Radiation-induced Void Swelling Research Articles

The influence of helium and heavy ion irradiations on radiation responses of single crystal Cu with nanovoids: An in situ TEM study

Helium is a common nuclear reaction product, and it plays an important role in radiation-induced void swelling. Although extensive studies have been conducted to investigate the helium effects in metals and alloys, most of them were based on the post-irradiation analyses or computer simulations. Conclusive evidence that can demonstrate how voids evolve in the presence of He under high-temperature radiation remains limited. In this work, we performed in situ heavy ion radiation studies in a transmission electron microscope to compare the radiation response of the preexisting nanovoids in single crystal copper subjected to a single beam of 1 MeV Kr ions and a sequential dual beam of 14 keV He ion and 1 MeV Kr ion at 350 °C. Our studies revealed that the nanovoids would contract continuously until being eliminated when irradiated by a single beam of Kr ions. In comparison, the nanovoids in helium-injected Cu could reach a stable state and eventually stopped shrinking. Moreover, the influence of helium on the kinetics of void/bubble evolution under heavy ion irradiation was discussed within the framework of a proposed critical bubble model and bubble coarsening model.

Effects of 3d electron configurations on helium bubble formation and void swelling in concentrated solid-solution alloys

Elemental specific chemical complexity is known to play a critical role in microstructure development in single-phase concentrated solid-solution alloys (SP-CSAs), including both He bubble formation and irradiation-induced void swelling. While cavity formation and evolution under ion irradiation at elevated temperature are complex nonequilibrium processes, chemical effects are revealed at the level of electrons and atoms herein in a simplified picture, using Ni and a special set of Ni-based SP-CSAs composed of 3d transition metals as model alloys. Based on Ni and the model alloys with minimized variables (e.g., atomic mass, size, and lattice structure), we discuss the effects of chemically-biased energy dissipation, defect energetics, sluggish diffusion, and atomic transport on cavity formation and evolution under both self-ion Ni irradiation and He implantation. The observed difference in microstructure evolution is attributed to the effects of d electron interactions in their integrated ability to dissipate radiation energy. The demonstrated impact of alloying 3d transition metals with larger differences in the outermost electron counts suggests a simple design strategy for tuning defect properties to improve radiation tolerance in structural alloys.

Atomistic simulation of defect-dislocation interactions in concentrated solid-solution alloys

The interaction between point defects and dislocations plays a crucial role in governing material properties and microstructural evolutions under external stimuli, such as mechanical deformation and irradiation. Here, we present an atomistic study of the interactions between point defects and dislocations in concentrated solid-solution alloys (CSAs). Using molecular statics and kinetic Monte Carlo methods, we demonstrate that the strain energy and stress field distribution induced by a dislocation in CSAs are highly inhomogeneous along the dislocation line, which leads to heterogeneity of defect-dislocation interactions. Specifically, the interactions are spatially different and screened by the random arrangement of different elemental species. Such localization of defect-dislocation interaction indicates that the ``dislocation-bias'' mechanism that is a driving force for radiation-induced void swelling can be suppressed in concentrated alloys.

Predicting the onset of void swelling in irradiated metals with machine learning

Radiation-induced void swelling is a serious mode of degradation in nuclear structural materials. Much effort has been spent to predict swelling resistance, with the goal of increasing the void swelling incubation dose so as to postpone the consequences of radiation damage. However, this trial-and-error approach is highly inefficient due to the time- and resource-intensive nature of both experiments and physics-based multiscale simulations. In this work, as a first attempt, machine learning is applied to perform this prediction based on available experimental data. Of the multiple techniques applied, the gradient boosting ensemble method best predicts experimental onset doses for swelling in test datasets, and identifies the main contributing factors such as temperature, Fe and Cr content, and dose rate, which are consistent with established understanding. This work demonstrates the feasibility of machine learning to predict macroscale radiation effects based on material and environmental parameters, and has practical significance in guiding further material optimization for nuclear applications.

HT9 swelling in high burnup fast reactor fuel pin components

The irradiation-induced void swelling of HT9 stainless steel has been observed many times, but there are little data available to quantify the amount of swelling expected as a function of neutron exposure and temperature. This is further complicated by potential effects of heat-to-heat chemical variations or heat treatment conditions during fabrication. This paper summarizes the results of most of the relevant previously completed studies, then focuses on the swelling behavior of HT9 mixed-oxide fuel cladding and wrapper wire irradiated to high neutron exposures. Note that the hexagonal duct used for the ACO-3 experiment (in the Fast Flux Test Facility, or FFTF) was also made from HT9 and has been studied previously by other researchers. The cladding enables a wider range of operating temperatures. Immersion density and transmission electron microscopy were used to identify a peak in the amount of swelling located axially, just below core centerline, for both the cladding and the wire. The peak roughly coincided with a peak seen in the cladding diameter profile. The wrapper wire showed enhanced swelling compared to the cladding, likely due to over-tempering during wire fabrication heat treatment. The grains showed decreased martensitic structure and were larger in size. These data do not provide for the means to fully characterize the swelling of HT9, but do provide further insight into the factors controlling it. The onset of breakaway swelling was not indicated in this data.

Radiation induced nanovoid shrinkage in Cu at room temperature: An in situ study

Radiation induced void swelling is widely observed in a variety of metallic materials. Here, by using sputtering deposition technique, we have introduced faceted nanovoids into Cu films. In-situ Kr ion irradiation was subsequently performed at room temperature to investigate the evolution of nanovoids. Most nanovoids found to shrink gradually with increasing dose. Irradiation induced high-density vacancy clusters exist mostly in the form of stacking fault tetrahedrons. Phase field modeling reveales that void shrinkage arises from biased absorption of interstitials. These findings provide insights to the physical mechanisms of radiation response of nanovoids in metallic materials.

Energetics of helium-vacancy complexes in Fe-9Cr alloys from first-principles calculations

The formation of helium-vacancy complexes under irradiation is crucial for the nucleation and early-stage growth of helium bubbles in reduced activation steels. The energetics of HenVm (V represents vacancy) complexes (n and m = 0–4) in Fe-9Cr alloy models and bcc Fe were investigated by first-principles calculations. A stronger self-trapping of He in Fe-9Cr alloys than in bcc Fe is found. The existence of Cr suppresses multiple He trapping in the vacancy to some extent. Besides, alloying element Cr strengthens lattice expansion induced by He atoms. Lower formation energy of HenVm complexes and stronger binding of a He atom to HenVm complexes in Fe-9Cr alloys as compared to in bcc Fe indicate that density of these small complexes can be higher in alloys. When m/n > 1, aggregation of a vacancy to HenVm complexes becomes more difficult in Fe-9Cr alloys than in bcc Fe. It is well known that the irradiation induced void swelling is related to the aggregation of small vacancy clusters or HenVm complexes. The results indicate that the addition of Cr atoms to the Fe matrix is beneficial to the irradiation swelling resistance aided by dispersed HeV complexes.

Nano-Oxide-Dispersed Ferritic Steel for Fusion Energy Systems

ABSTRACTThe role of oxide nanoparticles in cavity formation of a nano-oxide-dispersed ferritic steel subjected to (Fe + He) dual-ion and (Fe + He + H) triple-ion irradiations has been studied using transmission electron microscopy to elucidate the synergistic effects of helium and hydrogen on radiation tolerance of nano-oxide-dispersed ferritic steel for fusion energy systems. The effect of oxide nanoparticles on suppressing radiation-induced void swelling is clearly revealed from the observation of preferred trapping of helium bubbles at oxide nanoparticles, which results in a unimodal distribution of cavities in the (Fe + He) dual-ion irradiated specimen. An adverse effect of hydrogen implantation, however, is revealed from the observation of a bimodal distribution of cavities with large and facetted voids in association with the formation of HFe5O8-based hydroxide in local regions of the (Fe + He + H) triple-ion irradiated specimen.

Detecting self-ion irradiation-induced void swelling in pure copper using transient grating spectroscopy

Irradiation-induced void swelling remains a major challenge to nuclear reactor operation. Swelling may take years to initiate and often results in rapid material property degradation once started. Alloy development for advanced nuclear systems will require rapid characterization of the swelling breakaway dose in new alloys, yet this capability does not yet exist. We demonstrate that transient grating spectroscopy (TGS) can detect void swelling in single crystal copper via changes in surface acoustic wave (SAW) velocity. Scanning transmission electron microscopy (STEM) links the TGS-observed changes with void swelling-induced microstructural evolution. These results are considered in the context of previous work to suggest that in situ TGS will be able to rapidly determine when new bulk materials begin void swelling, shortening alloy development and testing times.

Irradiation damage of single crystal, coarse-grained, and nanograined copper under helium bombardment at 450 °C

Abstract

Irradiation Performance of Fast Reactor MOX Fuel Assemblies Irradiated to High Burnups

Mixed oxide fuel assemblies (MFA-1 and MFA-2 assemblies) were irradiated in the fast flux test facility to evaluate the irradiation performance of fast reactor core fuels at high burnups and high fast neutron fluences. The MFA-1 and MFA-2 assemblies achieved respective peak pellet burnups of 147 and 162GWd/t, and resisted to respective peak fast neutron fluences (E > 0:1 MeV) of 21:4 _ 1026 and 23:8 _ 1026 n/m2, without any indication of fuel pin breaching. Structural components of these assemblies were made of modified type 316 stainless steel and 15Cr-20Ni base advanced austenitic stainless steel. Postirradiation examinations of these assemblies revealed dimensional changes of fuel pins and assembly ducts due to irradiation-induced void swelling and irradiation creep, and fuel cladding local oval distortions due to bundle-duct interaction (BDI). The swelling resistance of 15Cr-20Ni base advanced austenitic stainless steel fuel pin cladding was almost the same as that of the modified type 316 stainless steel cladding, while the assembly duct of the former material had a slightly higher swelling resistance than that of the latter material. Analyses of fuel pin bundle deformations indicated that these assemblies likely mitigate BDI mainly by fuel pin bowings and cladding oval distortions.

An assessment by activation analysis of the influence of small nitrogen contents on electron-induced void swelling in Fe-Cr-Ni alloys

Abstract Radiation-induced void swelling was studied in high-purity and nitrogen-doped Fe-16Cr-20Ni alloys by irradiation in a high-voltage electron microscope at 500°C. The nitrogen concentrations were determined at various stages of preparation by neutron and photon activation analysis. Although some contamination occurred during sample preparation the nitrogen contents in the samples ready for irradiation are similar to those found in the massive starting alloys. They are about 90 and 400 wtppm in the doped alloys and about or less than 10 wtppm in the pure alloys. Nitrogen significantly reduces the swelling. For 90 wtppm, the effect is mainly a delay in the beginning of swelling, whereas there is an important decrease of the swelling rate for 400 wtppm with almost no delay. Such minor concentrations of metalloidic impurities should be considered in the interpretation of the swelling behaviours of materials usually considered as highly pure.

ChemInform Abstract: Heavy‐Ion Damage in Solids

Much of the impetus for studying irradiation damage in solids stems from the demands of nuclear technology. The radiation fields encountered in all critical areas of present and projected reactors present an intensely hostile environment. At typical reactor operating temperatures a whole range of largely irreversible and deleterious effects may occur in reactor components—creep, swelling, embrittlement, sputtering etc.—which lead to serious degredation in properties and impose severe limitations on reactor design. Perhaps the best-known example is radiation-induced void swelling, originally discovered in studies of metal fuel cladding by Cawthorne and Fulton (Ca 67), and subsequently found to be a major potential problem in structural components, which has had enormous implications in metallurgical design criteria for fast reactors. It has been estimated that the cost benefit of designing void-resistant low-swelling alloys for use in reactors into the next century is more than $10 billion!

In-situ observation of void swelling in molybdenum alloys by means of high voltage electron microscopy

Radiation induced void swelling is one of the key problems in alloy development for fusion reactors. The objectives of this study are to clarify the elemental process of void swelling, the effect of pre-injected helium, the effect of rhenium addition and the dependence on damage rate of void swelling in molybdenum alloys at elevated temperatures up to 1350K by means of in-situ observation under HVEM. Cascade damages due to He pre-injection make void nuclei together with He atoms and thus the incubation time for void growth becomes very short. Rhenium addition is effective in suppressing void swelling and also with the effectiveness in qualifying mechanical properties, Re is considered to be a very useful alloying element to molybdenum.

Irradiation-induced void swelling and solute segregation in a V-ion-irradiated V-15 wt % Cr bcc alloy

Specimens of a V-15 wt % Cr alloy were irradiated at 650°C with 3.5 MeV 51V + ions to peak dose levels of 5 to 60 dpa. At 650°C, the swelling increased sharply and monotonically with an increase in dose. At 55 dpa, the voids were ≈ 900 Å in diameter and the swelling (≈ 20%) was more than two orders of magnitude higher than for unalloyed V. Precipitates that are observed after irradiation of unalloyed V were not found in the V-15 wt % Cr alloy. A cellular dislocation structure was found to develop with peak doses to ≈ 30 dpa. Voids were generally observed to lie on the peripheries of the dense dislocation cell walls or in areas of relatively low dislocation density. Auger analysis showed that Cr segregates to the irradiated surface (and presumably to other defect sinks). Redistribution of Cr occurred to depths of ≈ 1000 Å, suggesting non-Gibbsian segregation. Cr segregation to defect sinks is consistent with the idea that undersize solute atoms preferentially migrate via interstitial fluxes.