermodynamically stable and unstable nano-features. The present work is focused on the stability of Cu-, Mn- and Ni-containing solute clusters in model alloys exposed to post-irradiation annealing. Fe0.1Cu, Fe1.2Mn0.7Ni and Fe1.2Mn0.7Ni0.1Cu (wt%) model alloys irradiated up to neutron exposures of 0.1 and 0.19dpa (displacements per atom) were annealed at stepwise increasing temperatures in the range from 300°C (i.e. near irradiation temperature) to 500°C and characterized by means of small-angle neutron scattering (SANS). We have found characteristic differences in the annealing behavior of the alloys. In particular, there is a non-trivial (synergistic–antagonistic) interplay of Mn/Ni and Cu.

Formation of TiO2 nanorods by ion irradiation

Ion beam irradiation is a powerful method to fabricate and tailor the nanostructured surface of materials. Nanorods on the surface of single crystal rutile TiO2 were formed by N+ ion irradiation. The dependence of nanorod morphology on ion fluence and energy was elaborated. With increasing ion fluence, nanopores grow in one direction perpendicular to the surface and burst finally to form nanorods. The length of nanorods increases with increasing ion energy under same fluence. The development of the nanorod structure is originated from the formation of the nanopores while N2 bubbles and aggregation of vacancies were responsible for the formation of nanopores and nanorods. Combining C+ ion irradiation and post-irradiation annealing experiments, two qualitative models are proposed to explain the formation mechanism of these nanorods.

Introducing thermally stable inter-tube defects to assist off-axial phonon transport in carbon nanotube films

Through integrated molecular dynamics (MD) simulations and experimental studies, we demonstrated the feasibility of an ion-irradiation-and-annealing based phonon engineering technique to enhance thermal conductivity of carbon nanotube (CNT) films. Upon ion irradiation of CNT films, both inter-tube defects and intra-tube defects are introduced. Our MD simulations show that inter-tube defects created between neighboring tubes are much more stable than intra-tube defects created on tube graphitic planes. Upon thermal annealing, intra-tube defects are preferentially removed but inter-tube defects stay. Consequently, axial phonon transport increases due to reduced phonon scattering and off-axial phonon transport is sustained due to the high stability of inter-tube defects, leading to a conductivity enhancement upon annealing. The modeling predictions agree with experimental observations that thermal conductivities of CNT films were enhanced after 2 MeV hydrogen ion irradiations and conductivities were further enhanced upon post irradiation annealing.

Evolution of the radiation-induced defect structure in 316 type stainless steel after post-irradiation annealing

The thermal stability of Frank loops, black dots, cavities and γ′ precipitates in an irradiated 316 stainless steel was studied by transmission electron microscopy. The samples were retrieved from a thimble tube irradiated at around 320°C up to 80dpa in a commercial nuclear power reactor, and thermally annealed, varying both annealing temperature and time. With increasing annealing temperature the density of all defects gradually decreased, resulting in the complete removal of Frank loops at 550°C. In contrast to other defects, the density of the γ′ precipitates sharply decreased with increasing annealing time, which indicates that the dissolution of the γ′ precipitates is governed by the iron diffusion length.

Thermally- and irradiation-induced sulphur aggregation in Ni–S ageing system studied by positron annihilation

Pure nickel and very dilute Ni–S systems (containing 50–340at.ppm sulphur), irradiated by 5MeV electrons at 270K, were studied by positron annihilation spectroscopy (PAS) and the electrical resistivity measurements. The ultrafine S-rich precipitates (clusters), which are formed by thermal ageing at about 650K in Ni–S, are observed clearly by PAS. The PAS data were interpreted by assuming that the positrons localize in the volume of the “defect-free” S-rich clusters due to affinity-induced confinement. Under irradiation, the accumulation of the vacancies in the Ni–S system is several times greater than that in pure Ni. This fact testifies the strong interaction between S atoms and self-interstitial atoms. During post-irradiation annealing, the formation and recovery process of the S-rich clusters are revealed: (i) the mobile vacancy–sulphur complexes, which are formed above 350K, aggregate into nanovoids; (ii) above 550K, the dissociation of vacancies from the nanovoids decorated with S-atoms leads to the formation of the S-rich clusters; and (iii) the S-rich clusters anneal out at about 870K.

Investigation of Sulphur Nanoclusters in Ageing Ni-S System after Irradiation and Thermal Treatment by Positron Annihilation

The pure Ni and Ni S ageing systems, thermally aged and also irradiated by 5 MeV electrons at 270 K with subsequent annealing, are studied by means of positron annihilation spectroscopy. The formation of ultra ne S-rich precipitates (clusters) is observed in Ni S during thermal ageing at about 650 K. It is assumed that these clusters do not contain vacancy-type defects and they are coherent to the Ni matrix. The positrons localize in sulphur clusters that is due to a nity-induced con nement. In irradiated Ni S system, the mobile vacancy-S atom complexes are formed at post-irradiation annealing above 350 K (stage III). These complexes form the three-dimensional vacancy clusters (nanovoids) decorated with S atoms. The radiation induced nucleation of S-rich clusters occurs when nanovoids dissociate.

Formation of NV centers in diamond: A theoretical study based on calculated transitions and migration of nitrogen and vacancy related defects

Formation and excitation energies as well charge transition levels are determined for the substitutional nitrogen (N${}_{\mathrm{s}}$), the vacancy (V), and related point defects (NV, NVH, N${}_{2}$, N${}_{2}$V, and V${}_{2}$) by screened nonlocal hybrid density functional supercell plane wave calculations in bulk diamond. In addition, the activation energy for V and NV diffusion is calculated. We find good agreement between theory and experiment for the previously well-established data and predict missing ones. Based on the calculated properties of these defects, the formation of the negatively charged NV center is studied, because it is a prominent candidate for application in quantum information processing and for nanosensors. Our results indicate that NV defects are predominantly created directly by irradiation, while simultaneously produced vacancies will form V${}_{2}$ pairs during postirradiation annealing. Divacancies may pin the Fermi level, making the NV defects neutral.

Effects of post-irradiation annealing and re-irradiation on microstructure in surveillance test specimens of the Loviisa-1 reactor studied by atom probe tomography and positron annihilation

This paper presents a microstructural study of a surveillance test specimen from the Loviisa-1 reactor in Finland, which is a Russian-type pressurized water reactor (VVER-440), after initial irradiation to a neutron fluence of 2.5×1019n/cm2 (E>1MeV), post-irradiation annealing at 475°C for 100h and re-irradiation to three different fluences up to 2.7×1019n/cm2. Atom probe tomography (APT) and positron annihilation spectroscopy (PAS) were used to characterize the test specimens. APT results showed the formation of Cu-rich solute clusters (SCs) during the initial irradiation and their subsequent coarsening during annealing. After re-irradiation, a small number of SCs formed once again. The hardening due to the SCs was estimated using the Russell-Brown model based on the APT results, and was in good agreement with the measured hardening after the initial irradiation and post-irradiation annealing. In contrast, during the first-step of re-irradiation, the estimated hardening due to the SCs was smaller than the measured hardening. This suggested that the hardening after re-irradiation was due to some microstructure other than the observed SCs. This difference was attributed to newly-formed matrix defects during re-irradiation, which was supported by the PAS results. However in subsequent steps of re-irradiation, the hardening was almost constant.

Structural modification of single-walled carbon nanotube induced by X-ray irradiation and subsequent annealing studied by Raman scattering spectroscopy in radial breathing mode

The formation of X-ray-induced defects changes the spectral shape of the radial breathing mode (RBM) and defect-induced mode (D band) in the Raman spectra of single-walled carbon nanotubes (SWNTs). X-ray-induced defects have been found to be annealed by thermal treatment, indicating that they are Frenkel pairs (vacancy and interstitial pairs). We found that the spectral shape of RBM is not entirely recovered after post-irradiation annealing. The temperatures for the complete annealing of X-ray-induced defects were within the range of 200–600 °C depending on the tube geometry. From these results, we suggest that the stability of X-ray-induced defects depends on the tube geometry and that the combination of X-ray irradiation and post-irradiation annealing causes a chirality change in SWNTs.

The role of microstructure on the optical performance of neutron irradiated dielectric mirrors

Dielectric mirrors of HfO2/SiO2 and Al2O3/SiO2 designed for optimum reflectivity at 248nm with 11 and 30 bi-layer coatings, respectively, survived irradiation to 0.1dpa at 448K without film cracking or delamination from their sapphire substrates. Subsequent annealing of the irradiated samples resulted in a loss of reflectivity in the HfO2/SiO2 mirror, while the Al2O3/SiO2 type remained unaffected. Microstructural changes that correlate to optical property changes of the mirror are investigated. The amorphous layers of the Al2O3/SiO2 mirror provide greater stability despite increased Al and Si interdiffusion across the film interfaces with increasing dose and post-irradiation annealing temperature. This interdiffusion may have limited the densification of SiO2 in the Al2O3/SiO2 mirror, but no interdiffusion was observed for the HfO2/SiO2 mirror. The thickness changes in the SiO2 layers of the HfO2/SiO2 mirror resulted in a shifting of the peak reflectivity to lower wavelengths. The formation of an amorphous Al–O layer within the substrate is observed in the 0.1dpa irradiated HfO2/SiO2 mirror, which on further annealing at 573 and 673K resulted in a buckling-type delamination failures in the mirrors producing a loss in reflectivity.

Helium solubility and bubble formation in a nanostructured ferritic alloy

The response of a nanostructured ferritic alloy to He implantation and post-irradiation annealing (PIA) at 750°C was characterized by atom probe tomography and transmission electron microscopy. The supersaturated He concentration in the ferrite at a dose of ∼2.1 displacements per atom was similar for the as-implanted, 75±7 appm, and a 10h PIA treatment, 71±7 appm, but decreased to 38±2 appm after a 100h PIA treatment. Approximately 91–97% of the He bubbles were present as isolated bubbles in the ferrite and ∼1–5% on the surface of the nanoclusters in the ferrite. The remainder were associated with the grain boundaries with a small fraction on the surface of Ti(N,C,O) precipitates. Their average size and number density were similar for the as-implanted and 10h PIA treatment with a small increase in the size and a significant increase in the number density after the 100h PIA treatment. Swelling in the high dose region increased from ∼1% in the as-implanted and 10h PIA conditions to ∼5% after the 100h PIA treatment but the estimated number of He atoms per unit volume in the He bubbles decreased by an order of magnitude. Number densities increased from ∼8×1023m−3 in the as-implanted to ∼15×1023m−3 in the 10h PIA condition, with little change (to ∼12×1023m−3) in the 100h PIA condition. This trend may indicate nucleation of new bubbles up to 10h, with growth and possible consumption of the smaller bubbles between 10 and 100h.

Separation of Ionization Traps in NPN Transistors Irradiated by Lower Energy Electrons

Effects of oxide-trapped charge and interface traps induced by ionizing radiation, on the degradation of the electrical properties in NPN bipolar junction transistors (BJTs), are identified in this paper. Ionization traps in 3DG110 NPN BJTs are caused by 110 keV electron irradiations for various fluences, and the key electrical parameters are measured in situ during irradiation. In order to separate the effects of the interface traps and oxide charge on the radiation response in BJTs, the isochronal and isothermal annealing are performed after irradiations. Behavior of the radiation-induced defects is detected by DLTS after the irradiation and the postirradiation annealing. Measurement results indicate that when the change in reciprocal of the gain (Δ(1/ β)) tends to level off, the concentration of oxide-trapped charge increases with increasing irradiation fluence, while that of the interface traps keeps unchangeable. A comparison of DLTS signals with changes in excess base current (ΔI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> ) versus base-emitter voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">BE</sub> ) is conducted to show the role of oxide-trapped charge and interface traps on the excess base current, illustrating that the DLTS analysis is suitable to separate the ionization traps in BJTs.

Formation of Single and Double Donor States of Trivacancy-Oxygen Complexes in P-Type Silicon

Deep level transient spectroscopy (DLTS) has been applied to study the formation of trivacancy-oxygen complexes in Si. Samples of p-type Czochralski grown (Cz) silicon, with a boron doping concentration of ~1x1015cm-3, have been irradiated at room temperature (RT) with 1.8 MeV protons to a dose of 5×1012cm-2. Two new energy levels at Ev+0.24 eV and Ev+0.11 eV (Evdenotes the valence band edge) emerge when the divacancy (V2) to divacancy-oxygen (V2O) transition takes place during post-irradiation annealing. The concentration is ~30% relative to V2(or V2O) and further, the two new levels exhibit an almost one-to-one correlation in strength. The present results strongly support that both levels are related to the same defect with a possible identification as single and double donor states of V3O.

Origin of the visible absorption in radiation-resistant optical fibers

In this work we investigated the point defects at the origin of the degradation of radiation-tolerant optical fibers used in the visible part of the spectrum for plasma diagnostics in radiation environments. For this aim, the effects of γ-ray irradiation up to the dose of 10 MGy(SiO2) and post-irradiation thermal annealing at 550°C were studied for a Fluorine-doped fiber. An absorption peaking around 2 eV is mainly responsible for the measured radiation-induced losses, its origin being currently debated in the literature. On the basis of the unchanging shape of this band with the radiation dose, its correlation with the 1.9 eV photoluminescent band and the thermal treatment results we assign the asymmetric absorption around 2 eV to an unique defect, the NBOHC, instead of a set of various defects.

Simulation of the nanostructure evolution under irradiation in Fe–C alloys

Neutron irradiation induces in steels nanostructural changes, which are at the origin of the mechanical degradation that these materials experience during operation in nuclear power plants. Some of these effects can be studied by using as model alloy the iron–carbon system.The Object Kinetic Monte Carlo technique has proven capable of simulating in a realistic and quantitatively reliable way a whole irradiation process. We have developed a model for simulating Fe–C systems using a physical description of the properties of vacancy and self-interstitial atom (SIA) clusters, based on a selection of the latest data from atomistic studies and other available experimental and theoretical work from the literature. Based on these data, the effect of carbon on radiation defect evolution has been largely understood in terms of formation of immobile complexes with vacancies that in turn act as traps for SIA clusters. It is found that this effect can be introduced using generic traps for SIA and vacancy clusters, with a binding energy that depends on the size of the clusters, also chosen on the basis on previously performed atomistic studies.The model proved suitable to reproduce the results of low (<350K) temperature neutron irradiation experiments, as well as the corresponding post-irradiation annealing up to 700K, in terms of defect cluster densities and size distribution, when compared to available experimental data from the literature. The use of traps proved instrumental for our model.

Small-angle neutron scattering of low-Cu RPV steels neutron-irradiated at 255 °C and post-irradiation annealed at 290 °C

Fast neutron irradiation of low-Cu reactor pressure vessel (RPV) steels gives rise to the formation of solute clusters and a degradation of mechanical properties. A remarkable acceleration of cluster formation was identified in a previous investigation of low-Cu steels irradiated at 255°C. The thermal stability of the observed features at a typical RPV operation temperature (290°C) is of particular interest. Small-angle neutron scattering experiments for three low-Cu materials irradiated at 255°C and exposed to post-irradiation annealing at 290°C are reported. The results indicate stability of the irradiation-induced features at a temperature of 290°C for all three materials. In two cases, even an increase in scattering intensity upon post-irradiation annealing has been observed. The effect of the annealing treatment on the nature of clusters is discussed and a model of the cluster–matrix interface is introduced to rationalize the observed increase in scattering intensity. The role of the residual elements is discussed.