Francis A. Garner

Influence of irradiation temperature and dose gradients on the microstructural evolution in neutron-irradiated 316SS

A cold worked 316SS baffle bolt was extracted from the Tihange pressurized water reactor and sectioned at three different positions. The temperature and dose at the 1-mm bolt head position were 593 K and 19.5 dpa respectively, whereas at two shank positions the temperature and dose was 616 K and 12.2 dpa at the 25-mm position and 606 K and 7.5 dpa at the 55-mm position. Microstructural characterization revealed that small faulted dislocation loops and cavities were visible at each position, but the cavities were most prominent at the two shank positions. Measurable swelling exists in the shank portions of this particular bolt, and accompanying this swelling is the retention of very high levels of hydrogen absorbed from the environment. The observation of cavities in the CW 316SS at temperatures and doses relevant to LWR conditions has important implications for pressurized water reactors since SA 304SS plates surround the bolts, a steel that usually swells earlier due to its lower incubation period for swelling.

The primary origin of dose rate effects on microstructural evolution of austenitic alloys during neutron irradiation

The effect of dose rate on neutron-induced microstructural evolution was experimentally estimated. Solution-annealed austenitic model alloys were irradiated at ≃400 °C with fast neutrons at seven different dose rates that vary more than two orders difference in magnitude, and two different doses were achieved at each dose rate. Both cavity nucleation and growth were found to be enhanced at lower dose rate. The net vacancy flux is calculated from the growth rate of cavities that had already nucleated during the first cycle of irradiation and grown during the second cycle. The net vacancy flux was found to be proportional to (dpa/s)1/2 up to 28.8 dpa and 8.4×10−7 dpa/s. This implies that mutual recombination dominates point defect annihilation in this experiment, even though point defect sinks such as cavities and dislocations were well developed. Thus, mutual recombination is thought to be the primary origin of the effect of dose rate on microstructural evolution.

Determination of helium and hydrogen yield from measurements on pure metals and alloys irradiated by mixed high energy proton and spallation neutron spectra in LANSCE

The confident design of accelerator-driven spallation neutron devices will require good estimates of the cross-sections for generation of helium and hydrogen in the mixed spectra of high energy protons and neutrons that will be experienced by the structural materials. Improved estimates of these cross-sections were derived from a series of irradiations that were conducted at relatively low temperatures (<100°C) in the Los Alamos Neutron Science Center (LANSCE) as part of the test program supporting the Accelerator Production of Tritium (APT) Program. In this irradiation campaign, a variety of candidate structural alloys and pure metal dosimeter foils were irradiated in various particle spectra, ranging from 800 MeV protons, to mixed energy distributions of both protons and spallation neutrons, and finally to distributions consisting primarily of high energy neutrons. At proton energies on the order of hundreds of MeV, exceptionally high levels of gas atoms are generated in all elemental constituents of typical iron-based and nickel-based structural alloys, with helium typically on the order of ∼150 appm per dpa and hydrogen at approximately a factor of 3–5 higher. Most of the gas production is due to proton and helium recoils from the proton beam interactions with the specimens, although gas and especially damage production from lower-energy spallation neutrons becomes increasingly significant at locations farther from the beam center. The results show that helium production rate per dpa by protons in elements typically found in structural alloys is relatively insensitive to elemental composition. The measured helium concentrations and the derived cross-sections are larger by about a factor of two, however, than those calculated using the LAHET code which was optimized for prediction of neutron/proton ratios in the target tungsten source rods of the APT test. Unlike helium, the retained hydrogen levels are somewhat sensitive to composition, reflecting primarily different levels of diffusional loss, but hydrogen is still retained at rather high concentrations, allowing a lower bound estimate of the hydrogen generation rates.

COMPOSITIONAL AND TEMPERATURE DEPENDENCE OF VOID SWELLING IN MODEL Fe-Cr BASE ALLOYS IRRADIATED IN THE EBR-II FAST REACTOR

Abstract A series of annealed and aged Fe– x Cr, Fe–12Cr– y C and Fe–12Cr–0.1C– z Mo model alloys were irradiated in EBR-II at eight temperatures between 400°C and 650°C and dose levels ranging from 35 to 131 dpa. Swelling-induced density changes observed in the binary alloys generally peaked at mid-chromium levels, with the chromium and temperature dependence expressed primarily in the duration of the transient regime. The steady-state swelling rate at the lower irradiation temperatures was much higher than previous estimates, reaching ∼0.2%/dpa and possibly still climbing at higher neutron exposures. The dependence of swelling on molybdenum and carbon was more complex, depending on whether the temperature was relatively low or high. At temperatures of 482°C and above, the effect of carbon additions was very pronounced with swelling of Fe–12Cr jumping dramatically from near zero at 0.002% C to 6–10% at 0.1% C. This indicates that the major determinant of the composition and temperature dependence probably lies in the duration of the nucleation-dominated transient regime of swelling and not primarily in the steady-state swelling rate as previously envisioned. This raises the possibility that significant swelling may occur earlier in fusion and spallation neutron spectra where high gas generation rates may assist void nucleation.

Implications of neutron spectrum and flux differences on fission-fusion correlations at high neutron fluence

Abstract The application to fusion environments of materials data derived from fission reactors involves considerations related not only to neutron spectra but also the often dominant effect of displacement rate. It is shown in this paper that fission-fusion correlation experiments directed toward helium effects and PKA recoil spectra are frequently difficult to interpret due to the strong influences of displacement rate, low energy recoils from thermal neutron absorption and in some cases a large influence of solid transmutants. It is also shown that materials data published in earlier decades must be reevaluated in light of recent advances in defining irradiation parameters.

Neutron irradiation effects in fusion or spallation structural materials: Some recent insights related to neutron spectra

Abstract A review is presented of recent insights on the role of transmutation in the development of radiation-induced changes in dimension or radiation-induced changes in physical or mechanical properties. It is shown that, in some materials and some neutron spectra, transmutation can significantly affect or even dominate a given property change process. When the process under study is also sensitive to displacement rate, and especially if it involves radiation-induced segregation and precipitation, it becomes much more difficult to separate the transmutation and displacement rate dependencies. This complicates the application of data derived from “surrogate” spectra to predictions in other flux-spectra environments. It is also shown in this paper that one must be sensitive to the impact of previously-ignored “small” variations in neutron spectra within a given reactor. In some materials these small variations have major consequences.

Microstructural evolution of Alloy 718 at high helium and hydrogen generation rates during irradiation with 600–800 MeV protons

When precipitation hardened Alloy 718 is irradiated with high-energy protons (600–800 MeV) and spallation neutrons at temperatures below ∼60∘C, it quickly hardens and loses almost all uniform elongation. It later softens somewhat at higher exposures but does not regain any elongation. This behavior is explained in terms of the evolution of Frank loop formation, disordering and eventual dissolution of the γ′ and γ″ strengthening phases, and the steady accumulation of very large levels of helium and hydrogen. These gases must be dispersed on a very fine scale in the matrix since no cavities could be found.

Helium and hydrogen generation in pure metals irradiated with high-energy protons and spallation neutrons in LANSCE

Abstract High-power spallation neutron sources will require accurate estimates of cross-sections for generation of He and H in structural materials. At high-proton energies, very high levels of gas atoms are generated in all constituents of typical iron-based and nickel-based structural alloys, with He typically ∼150 appm/dpa and H at levels ∼3–5 times higher. Improved estimates of these cross-sections have been derived from a series of irradiations conducted at relatively low temperatures (

High-sensitivity quadrupole mass spectometry system for the determination of hydrogen in irradiated materials

High levels of helium and hydrogen are generated in metals in fusion reactors, and fusion simulations in mixed-spectrum fission reactors, spallation neutron sources, and high-energy charged particle environments. Although hydrogen generation rates are generally higher than those of helium, hydrogen is thought to quickly diffuse out of metals, especially at elevated temperatures. There appear to be some conditions, however, where significant hydrogen retention can occur. To assess this potential, a high-sensitivity analysis system has been developed for the measurement of hydrogen in small samples of irradiated materials. The system is based on a low-volume extraction furnace in combination with a quadrupole mass spectrometer, and has a detection limit of ∼1 appm for steel. Hydrogen levels have been measured in high-energy proton-irradiated tungsten and Inconel 718, in iron-based alloys and vanadium alloys from fusion materials programs, and in stainless steels and pure metals irradiated in light water reactors. Details of the system and typical results are presented.

Void swelling in copper and copper alloys irradiated with fission neutrons

Abstract Specimens of pure copper and copper alloys (Cuue5f85Ni, Cuue5f8Crue5f8Zr) were irradiated with fission neutrons. Radiation damage microstructures were determined by transmission electron microscopy (TEM). The change in volume due to irradiation was obtained by density change measurements. The void nucleation and growth are found to be significantly affected by the presence of 5% Ni; the effects observed in the present experiments are directly opposite to the findings reported earlier on the Cu-5% Ni alloy irradiated with 1 MeV electrons. The swelling is found to be significantly reduced in Cuue5f8Crue5f8Zr alloy. The implications of these results are discussed in terms of spectrum and rate effects.