Los Alamos Spallation Radiation Effects Research Articles

Shear punch and tensile measurements of mechanical property changes induced in various austenitic alloys by high-energy mixed proton and neutron irradiation at low temperatures

Tensile and shear punch tests of Inconel 718, 316L and 304L specimens irradiated with very high-energy protons and spallation neutrons showed that irradiation caused significant hardening and loss of uniform elongation. The specimens were irradiated at temperatures from room temperature to 150°C in the Los Alamos Spallation Radiation Effects Facility (LASREF) at the Los Alamos Neutron Scattering Center (LANSCE). Dose levels ranged from 0.01 to 12 dpa. Most of the observed hardening for all three alloys occurred within the first 0.1 dpa (i.e., low dose) and before significant gas accumulation, indicating that gas generation may not play a significant role in the hardening or ductility loss. The correlations between tensile and shear punch data were as expected for yield strength but were unusual for the maximum strength in comparison with previous correlations based on fission-neutron-irradiated alloys.

An evaluation of multilayer mirrors for the soft x ray and extreme ultraviolet wavelength range that were irradiated with neutrons

The Plasma Spectroscopy Group at the Johns Hopkins University develops high photon throughput multilayer mirror (MLM) based soft x ray and extreme ultraviolet (XUV 10 Å<λ<304 Å) spectroscopic diagnostics for magnetically confined fusion plasmas. The D-T reactions in large fusion reactor type devices such as the International Thermonuclear Experimental Reactor will produce neutrons at a rate as high as 5×1019 n s−1. The MLMs, which are used as dispersive and focusing optics, will not be shielded from these neutrons. In an effort to assess the potential radiation damage, four MLMs (No. 1: Mo/Si, d=87.8 Å, Zerodur substrate with 50 cm concave spherical curvature; No. 2: W/B4C, d=22.75 Å, Si wafer substrate; No. 3: W/C, d=25.3 Å, Si wafer substrate; and No. 4: Mo/Si, d=186.6 Å, Si wafer substrate) were irradiated with fast neutrons at the Los Alamos Spallation Radiation Effects Facility (LASREF). The neutron beam at LASREF has an energy distribution that peaks at 1–2 MeV with a tail that extends out to 100 MeV. The MLMs were irradiated to a fast neutron fluence of 1.1×1019 n cm−2 at 270–300 °C. A comparison between the dispersive and reflective characteristics of the irradiated MLMs and the corresponding qualities of control samples will be given.

Optical absorption of neutron-irradiated silica fibers

Silica-based optical fibers capable of transmitting light in the ultraviolet, visible and infrared regions of the electromagnetic spectrum are gaining widespread use in fusion reactor diagnostic systems. To assess radiation damage in these optical materials, the optical absorption has been measured at room temperature in the interval 250–2000 nm of neutron-irradiated silica fibers containing low or high hydroxyl concentrations. Fiber irradiations were done in either low (1.4 × 1021 n/m2) or high (1.1 × 1023 n/m2) neutron fluence (E > 0.1 MeV) regions of the Los Alamos Spallation Radiation Effects Facility. Attenuation in high-fluence irradiated fibers exceeds 104 dB/km for wavelengths less than about 700 nm, presumably due to radiation-induced absorption of peroxy radical, non-bridging oxygen hole and E′-type centers. Even the low fluence exposures induced considerable fiber absorption in this spectral region. High-OH content fibers exhibit significant radiation-induced absorption above 1500 nm, in addition to the intrinsic absorption due to OH stretching modes.

Electrical properties of Al 2O 3 during irradiation with spallation neutrons

DC and AC conductivity of Al 2O 3 ceramics were measured at elevated temperatures during irradiation with neutrons and gamma rays at the Los Alamos Spallation Radiation Effects Facility. DC conductivity increased at start of irradiation, but subsequently reduced as displacement damage was accumulated. AC conductivity appeared to increase at high levels of damage. The observed DC behavior is attributed to excitation of electrons into the conduction band by ionizing radiation, followed by charge trapping and recombination at damage sites. The AC behavior, which resembles RIED, is attributed to a combination of surface conductivity and conductivity in the residual gas surrounding the samples. These results have design implications for fusion-reactor systems.

Search for radiation-induced electrical degradation in alumina during spallation-neutron irradiation

dc and ac electrical conductivity of alumina and sapphire were measured during irradiation with spallation neutrons at the Los Alamos Spallation Radiation Effects Facility to a fluence of about 0.02 dpa. Both ac and dc voltages were applied during irradiation at 395, 615 and 655° C in an attempt to measure radiation-induced electrical degradation (RIED). RIED-like effects were observed in the evacuated AC capsules, but no RIED was observed with the dc capsules in about 1 atm of argon gas. The dc measurements showed an initial high increase in conductivity followed by a continual decrease with increasing fluence. We believe that postirradiation conductivity measurements must be made to confirm the observed current increases in the ac capsules.

Fast-neutron radiation effects in a silica-core optical fiber studied by a CCD-camera spectrometer

A simple CCD-camera spectrometer was deployed at the Los Alamos Spallation Radiation Effects Facility to characterize fast-neutron irradiation effects in several silica-based optical fibers over the wavelength range ~ 450-1100 nm. The experimental arrangement allowed optical loss spectra to be developed from remotely recovered frame grabs at various times during irradiation without it being necessary to resort to cutback methods. Data recorded for a pure-silica-core/F-doped-silica-clad fiber displayed a peculiar artifact, which is described and mathematically modeled in terms of leaky modes propagating in an optical cladding that is substantially less susceptible to radiation-induced optical attenuation than is the core. Evidence from optical time-domain reflectometry supports the postulate that mode leakage into the cladding may be a result of light scattering from the tracks of ions displaced by the 14-MeV neutrons. These results suggest that fibers with fluorine doping in the core, as well as in the cladding, would be relatively resistant to radiation-induced attenuation in the UV-visible spectral region.

Neutron flux enhancement at LASREF

The accelerator at the Los Alamos Meson Physics Facility produces a 1 mA beam of protons at an energy of 800 MeV. Since 1985, the Los Alamos Spallation Radiation Effects Facility (LASREF) has made use of the neutron flux that is generated as the incident protons interact with the nuclei in targets and a copper beam stop. A variety of basic and applied experiments in radiation damage and radiation effects have been completed. Recent studies indicate that the flux at LASREF can be increased by at least a factor of 10 from the present level of about 5 E + 17 m −2 s −1. This requires changing the beam stop material from Cu to W and optimizing the geometry of the beam-target interaction region. These studies are motivated by the need for a large volume, high energy, and high intensity neutron source in the development of materials for advanced energy concepts such as fusion reactors.

Tensile property changes of metals irradiated to low doses with fission, fusion and spallation neutrons

Radiation effects due to low doses of spallation neutrons are compared directly to those produced by fission and fusion neutrons. Yield stress changes of pure Cu, alumina dispersion strengthened Cu, and AISI 316 stainless steel irradiated at 36–55°C in the Los Alamos Spallation Radiation Effects Facility (LASREF) are compared with earlier results of irradiations at 90°C using 14 MeV D-T fusion neutrons at the Rotating Target Neutron Source and fission reactor neutrons in the Omega West Reactor. At doses up to 0.04 displacements per atom (dpa), the yield stress changes due to the three quite different neutron spectra correlate well on the basis of dpa in the stainless steel and the Cu alloy. However, in pure Cu, the measured yield stress changes due to spallation neutrons were anomalously small and should be verified by additional irradiations. With the exception of pure Cu, the low dose, low temperature experiments reveal no fundamental differences in radiation hardening by fission, fusion or spallation neutrons when compared on the basis of dpa.