Lawrence R. Greenwood

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.

Evaluation of the new cesium‐131 seed for use in low‐energy x‐ray brachytherapy

Characterization measurements and calculations were performed on a new medical seed developed by IsoRay Inc. in Richland, Washington, that utilizes the short-lived isotope 131Cs. This model has recently received FDA 510(k) clearance. The objective of this work was to characterize the dosimetric properties of the new seed according to the AAPM Task Group 43 recommendations. Cesium-131 is a low-energy x-ray emitter, with the most prominent peaks in the 29 keV to 34 keV region. The intended application is brachytherapy for treating cancers in prostate, breast, head and neck, lung, and pancreas. The evaluations performed included air-kerma strength, radial dose function, anisotropy in phantom, half-life, energy spectra, and internal activity. The results indicate the CS-1 seeds have a dose-rate constant of 0.915 cGy hr(-1) U(-1) in water, dose penetration characteristics similar to 125I and 103Pd, anisotropy function values on the order of 0.71 at short distances and small angles, and an average anisotropy factor of 0.964. The overall dosimetric characteristics are similar to 125I and 103Pd seeds with the exception of half-life, which is 9.7 days, as compared to 17 days for 103Pd and 60 days for 125I. The shorter half-life may offer significant advantages in biological effectiveness.

Neutron irradiation of MgB2 bulk superconductors

Sintered samples of MgB2 were irradiated in a fission reactor. Defects in the bulk microstructure are produced during this process mainly by the 10B(n,α)7Li reaction while collisions of fast neutrons with the lattice atoms induce much less damage. Self-shielding effects turn out to be very important and lead to a highly inhomogeneous defect distribution in the irradiated samples. The resulting disorder enhances the normal state resistivity and the upper critical field. The irreversibility line shifts to higher fields at low temperatures and the measured critical current densities increase following irradiation.

Subcascade formation in displacement cascade simulations: Implications for fusion reactor materials

Displacement cascade formation in iron has been investigated by the method of molecular dynamics (MD) for cascade energies up to 40 keV, corresponding to PKA energies up to 61 keV. The results of these simulations have been used in the SPECOMP code to obtain effective, energy-dependent cross sections for two measures of primary damage production: (1) the number of surviving point defects expressed as a fraction of the those predicted by the standard secondary displacement model by Norgett, Robinson, and Torrens (NRT), and (2) the fraction of the surviving interstitials contained in clusters that formed during the cascade event. The primary knockon atom spectra for iron obtained from the SPECTER code have been used to weight these MD-based damage production cross sections in order to obtain spectrally averaged values for several locations in commercial fission reactors, materials test reactors, and a DT fusion reactor (ITER) first wall. An evaluation of these results indicates that neutron energy spectrum differences between the various environments do not lead to significant differences between the average primary damage formation parameters. This conclusion implies that the displacement damage component of radiation damage produced in a DT fusion reactor should be well simulated by irradiation in a fission reactor neutron spectrum, and that differences in nuclear transmutation production may be the primary source of uncertainty in the prediction of material performance at high doses in DT fusion reactors.

Transmutation of MO, Re, W, Hf, and V in various irradiation test facilities and STARFIRE

Abstract With the exception of V and Cu alloys, the formation of solid transmutants has not been thought to play a large role in the response of metals to irradiation. For a number of refractory elements, however, it appears that very large levels of solid transmutation can occur both in fission and fusion spectra. The phase stability and other properties of alloys can be strong functions of the changing composition. The spectral sensitivity of transmutation also poses some difficulty in correlating data from various neutron spectra. Transmutation of Mo, Re, W, Hf and V have been calculated for the Fast Flux Test Facility (FFTF), the High Flux Isotope Reactor (HFIR), and the STARFIRE fusion reactor. Results show that Re and W undergo sizeable transmutation in these spectra, and may limit the use of these elements in fusion reactor materials. Vanadium and hafnium have high transmutation rates in HFIR and may require spectral tailoring.

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.

Defect structure and evolution in silicon carbide irradiated to 1 dpa-SiC at 1100 °C

Transmission electron microscopy (TEM), swelling measurements, isochronal annealing, and thermal diffusivity testing were used to characterize the effects of radiation damage in SiC. Together, these techniques provided a comprehensive set of tools for observing and characterizing the structure and evolution of radiation-induced defects in SiC as a function of irradiation temperature and dose. In this study, two types of dense, crystalline, monolithic SiC were subjected to irradiation doses up to 1 dpa-SiC at a temperature of 1100 °C, as well as post-irradiation annealing up to 1500 °C. The microscopic defect structures observed by TEM were correlated to changes in the macroscopic dimensions, thermal diffusivity and thermal conductivity. The results demonstrated the value of using ultrapure β-SiC as an effective reference material to characterize the nature of expected radiation damage in other, more complex, SiC-based materials such as SiC/SiC composites.

A shallow underground laboratory for low-background radiation measurements and materials development.

Pacific Northwest National Laboratory recently commissioned a new shallow underground laboratory, located at a depth of approximately 30 meters-water-equivalent. This new addition to the small class of radiation measurement laboratories located at modest underground depths houses the latest generation of custom-made, high-efficiency, low-background gamma-ray spectrometers and gas proportional counters. This paper describes the unique capabilities present in the shallow underground laboratory; these include large-scale ultra-pure materials production and a suite of radiation detection systems. Reported data characterize the degree of background reduction achieved through a combination of underground location, graded shielding, and rejection of cosmic-ray events. We conclude by presenting measurement targets and future opportunities.

Analysis of data from sensitive U.S. monitoring stations for the Fukushima Dai-ichi nuclear reactor accident

The March 11, 2011 9.0 magnitude undersea megathrust earthquake off the coast of Japan and subsequent tsunami waves triggered a major nuclear event at the Fukushima Dai-ichi nuclear power station. At the time of the event, units 1, 2, and 3 were operating and units 4, 5, and 6 were in a shutdown condition for maintenance. Loss of cooling capacity to the plants along with structural damage caused by the earthquake and tsunami resulted in a breach of the nuclear fuel integrity and release of radioactive fission products to the environment. Fission products started to arrive in the United States via atmospheric transport on March 15, 2011 and peaked by March 23, 2011. Atmospheric activity concentrations of (131)I reached levels of 3.0×10(-2) Bqm(-3) in Melbourne, FL. The noble gas (133)Xe reached atmospheric activity concentrations in Ashland, KS of 17 Bqm(-3). While these levels are not health concerns, they were well above the detection capability of the radionuclide monitoring systems within the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty.