G.F. Hurley

Neutron irradiation damage in MgO, Al2O3 and MgAl2O4 ceramics☆

The consequences of point defect aggregation in single-crystal and polycrystalline Al2O3 and MgAl2O4 and polycrystalline MgO, irradiated to fast-neutron (>0.1 MeV) doses ⩽2–3 × 1026nm−2, have now been investigated over a range of temperatures using macroscopic density measurements of swelling and transmission electron microscopy of defect aggregates. Al2O3 irradiated between 925 and 1100 K exhibits 2–4 vol% swelling, dense dislocation networks and aligned voids, while single-crystal MgAl2O4 spinel irradiated under similar conditions, does not swell and forms only isolated interstitial dislocation loops which do not unfault. Planar arrays of voids form either side of grain boundaries in polycrystalline MgAl2O4 spinel, however. MgO irradiated at 430 K swells 2.8% and contains a high density of small elongated loops and exhibits a tendency towards dislocation network formation despite the low irradiation temperature. Al2O3 and MgAl2O4 irradiated at 430 K exhibits dense arrays of small loops. The nature and origin of the observed aggregate defect structures are described and correlated with swelling behaviour and with previous studies which are reviewed.

Structural performance of ceramics in a high-fluence fusion environment

The ceramics MgAl2O4, A12O3 (single crystal), Si3N4, and a SiC/graphite laminate were irradiated to ∼2 × 1026 n/m2 (E > 0.1 MeV) at 680 and 815K. Spinel exhibited near-zero dimensional change, while Al2O3 and Si3N4 swelled ∼3 vol% and 1 vol% respectively. Strength of MgAl2O4 was increased, while strength of Al2O3 and Si3N4 were not greatly altered. The SiC/graphite composite, tested only at 680K, suffered almost complete delamination as a result of swelling of the SiC and densification of the graphite. These results are discussed in terms of microstructural alterations and related to various fusion applications.

Structural properties of MgO AND MgAl2O4, after fission neutron irradiation near room temperature

Polycrystalline MgO and MgAl/sub 2/O/sub 4/ samples were irradiated at 430 +- 5 K in HFIR to a fast neutron fluence of 2.1 x 10/sup 26/ n/m/sup 2/, E > 0.2 MeV, and 4.6 x 10/sup 26/ thermal n/m/sup 2/. Following irradiation, swelling, microstructure and mechanical strength were evaluated relative to control samples. Both materials swelled substantially, 2.6-3.0% in the case of MgO, and 0.8% in the case of MgAl/sub 2/O/sub 4/. The substructure of the MgO was found to contain a dense array of dislocation loops while the spinel showed heavy but unresolved damage. Results of mechanical strength evaluation by diametral compression testing showed significant strengthening for both materials. This result, which has important implications for use of cubic technological ceramics, is discussed in terms of the observed fracture modes and microstructural damage.

Ceramic and organic insulators for fusion applications

Abstract Fusion insulator requirements, operating conditions, and anticipated performance of candidate materials are reviewed, with emphasis on recent developments. Critical problem areas are highlighted, and research and development activities needed to meet the demands of advanced fusion devices are identified.

The effect of elevated-temperature neutron irradiation on fracture toughness of ceramics☆

Single-crystal forms of MgAl/sub 2/O/sub 4/, Y/sub 3//Al/sub 5/O/sub 12/, and Al/sub 2/O/sub 3/ were irradiated in the EBR-II fast fission reactor to a fluence of 0.3 x 10/sup 26/ n/m/sup 2/ at 1015/sup 0/K, and to /sup -/1 to 2 x 10/sup 26/ n/m/sup 2/ at 925 and 1100/sup 0/K. Fracture toughness was subsequently measured at room temperature by an indentation technique, and radiation-induced defect aggregates were characterized by transmission electron microscopy. A slight increase in toughness of MgAl/sub 2/O/sub 4/ was observed, and attributed to interaction of cracks with strain fields around dislocation loops. No significant change was noted for Y/sub 3/Al/sub 5/O/sub 12/, despite the presence of a high concentration of unresolved defect clusters. Fracture toughness of Al/sub 2/O/sub 3/ was markedly increased, with the enhancement apparently attributable in large part to impedance of crack propagation by interaction with the irradiation-induced void lattice.

High dose neutron irradiation damage in alpha alumina

Bulk samples of single crystalline and polycrystalline alpha alumina have been neutron-irradiated in the Experimental Breeder Reactor-II (EBR-II) to doses of 10{sup 26} n/m{sup 2} at temperatures of 925 K and 1100 K. The samples were found to swell macroscopically between 3% and 6%, depending on the temperature of irradiation and the form of the material. The damaged microstructures were investigated via transmission electron microscopy in order to understand the origin of the macroscopic swelling. In both single crystals and polycrystals the damage consists of a high density of dislocations containing predominately {bold b}=1/3{l angle}10{bar 1}1{r angle} dislocation loops on the (0001) planes coexistent with a high density of voids, which are aligned along the c-axis in this rhombohedral material. The established theory of void formation in metals is utilized to explain the formation of voids in alumina. The polycrystalline samples were extensively microcracked, and this is thought to be due to anisotropic swelling of the grains which in turn leads to stresses and fracturing at the grain boundaries.

14 MeV neutron irradiation effects in macor glass-ceramic

Abstract Samples of MACOR machinable glass-ceramic were irradiated at room temperature to fluences of 10 20 and 10 22 n/m 2 . No significant volume changes or deterioration in thermal conductivity were detected, while strength appeared to increase slightly. A slight increase in electrical conductivity, which disappeared with annealing, was noted for the higher fluence. Examination of ion-thinned samples by TEM showed only minor damage from the high-fluence neutron irradiation, where ionizing doses are estimated to be 10 7 Gy. Exposure to higher doses from 200-KeV electrons in the TEM caused the formation of pore-like aggregates.

Organic materials for fusion reactor applications

Abstract Organic materials requirements for fusion reactor magnets are described with reference to the temperature, radiation, and electrical and mechanical stress environment expected in these magnets. A review is presented of the response to gamma-ray and neutron irradiation at low temperatures of candidate organic materials; i.e. laminates, thin films, and potting compounds. Lifetime-limiting features-of this response as well as needed testing under magnet operating conditions not yet adequately investigated are identified and recommendations for future work are made.

Properties of polymers after cryogenic neutron irradiation

Abstract Organic matrix insulation has been specified for use in fusion reactor superconducting coils due to lower material and fabrication costs compared to other forms of insulation. Glass fabric filled resins are prime candidates since they provide not only electrical insulation, but also the mechanical strength necessary to withstand the high magnetic forces present during coil operation. Two epoxy- and two polyimide-based fiber reinforced materials were irradiated to neutron fluences of 4.1 × 10 21 n/m 2 , E > 0.1 MeV at 4.2 K. Post-irradiation testing included flexural (3 pt. bend), as well as DC conductivity and dielectric breakdown strength. Measurements were made at 77 K and 300 K. Flexural strength was observed to decrease for the epoxy-based materials while the polyimide-based materials exhibited essentially no change in strength. Both DC conductivity and dielectric breakdown strength revealed little or no pattern of degradation at all levels of radiation exposure.

A comparison of the doppler-broadened positron annihilation spectra of neutron irradiated Al2O3 and MgAl2O3

Radiation damage studies of oxides and ceramics have become of increasing importance due to the projected use of these materials in thermonuclear fusion reactors as electronic insulators and first wall materials. In addition these materials are important in RAD waste disposal. As part of a study of the defect structure in radiation damaged ceramics Doppler-broadened positron annihilation spectra have been obtained for a series of single crystal sapphire (α-Al2O3) and polycrystal (1:1) and (1:2) magnesium aluminate spinel (MgO·Al2O3 and MgO-2Al2O3) samples. These samples were irradiated in EBR-II to a fluence of 3 × 1025 n/m2 (E > 0.1 MeV) at 740°C, and 2 × 1026 n/m2 (E > 0.1 MeV) at ~ 550°C respectively. Positron annihilation spectra lineshapes for the irradiated, annealed, and as-received samples of both materials were compared using S parameter analysis. These calculations were made on deconvoluted gamma ray spectra that were free of any instrumental broadening effects. In this way, absolute S parameter changes could be calculated. The observed changes in the S parameter are consistent with independent volume swelling measurements for both the α-A12O3 and the (1:2) MgAl2O4 samples. However, the change in S parameter measured for the (1:1) spinel is contrary to the measured volume change. This apparent anomaly indicates a predominence of interstitial as opposed to vacancy type defects in this material.