T. R. Allen

Emulation of neutron irradiation effects with protons: validation of principle

Abstract This paper presents the results of the irradiation, characterization and irradiation assisted stress corrosion cracking (IASCC) behavior of proton- and neutron-irradiated samples of 304SS and 316SS from the same heats. The objective of the study was to determine whether proton irradiation does indeed emulate the full range of effects of in-reactor neutron irradiation: radiation-induced segregation (RIS), irradiated microstructure, radiation hardening and IASCC susceptibility. The work focused on commercial heats of 304 stainless steel (heat B) and 316 stainless steel (heat P). Irradiation with protons was conducted at 360 °C to doses between 0.3 and 5.0 dpa to approximate those by neutron irradiation at 275 °C over the same dose range. Characterization consisted of grain boundary microchemistry, dislocation loop microstructure, hardness as well as stress corrosion cracking (SCC) susceptibility of both un-irradiated and irradiated samples in oxygenated and de-oxygenated water environments at 288 °C. Overall, microchemistry, microstructure, hardening and SCC behavior of proton- and neutron-irradiated samples were in excellent agreement. RIS analysis showed that in both heats and for both irradiating particles, the pre-existing grain boundary Cr enrichment transformed into a `W shaped profile at 1.0 dpa and then into a `V shaped profile between 3.0 and 5.0 dpa. Grain boundary segregation of Cr, Ni, Si, and Mo all followed the same trends and agreed well in magnitude. The microstructure of both proton- and neutron-irradiated samples was dominated by small, faulted dislocation loops. Loop size distributions were nearly identical in both heats over a range of doses. Saturated loop size following neutron irradiation was about 30% larger than that following proton irradiation. Loop density increased with dose through 5.0 dpa for both particle irradiations and was a factor of 3 greater in neutron-irradiated samples vs. proton-irradiated samples. Grain boundary denuded zones were only observed in neutron-irradiated samples. No cavities were observed for either irradiating particle. For both irradiating particles, hardening increased with dose for both heats, showing a more rapid increase and approach to saturation for heat B. In normal oxygenated water chemistry (NWC) at 288 °C, stress corrosion cracking in the 304 alloy was first observed at about 1.0 dpa and increased with dose. The 316 alloy was remarkably resistant to IASCC for both particle types. In hydrogen treated, de-oxygenated water (HWC), proton-irradiated samples of the 304 alloy exhibited IG cracking at 1.0 dpa compared to about 3.0 dpa for neutron-irradiated samples, although differences in specimen geometry, test condition and test duration can account for this difference. Cracking in heat P in HWC occurred at about 5.0 dpa for both irradiating particles. Thus, in all aspects of radiation effects, including grain boundary microchemistry, dislocation loop microstructure, radiation hardening and SCC behavior, proton-irradiation results were in good agreement with neutron-irradiation results, providing validation of the premise that the totality of neutron-irradiation effects can be emulated by proton irradiation of appropriate energy.

The effects of low dose rate irradiation and thermal aging on reactor structural alloys

Abstract As part of the EBR-II reactor materials surveillance program, test samples of fifteen different alloys were placed into EBR-II in 1965. The surveillance (SURV) program was intended to determine property changes in reactor structural materials caused by irradiation and thermal aging. In this work, the effect of low dose rate (approximately 2xa0×xa010 −8 dpa/s) irradiation at 380–410°C and long term thermal aging at 371°C on the properties of 20% cold worked 304 stainless steel, 420 stainless steel, Inconel X750, 304/308 stainless weld material, and 17-4 PH steel are evaluated. Doses of up to 6.8 dpa and thermal aging to 2994 days did not significantly affect the density of these alloys. The strength of 304 SS, X750, 17-4 PH, and 304/308 weld material increased with irradiation. In contrast, the strength of 420 stainless steel decreased with irradiation. Irradiation decreased the impact energy in both Inconel X750 and 17-4 PH steel. Thermal aging decreased the impact energy in 17-4 PH steel and increased the impact energy in Inconel X750. Tensile property comparisons of 304 SURV samples with 304 samples irradiated in EBR-II at a higher dose rate show that the higher dose rate samples had greater increases in strength and greater losses in ductility.

Microstructural changes induced by post-irradiation annealing of neutron-irradiated austenitic stainless steels

Abstract Irradiated EBR-II hexagonal ‘hex’ duct material fabricated from 304 stainless steel (SS) is characterized prior to and following in situ annealing in the transmission electron microscope (TEM) at temperatures between 400°C and 600°C for various lengths of time. The hex duct samples were irradiated at temperatures between 375°C and 389°C to doses up to 29 dpa over a range of dose rates. The pre-annealed microstructure of the irradiated hex ducts exhibited substantial radiation-induced dislocation development (networks and loops) and cavity formation (bubbles and voids). Following annealing at all temperatures, dislocation loop densities decrease significantly and the dislocation network density increases. Annealing at 500°C and 600°C led to the shrinking or disappearance of many larger faceted voids. In addition to shrinkage of larger voids, small spherical bubbles formed leading to an increase in the overall cavity density and a decrease in the average cavity size.

The Correlation Between Swelling and Radiation-Induced Segregation in Iron-Chromium-Nickel Alloys

The magnitudes of both void swelling and radiation-induced segregation (RIS) in iron-chromium-nickel alloys are dependent on bulk alloy composition. Because the diffusivity of nickel via the vacancy flux is slow relative to chromium, nickel enriches and chromium depletes at void surfaces during irradiation. This local composition change reduces the subsequent vacancy flux to the void, thereby reducing void swelling. In this work, the resistance to swelling from major element segregation is estimated using diffusivities derived from grain boundary segregation measurements in irradiated iron-chromium-nickel alloys. The resistance to void swelling in iron- and nickel-base alloys correlates with the segregation and both are functions of bulk alloy composition. Alloys that display the greatest amount of nickel enrichment and chromium depletion are found to be most resistant to void swelling, as predicted. Additionally, swelling is shown to be greater in alloys in which the RIS profiles are slow to develop.

Properties of 20% cold-worked 316 stainless steel irradiated at low dose rate.

To assess the effects of long-term, low-dose-rate neutron exposure, tensile, hardness, and fracture properties were measured and microstructural characterization performed on irradiated 20% cold-worked Type 316 stainless steel. Samples were prepared from reactor core components retrieved from the EBR-II reactor following final shutdown. Sample locations were chosen to cover a dose range of 1-56 dpa at temperatures from 371-390 C and dose rates from 0.8-3.3 x 10{sup -7} dpa/s. Irradiation caused hardening, with the ultimate tensile strength (UTS) reaching about 800 MPa near 20 dpa and appearing to saturate at higher doses. The yield strength (YS) follows approximately the same trend as the ultimate tensile strength. At higher dose, the difference between the UTS and YS decreases, suggesting the work-hardening capability of the material is decreasing with increasing dose. The hardness and yield strength increases occur roughly over the same range of dose. While the material retained respectable ductility at 20 dpa, the uniform and total elongation decreased to <1 and <3%, respectively, at 47 dpa. Fracture in the 30 dpa specimen is mainly ductile but with local regions of mixed-mode failure, consisting mainly of dimples and microvoids. The fracture surface of the higher-exposure 47 dpa specimen displays more brittlemorexa0» features. Changes in yield strength predicted from the microstructural components are roughly consistent with the measured changes in yield strength.«xa0less

Swelling and Microstructural Evolution in 316 Stainless Steel Hexagonal Ducts Following Long-Term Irradiation in EBR-II

Swelling behavior and microstructural evolution of 12% cold-worked 316 SS hexagonal ducts following irradiation in the outer rows of EBR-II is described. Immersion density measurements and transmission electron microscopy (TEM) examination were performed on a total of seven irradiation conditions. The samples were irradiated to temperatures between 375 and 430 C to doses between 23 and 51 dpa and at dose-rates ranging from 1.3 x 10{sup -7} to 5.8 x 10{sup -7} dpa/s. Dose-rates and temperatures approach conditions experienced by a variety of components in pressurized water reactors (PWRs) and those which may be present in future advanced reactors designs. TEM analysis was employed to elucidate the effect of radiation on the dislocation, void and precipitate structures as a function of irradiation conditions. A moderate dose-rate effect was observed for samples which were irradiated at dose-rates differing by a factor of two. Lower dose-rate samples contained voids of larger diameter and typically swelled more in the bulk. The dislocation and precipitate structure was not visibly influenced by a dose-rate decrease.

The effects of long-time irradiation and thermal aging on 304 stainless steel

Abstract The effect of long-time irradiation and thermal aging on the tensile, fracture, and swelling properties of 304 stainless steel were studied. Samples of cold-worked and annealed 304 were irradiated in EBR-II at temperatures between 371–400°C or thermally aged in the EBR-II primary core basket at 371°C for times up to 18 years. Samples of annealed steel were irradiated in rows 4 and 12 of EBR-II to determine the effect of dose rate. No significant changes in tensile properties or density occurred in cold-worked thermally aged steel. For samples irradiated near 370°C, decreasing the dose rate caused an increase in swelling and had no measurable effect on tensile properties.

Radiation-induced segregation and void swelling in 304 stainless steel.

. Void swelling and radiation-induced segregation have been measured in 304 stainless steel. Samples were irradiated in the outer regions of the EBR-11reactor where displacement rates of 2.OX10-8and 6.6 xl 0-8dpds are comparable to those in pressurized water reactor components. Samples were inadiated at temperatures from 371-390°C to total doses of up to 20 dpa. Void swelling reached a maximum of 2 0/0 at 20 dpa. Nickel enrichment and chromium dep~etion of up to of 20 at% and 12 atO/Orespectively were measured. Both void swelling and radiation-induced segregation were dependent on dose rate, increasing as the dose rate decreased. Grain boundary compositions were measured both near and in areas free of precipitates. The presence of a precipitate significantly changes the grain boundary compositions near the precipitate.

Microstructure and Property Evolution in Advanced Cladding and Duct Materials Under Long-Term Irradiation at Elevated Temperature: Critical Experiments

The in-service degradation of reactor core materials is related to underlying changes in the irradiated microstructure. During reactor operation, structural components and cladding experience displacement of atoms by collisions with neutrons at temperatures at which the radiation-induced defects are mobile, leading to microstructure evolution under irradiation that can degrade material properties. At the doses and temperatures relevant to fast reactor operation, the microstructure evolves by microchemistry changes due to radiation-induced segregation, dislocation loop formation and growth, radiation induced precipitation, destabilization of the existing precipitate structure, as well as the possibility for void formation and growth. These processes do not occur independently; rather, their evolution is highly interlinked. Radiation-induced segregation of Cr and existing chromium carbide coverage in irradiated alloy T91 track each other closely. The radiation-induced precipitation of Ni-Si precipitates and RIS of Ni and Si in alloys T91 and HCM12A are likely related. Neither the evolution of these processes nor their coupling is understood under the conditions required for materials performance in fast reactors (temperature range 300-600°C and doses to 200 dpa and beyond). Further, predictive modeling is not yet possible, as models for microstructure evolution must be developed along with experiments to characterize these key processes and provide tools for extrapolation. To extend the range of operation of nuclear fuel cladding and structural materials in advanced nuclear energy and transmutation systems to that required for the fast reactor, the irradiation-induced evolution of the microstructure, microchemistry, and the associated mechanical properties at relevant temperatures and doses must be understood. This project builds upon joint work at the proposing institutions, under a NERI-C program that is scheduled to end in September, to understand the effects of radiation on these important materials. The objective of this project is to conduct critical experiments to understand the evolution of microstructural and microchemical features (loops, voids, precipitates, and segregation) and mechanical properties (hardening and creep) under high temperature and full dose range radiation, including the effect of differences in the initial material composition and microstructure on the microstructural response, including key questions related to saturation of the microstructure at high doses and temperatures.

Effect of Zr on the Irradiated Microstructure and Hardening in 304 Stainless Steel

ABSTRACT Model alloys of 304 Stainless Steels (SS) (Fe-18Cr-9.5Ni-1.75Mn) and 304 SS+Zr (Fe-18Cr-9.5Ni-1.75Mn+0.04Zr and Fe-18Cr-9.5Ni-1.75Mn+ 0.16Zr) were irradiated with 3.2 MeV protons to a dose of 1.0 dpa at 400°C. Following irradiation, the microstructure was characterized. The number density, defect size, and size distributions for faulted loops and voids were determined. Swelling for each irradiation condition was calculated based on the void measurements. The effect of Zr addition on the irradiated microstructure and hardening is clearly demonstrated. The number density of defects decreased with the Zr addition while the size change of faulted loops and voids is less pronounced. Radiation hardening was reduced by Zr addition. Void swelling is decreased with Zr addition. The reduction in void density and swelling may be caused by the enhanced recombination of defects at oversized Zr solute atoms, suppressing the vacancy super saturation and therefore directly suppressing void nucleation. The reduction in loop density is believed due to the enhanced point defects recombination caused by oversized solute Zr.