Post-irradiation Annealing Treatments Research Articles

Investigation of Irradiation Hardening and Effectiveness of Post-Irradiation Annealing on the Recovery of Tensile Properties of VVER-1000 Realistic Welds Irradiated in the LYRA-10 Experiment

Assessing the embrittlement and hardening of reactor pressure vessel steels is critical for the extension of the service lifetime of nuclear power plants. This paper summarises the tensile test results on the irradiation behaviour of realistic VVER-1000 welds from the STRUMAT-LTO project. The welds were irradiated at the HFR (Petten, the Netherlands) to a fluence of up to 1.087 × 1020 n·cm−2, and their irradiation hardening was studied by means of tensile testing. The four grades, with different Mn and Ni contents, show different hardening behaviours. The highest degree of irradiation hardening is observed for the weld that has the highest combined Ni + Mn content. The results show that there is a synergetic effect of Mn and Ni on the irradiation hardening behaviour of the VVER-1000 welds. Besides irradiation hardening, the effectiveness of post-irradiation annealing treatments on the recovery of the tensile properties is studied in the present work. Post-irradiation annealing treatments conducted at 418 °C and at 475 °C proved to be effective for three of the four investigated welds. For the realistic weld with the highest combined Ni + Mn, only the annealing at 475 °C led to the complete recovery of the tensile properties.

Irradiation hardening of stainless steel model alloy after Fe-ion irradiation and post-irradiation annealing treatment

Microstructural observation with atom probe tomography, transmission electron microscopy, and mechanical examination with a nano-indenter were performed on stainless steel model alloy that had been irradiated with Fe ions and subjected to post-irradiation annealing treatment (PIA). Significant irradiation hardening occurred in the as-irradiated specimen and irradiation hardening recovery occurred at 400 °C of cumulative PIA, Tiny black dots, Frank loops and Ni-Si clusters formed in the as-irradiated specimen and their recovery was observed with the cumulative PIA. A classic dispersed-barrier hardening model was used to evaluate irradiation hardening connected to microstructural information by applying the density and size of defect variants. Ni-Si clusters and Frank loops contributed substantially to the increase of irradiation hardening, whereas the black dots contributed little for the stainless steel model alloy. The barrier strength factor of Ni-Si clusters, αSC, was obtained as 0.029 and 0.015 for two different obstacle strength model, which suggests that an individual Ni-Si cluster is a weak barrier obstacle to mobile dislocation on the dispersed-barrier hardening model, but that the dense formation of Ni-Si clusters controls irradiation hardening behavior. A quantitative analysis was performed of the increase in strength from the Ni-Si clusters that was caused by coherency strains, and an irradiation hardening mechanism for the stainless steel, including the solute atom clusters, is discussed.

The damage investigations of 4H–SiC after P-ion irradiation

In this work, a single crystal of 4H–SiC was subjected to phosphorus irradiation at 100 keV with four different fluences at room temperature (RT) and post-irradiation annealing treatments at temperature range from 200 to 600 °C. The measured effective refractive index reflected that the disorder increased with the increasing fluences and partially recovered after our annealing treatments. Rutherford backscattering /channeling (RBS/C) experiments displayed that above the fluence of 5.0 × 1014 ions/cm2, severe damage caused in the surface of 4H-SiC crystal. The amorphous layer about 136 nm induced when the fluence is up to 1.0 × 1015 ions/cm2 according to the transmission electron microscopy (TEM) results. The amorphous phenomenon was explained by lattice swelling. We obtained the damage threshold value for the formation of amorphous in 4H–SiC crystal is about 0.3 dpa. Phosphor is one of the important semiconductor dopant elements. This work provides reference data on selective doping of SiC-based electronic devices.

Proton irradiation damage in cold worked Nb-stabilized 20Cr-25Ni stainless steel

Micro-scale damage with a topographical contrast has been observed in cold-worked Nb-stabilised 20%Cr-25%Ni stainless steel, following irradiation with 2.2 MeV protons at 400 °C and a dose rate of ∼10−5 dpa/s. After an irradiation dose of 3 and 5 dpa, microstructural changes were found to a depth of 22 μm below the irradiated surface, coincident with the estimated mean range of protons in the material as predicted by the TRIM code. Large variations of intragranular mis-orientations, in combination with a depletion of chromium at grain boundaries, were observed in the proton-irradiated layer. This was accompanied by an increase in nano-hardness, which was linked to a partial transformation of tangled dislocation networks into dislocation loops. Such visible microstructural changes have not been reported before in the absence of post-irradiation annealing treatments.

On α′ precipitate composition in thermally annealed and neutron-irradiated Fe- 9-18Cr alloys

Ferritic-martensitic steels are leading candidates for many nuclear energy applications. However, formation of nanoscale α′ precipitates during thermal aging at temperatures above 450 °C, or during neutron irradiation at lower temperatures, makes these Fe-Cr steels susceptible to embrittlement. To complement the existing literature, a series of Fe-9 to 18 Cr alloys were neutron-irradiated at temperatures between 320 and 455 °C up to doses of 20 dpa. In addition, post-irradiation annealing treatments at 500 and 600 °C were performed on a neutron-irradiated Fe-18 Cr alloy to validate the α-α′ phase boundary. The microstructures were characterized using atom probe tomography and the results were analyzed in light of the existing literature. Under neutron irradiation and thermal annealing, the measured α′ concentrations ranged from ∼81 to 96 at.% Cr, as influenced by temperature, precipitate size, technique artifacts, and, possibly, cascade ballistic mixing.

Ionizing radiation exposure reveals instability of purified domains in polymer/fullerene solar cells

Solar cells based on conjugated polymer and fullerene blends exhibit morphology-dependent stability towards ionizing radiation exposure. Blend thin film solar cells of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C60-butyric acid methyl ester (PCBM) were irradiated up to 1Mrad with X-rays (10–60KeV) and investigated with resonance Raman and photocurrent/photovoltage modulation spectroscopy and imaging. Bulk blend morphological characteristics were pre-selected by controlling film processing conditions (i.e., solvent and thermal annealing). As-cast P3HT/PCBM devices with a large fraction of a well-mixed phase display improvements in performance upon application of post-irradiation annealing treatments. On the other hand, devices annealed prior to irradiation showed irreversible performance degradation that could not be recovered following annealing treatments after exposure. Resonance Raman spectroscopy and imaging following irradiation up to 1Mrad reveals virtually no changes in the P3HT structural and packing integrity for both types of devices. Intensity modulated photocurrent/photovoltage spectroscopy measurements indicate that radiation-induced performance degradation is concentrated in the vicinity of device contacts, evident from large alterations in charge extraction kinetics. We propose that charge transfer from the aluminum cathode leads to formation of a space charge zone which becomes more pronounced in devices with greater phase purification (i.e., larger PCBM crystallites in the vicinity of the cathode). Importantly, well-mixed P3HT/PCBM regions, which are more plentiful in as-cast devices, appear to mitigate radiation-induced performance degradation since application of post-irradiation annealing treatments restores device performances to pristine levels. Our results suggest that greater morphological stability, such as polymer/fullerene systems forming co-crystals, is key for suppressing performance losses from ionizing radiation exposure.

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.

Recovery process of neutron-irradiated vanadium alloys in post-irradiation annealing treatment

Experiments to determine the influence of post-irradiation annealing on the mechanical properties and microstructures of neutron-irradiated V–4Cr–4Ti alloys were conducted. Two groups of specimens (as-irradiated specimens and specimens which underwent the post-irradiation annealing treatment) were subjected to tensile tests at room temperature and 773K. Post-irradiation annealing experiments carried out over periods of up to 50h were used to restore strength and ductility. As annealing time was extended, ductility was recovered up to 5% at 50h anneal; however irradiation hardening was not recovered completely. Microstructural changes due to post-irradiation annealing corresponded to the amount that yield stress increased in tensile behavior in the irradiated specimen. The recovery in ductility was likely caused by the dissolution of interstitial impurities from defect clusters and dislocation cores produced by neutron irradiation during post-irradiation anneal treatment. A 3% elongation recovery in V–4Cr–4Ti alloys was achieved by annealing at 773K for 20h in a vacuum for neutron-irradiated samples at low temperature.

Analysis and optimization of propagation losses in LiNbO3 optical waveguides produced by swift heavy-ion irradiation

The propagation losses (PL) of lithium niobate optical planar waveguides fabricated by swift heavy-ion irradiation (SHI), an alternative to conventional ion implantation, have been investigated and optimized. For waveguide fabrication, congruently melting LiNbO3 substrates were irradiated with F ions at 20 MeV or 30 MeV and fluences in the range 1013–1014 cm−2. The influence of the temperature and time of post-irradiation annealing treatments has been systematically studied. Optimum propagation losses lower than 0.5 dB/cm have been obtained for both TE and TM modes, after a two-stage annealing treatment at 350 and 375∘C. Possible loss mechanisms are discussed.

Development of an instrumented indentation device for further characterization of irradiated steels at high temperature

Abstract Considering its ability to characterize the tensile behavior of irradiated RAFM steels, the technique of indentation is of particular interest for use at both room- and high temperatures. In the present work, room temperature indentation tests indicate that post-irradiation annealing treatments realized at 500/550 ° C lead to a partial recovery of the embrittlement and strengthening effects observed in samples highly affected by the irradiation temperature of 250 ° C at 15 dpa. Further characterizations of such irradiated steels by indentation at high temperatures are required for a more complete description of the mechanical behavior. To enable such investigations, a new instrumented indentation device has been conceived for testing samples at up to 650 ° C in a high-vacuum atmosphere in a Hot Cell of the Karlsruhe Fusion Materials Laboratory. The final design of the device conceived is presented here. In addition to the constraints of remote handling to be taken into account for operation in a Hot Cell, the design of the test facility notably results from the development of an adapted heating system. Moreover, a digital image processing-based technique was selected according to its ability to measure indentation depths at sub-micrometer resolutions. The corresponding optical setup was defined for the adaptation to the testing device.

Determining irradiation damage and recovery by instrumented indentation in RAFM steel

The mechanical behavior of specific reduced-activation ferritic-martensitic steels (RAFM), designed for fusion applications, is investigated in the present study. Eurofer97 and F82H-mod specimens have been neutron-irradiated in the High Flux Reactor at Petten (HFR) up to a dose of 15dpa at 250–450°C. An indentation method [1], making use of a data post-analysis program based on neural networks, has been employed to identify tensile properties by multistage indents. The significant irradiation-induced hardening effect present at 15dpa in the range of 250–350°C could be observed in Vickers hardness and in the calculated material’s strength parameters, the influence of the pre-irradiation heat treatment could be identified. Post-irradiation annealing treatments have been realized at 500°C and 550°C and leads to a partial recovery of the irradiation damage.

Comparison of neutron and gamma irradiation effects on KU1 fused silica monitored by electron paramagnetic resonance

Electron paramagnetic resonance (EPR) studies have been carried out on KU1 fused silica irradiated with neutrons at fluences 10 21 and 10 22 n/m 2, and gamma-ray doses up to 12 MGy. The effects of post-irradiation thermal annealing treatments, up to 850 °C, have also been investigated. Paramagnetic oxygen-related defects (POR and NBOHC) and E′-type defects have been identified and their concentration has been measured as a function of neutron fluence, gamma dose and post-irradiation annealing temperature. It is found that neutrons at the highest fluence generate a much higher concentration of defects (mainly E′ and POR, both at concentrations about 5 × 10 18 spins/cm 3) than gamma irradiations at the highest dose (mainly E′ at a concentration about 4 × 10 17 spins/cm 3). Moreover, for gamma-irradiated samples a lower treatment temperature (about 400 °C) is required to annihilate most of the observed defects than for neutron-irradiated ones (about 600 °C).

Optical channel waveguides with trapezoidal-shaped cross sections in KTiOPO 4 crystal fabricated by ion implantation

Optical channel waveguides were fabricated in KTiOPO 4 crystal by He +-ion implantation using photoresist masks with wedged-shaped cross sections. Semi-closed barrier walls with reduced refractive indices inside the crystal constructed the enclosed regions to be channel waveguides with trapezoidal-shaped cross sections. The m-line as well as end-fire coupling arrangements were performed to characterize the waveguides with light at wavelength of 632.8 nm. The propagation loss of the channel waveguides was determined to be as low as ∼2 dB/cm after simple post-irradiation thermal annealing treatment in air.

Carbon ion-implanted optical waveguides in Nd:YLiF 4 crystal: Refractive index profiles and thermal stability

We report on the optical planar waveguides in Nd:YLiF 4 laser crystals fabricated by 6.0 MeV C 3+ ion implantation at doses of 1 × 10 15 or 2.5 × 10 15 ions/cm 2, respectively. The refractive index profiles, which are reconstructed according to the measured dark mode spectroscopy, show that the ordinary index had a positive change in the surface region, forming non-leaky waveguide structures. The extraordinary index is with a typical barrier-shaped distribution, which may be mainly due to the nuclear energy deposition of the incident ions into the substrate. In order to investigate the thermal stability of the waveguides, the samples are annealed at temperature of 200–300 °C in air. The results show that waveguide produced by higher-dose carbon implantation remains relatively stable with post-irradiation annealing treatment at 200 °C in air.

Fracture Toughness, Thermo-Electric Power, and Atom Probe Investigations of JRQ Steel in I, IA, IAR, and IARA Conditions

Abstract The International Atomic Energy Agency has sponsored a number of studies involving a specific plate of A533 grade B class 1 steel designated heat JRQ. In this cooperative study between the Paul Scherrer Institute (PSI) and the Heavy-Section Steel Irradiation Program of Oak Ridge National Laboratory (ORNL), groups of Charpy impact, tensile, and precracked Charpy specimens of the JRQ plate were irradiated by PSI to four different fast neutron fluences [from 0.39 to 5.0 × 1023 n/m2 (>1 MeV)] in a test reactor. Additional specimens were given a post-irradiation thermal annealing treatment at 460°C for 18 h when 50 % of the target fluence was reached, followed by reirradiation to the same target fluences for the four groups of irradiated specimens. Additionally, ORNL thermally annealed some of the reirradiated specimens, as well as some of those in the irradiated only condition. Charpy impact, tensile, fracture toughness, and hardness tests have been performed to evaluate material response in the various conditions and to compare with the unirradiated material. Additionally, thermo-electric power experiments have been conducted by PSI, while atom probe tomography evaluations have been conducted by ORNL. Except at the highest fluence, the results show that the material given an intermediate annealing treatment exhibited irradiation-induced transition temperature shifts about the same as those that were only irradiated. However, the upper-shelf energies were generally higher and the yield strengths were generally lower for the reirradiated groups. The intermediate thermal annealing resulted in less reirradiation embrittlement of fracture toughness than Charpy impact toughness, while annealing after reirradiation resulted in significant increases in Charpy upper-shelf energy above that in the unirradiated condition. Irradiation-induced and IAR Charpy impact transition temperature shifts exhibit a nearly linear correlation with Seebeck coefficient in the I and IAR conditions. ORNL has also sent additional specimens from I, IA, IAR, and IARA test of JRQ for additional TEP testing. After irradiation, a high number density of ultrafine Cu-, Mn-, Ni-, Si-, and P-enriched precipitates were observed by atom probe tomography. Phosphorus segregation to dislocations was also observed. A significantly lower number density of larger Cu-, Mn-, and Ni- enriched precipitates was observed after the second annealing treatment.

Effect of neutron irradiation and post-irradiation annealing on microstructure and mechanical properties of OFHC-copper

Specimens of oxygen-free high conductivity (OFHC) copper were irradiated in the DR-3 reactor at Risø at 100 °C to doses in the range 0.01–0.3 dpa (NRT). Some of the specimens were tensile tested in the as-irradiated condition at 100 °C whereas others were given a post-irradiation annealing treatment at 300 °C for 50 h and subsequently tested at 100 °C. The microstructure of specimens was characterized in the as-irradiated as well as irradiated and annealed conditions both before and after tensile deformation. While the interstitial loop microstructure coarsens with irradiation dose, no significant changes were observed in the population of stacking fault tetrahedra (SFT). The post-irradiation annealing leads to only a partial recovery and the level of recovery depends on the irradiation dose level. However, the post-irradiation annealing eliminates the yield drop and reinstates enough uniform elongation to render the material useful again. These results are discussed in terms of the cascade-induced source hardening (CISH) model.

Re-embrittlement behaviour of VVER-440 reactor pressure vessel weld material after annealing

The radiation embrittlement behaviour of Cr–Mo–V alloyed weld metal (VVER-440 RPV weld) after post-irradiation thermal annealing treatment is described in this paper. The experimental program includes two weld materials, two irradiation conditions, three different annealing temperatures and two subsequent annealing and re-irradiation cycles. The effects of these parameters on the embrittlement behaviour are determined by impact and fracture toughness testing. The results of the work show that the thermal annealing treatment can effectively restore the mechanical properties of Cr–Mo–V alloyed weld materials that was embrittled by neutron radiation. Repeated annealing treatment after re-irradiation show similar recovery of toughness properties as the first annealing. The phosphorus content has a clear influence on the embrittlement behaviour of the studied welds. The results indicate that the post-annealing embrittlement rate may be lower than the pre-annealing embrittlement rate and that an increase of the annealing temperature tends to reduce total post-annealing embrittlement. Based on the results of this work, a new approach for predicting the post-annealing re-embrittlement rate of Cr–Mo–V alloyed weld metals is proposed. The approach is based on the hypothesis that the post-annealing re-embrittlement is mainly governed by radiation induced precipitation of phosphorus-rich features. The proposed approach seems to describe the experimental findings of the work with relatively good accuracy.

Recovery characteristics of neutron-irradiated V–Ti alloys

The recovery characteristics of neutron-irradiated pure V and V–Ti alloys with 1.0 and 4.5 at.% Ti have been investigated by positron annihilation spectroscopy. Microvoid formation during irradiation at 320 K is produced in pure V and V–1Ti but not in V–4.5Ti. The results are consistent with a model of swelling inhibition induced by vacancy trapping by solute Ti during irradiation. The temperature dependencies of the parameter S in the range 8–300 K indicate a large dislocation bias for vacancies and solute Ti. This dislocation bias prevents the microvoid nucleation in V–4.5Ti, and the microvoid growth in V–1Ti, when vacancies become mobile during post-irradiation annealing treatments. A characteristic increase of the positron lifetime is found during recovery induced by isochronal annealing. It is attributed to a vacancy accumulation into the lattice of Ti oxides precipitated during cooling down, or at their matrix/precipitate interfaces. These precipitates could be produced by the decomposition of metastable phases of Ti oxides formed during post-irradiation annealing above 1000 K.

Atom probe characterization of the microstructure of nuclear pressure vessel surveillance materials after neutron irradiation and after annealing treatments

Microstructural changes due to neutron irradiation of weld and forging materials were characterized using the atom probe field ion microscope (APFIM). Neutron-induced clusters containing Cu, P, Ni, Mn and Si were detected in the high copper weld (0.24 at.% Cu) after irradiation to fluences of 6.6 × 10 22 and 3.47 × 10 23 n m −2; only phosphorus atmospheres were observed in the low copper forging material (0.02 at.% Cu) irradiated to an intermediate fluence of 1.5 × 10 23 n m −2. These results are in agreement with previous studies and with their respective measured transition temperature shifts. In addition, APFIM experiments were carried out on the high fluence weld material after two post-irradiation annealing treatments. The first annealing treatment of 168 h at 454°C is similar to the proposed condition for in situ pressure vessel annealing and the second, 29 h at 610°C, is similar to the final stress relief heat treatment employed in vessel fabrication. Annealing at 454°C led to coarsening of the copper-enriched precipitates and a 92% recovery of the radiation-induced transition temperature shift. Essentially complete rehomogenization of the solutes was obtained in the simulated stress relief treatment at 610°C.

Sensitization of 220°C TL peak in quartz separated from sand

In the present paper we have investigated sensitization of the 220°C TL peak in quartz, separated from sand, as a function of pre-gamma dose and post-irradiation annealing treatment at a test gamma dose of 50 Gy. A pre-gamma dose of 2.50 kGy followed by 400°C post-irradiation annealing treatment yielded the maximum sensitization factor of 35. The sensitization decreases for pre-doses higher than 2.50 kGy as well as for post-irradiation annealing temperatures higher than 400°C. The sensitization of the peak in quartz due to accrued geological dose of 72 Gy and thermal annealing treatment was estimated to be 2.36. The position and shape of the TL glow curve in sensitized and unsensitized samples was nearly the same.