E.H. Farnum

Radiation-induced electrical degradation experiments in the Japan materials testing reactor

Abstract An experiment to measure radiation-induced electrical degradation (RIED) in a sapphire sample and in three MgO-insulated cables was conducted at the JMTR light water reactor. The materials were irradiated at about 260°C to a fluence of 3 × 1024 n/m 2 ( E > 1 MeV) with an applied DC electric field between 100 kV/m and 500 kV/m. Even though the results for the sapphire sample are somewhat ambiguous because of an unexplained offset current of about 0.6 μA substantial degradation was not observed in the sapphire: instead, radiation-induced conductivity (RIC) seemed to decrease slightly during the experiment. Substantial increase in leakage current, that increased with applied electric field, occurred in the MgO-insulated cables. This increased conductivity disappeared when the reactor was shut down and sample temperature returned to ambient. However, the physical degradation apparently remained in the material while the reactor was off because restarting the irradiation brought the conductivity back to its previous, degraded, reactor-on value. This effect is different from the RIED effect reported by Hodgson but is similar to previous results reported by Shikama et al. Considerable data were taken to determine the sample temperature and leakage currents during the irradiation.

Electrical properties of ceramics during reactor irradiation

Twelve different types of polycrystal and single crystal Al 2 O 3 (alumina and sapphire) specimens of varying grades of purity were irradiated for three reactor cycles in a removable beryllium position in a High Flux Isotopes Reactor (HFIR) at Oak Ridge National Laboratory at a temperature of 720-760 K up to a maximum dose of 3 dpa while a dc electric field of 200 V/mm was applied. The recently completed Temperature Regulated In Situ Test (TRIST) facility in the HFIR was used to perform in situ measurements of electrical conductivity. In addition, three Al 2 O 3 specimens were simultaneously irradiated without a continuously applied dc electric field. In situ electrical conductivity measurements were performed on the specimens before, during and following each irradiation cycle. Behavior of electrical conduction in Al 2 O 3 was studied, with special emphasis on detection of any long-term increase of the electrical conductivity.

Electrical properties of Al2O3 during irradiation with spallation neutrons

DC and AC conductivity of Al{sub 2}O{sub 3} ceramics were measured at elevated temperatures during irradiation with neutrons and gamma rays at the Los Alamos Spallation Radiation Effects Facility. DC conductivity was increased at start of irradiation, but was 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 conductivity in the residual gas surrounding the samples. Surface conductivity, while not the source of the apparent RIED, nevertheless is of sufficient magnitude to be of concern for fusion applications.

Neutron damage to diagnostic mirrors

Abstract Diagnostic systems for fusion reactors will require mirrors capable of reflecting electromagnetic radiation in the soft X-ray, near UV, visible, and IR wavelengths. These components will be exposed to significant fluences of fast neutrons during use. Mirrors made from alternating layers of low-Z and high-Z materials were irradiated to a fast neutron fluence of 1.1 × 1023 n/m2 at 270–300°C, and subsequently evaluated for structural changes and in some cases changes in optical properties. Short-wavelength mirrors retained their structural integrity while exhibiting slight changes in reflectance; some long-wavelength mirrors showed structural degradation, while others did not. These results are discussed in terms of materials damage effects and possibilities for improvement of mirror performance under severe operating conditions.

Materials issues in diagnostic systems for BPX and ITER

Abstract Diagnostic systems in advanced D-T-burning fusion devices will be subjected to intense fluxes and fluences of high-energy neutrons and gamma rays. Materials used in these systems may suffer significant degradation of structural, optical, and electrical properties, either promptly upon irradiation or after accumulation of structural damage. Of particular concern are windows, optical fibers, reflectors, and insulators. Many materials currently specified for these components are known to degrade under anticipated operating conditions. However, materials selection and modification based on results from an appropriate irradiation testing program, when combined with design optimization for location, shielding, and ease of replacement, should point the way to development of acceptable systems.

Optical absorption of neutron-irradiated silica fibers

Abstract 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 × 10 21 n/m 2 ) or high (1.1 × 10 23 n/m 2 ) neutron fluence (E > 0.1 MeV) regions of the Los Alamos Spallation Radiation Effects Facility. Attenuation in high-fluence irradiated fibers exceeds 10 4 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 breakdown of insulating ceramics in a high-radiation field

Abstract This report addresses a recently reported phenomenon — that a simultaneous application of energetic particle radiation, electric field, and elevated temperature for an extended period of time has a permanent adverse effect on insulating ceramics, including electrical breakdown. This behavior poses a serious challenge for fusion devices, which require electrical insulators in several key components. The summary and recommendations developed here are based largely on the proceedings of a research assistance task force meeting entitled “Electrical Breakdown of Ceramics in a High-Radiation Field”. Since this is a rapidly expanding field, this report attempts to include highlights of pertinent studies reported at recent international meetings. In one recent meeting the fusion materials community recommended that this effect be referred to as radiation-induced electrical degradation (RIED).

Nondestructive fuel assay of laser targets. I. X-ray method

Abstract A convenient method of nondestructively assaying the tritium content of gas-filled glass or metal laser targets is described. Calibration factors for various target tyoes have been obtained, permitting the fast and accurate determination of ng quantities of tritium using a simple assembly of gas proportional X-ray counter tubes.

Radioluminescence in amorphous silica: temperature dependence and relaxation

Abstract Radioluminescence measurements on preform and fiber amorphous silica specimens have been made in the temperature interval 6 to 300 K and wavelength regime 300 to 800 nm. A typical spectrum consists of peaks at ∼550 (main band) and ∼650 (secondary band) nm, which are associated with self-trapped excitons and nonbridging oxygen hole centers of the silica lattice, respectively. Radioluminescence from the main band decays with increasing temperature and becomes undetectable in the 130 to 170 K range, consistent with the decay of self-trapped holes. Assuming the decrease in radioluminescence with temperature to be due to quenching of self-trapped exciton radiative recombination, with a spread in quenching barrier energy, we derive an expression of the form L(T)=L(0)/[1+AT+BT2+CT3], which yields an excellent fit to the experimental data. An observed time-dependent relaxation of radioluminescence is associated with the enhancement of nonradiative recombination.

Nondestructive fuel assay of laser targets: II. Photonuclear [D(γ,n)] method☆

Abstract A non-destructive photonuclear technique has been developed to assay the deuterium content of laser-fusion-project targets, such as gas-filled D 2 and DT micro-balloons, and solid CD 2 , LiD, or LiDT micro-spheres. Individual targets are irradiated by 2.754 MeV gamma rays from thermal-neutron-activated 24 Na. Gamma-flux uniformity and maximization at the target position are achieved by inserting the target into the center of a specially designed 0.6 cm o.d. Na 2 CO 3 spherical source, after the 24 Na has been activated in a reactor to a level of about 15 Ci/g. The spherical source in its aluminum container is inserted in the center of a tungsten or lead gamma shield, which is surrounded by a region of polyethylene. Neutrons of 0.26 MeV energy are produced in the deuterium by the D(γ, n) reaction. These neutrons are moderated in the polyethylene and counted by 3 He or 10 BF 3 detectors located in the polyethylene region. An assay sensitivity of ≤10 −7 g deuterium with essential linearity of response over at least a mass range of 10 −7 to 10 −5 g deuterium has been demonstrated.