J.C. Kennedy

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.

Porosity swelling and transmutation contributions to conductivity changes in some neutron-irradiated copper alloys

Fast-neutron irradiation of alloys for fusion-reactor applications produces bulk changes in the density and composition via porosity swelling and transmutation which affect the do volume electrical and thermal conductivities ( σ = 1/ ϱ e and K ). For the Cu materials of our study, neutron fluences of 2 × 10 26 n/m 2 ( E > 0.1 MeV) produced increases in Ni and Zn concentrations of about 0.05 and 0.09 wt%, respectively, and porosity swelling of 0–7%; ϱ e accordingly increased as much as 18%. We also determined the individual ϱ e changes due to both swelling and transmutation via use of an appropriate mixing rule and of Matthiessens law to unmask any residual effects present, e.g., phase or microstructural changes. For four materials — two pure copper and two alumina-dispersion-strengthened (ADS) alloys — subtraction of these δϱ e s from the irradiated values yielded or nearly yielded the respective control values. In contrast, the two precipitation-strengthened (PS) alloys studied, MZC and AMZIRC, had relatively large negative residues, apparently indicating effective radiation-induced conductivities.

Effects of neutron-irradiation on MgAl2O4 and Al203

Specimens of MgAl 2 O 4 and Al 2 0 3 were irradiated to a neutron fluence of approximately 2×10 26 n/m 2 , E > 0.1 MeV at 660 K. Tensile strength was seen to decrease by 25.6% for Al 2 O 3 and to increase by 24.4% for MgAl 2 O 4 . Density measurements revealed that Al 2 O 3 swelled by 3.22% while MgAl 2 O 4 densified slightly (0.316%). Both exhibited mixed failure modes in both control and irradiated samples with a shift from intergranular to transgranular for MgAl 2 O 4 and vice-versa for Al 2 O 3 upon irradiation. The MgAl 2 O 4 damage microstructure consisted of interstitial dislocation loops and denuded grain boundaries. The microstructure of irradiated Al 2 O 3 revealed fine areas of damage assumed to be interstitial dislocation loops and voids. Strength and structural changes are correlated with the results and compared with previously obtained results.

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.

The effect of ZrO2 doping on mechanical and dielectric properties of Al2O3 and MgAl2O4

Abstract Different samples of zirconia-doped alumina and spinel ceramics have been obtained and characterized. Their mechanical and dielectric properties at high frequencies have been measured. The dependence of these properties on the concentrations of the different zirconia phases is discussed. From the results obtained it is concluded that the dispersion of tetragonal zirconia particles in the ceramics causes strength and toughness to be improved without almost any degradation in dielectric properties, while monoclinic zirconia increases the dielectric loss.

On the development of a laser detonator

The initiation of explosives by laser illumination has been known for many years. In this paper we will discuss the development of a working detonator design that reduces the energy required for detonation in a low-density secondary explosive by vaporizing a thin metal coating. We present data on the development of the design for a workhorse laser detonator that provides enhanced safety over existing exploding bridgewire detonators (EBWs). Comparison of this laser initiated data to an exploding-bridgewire (EBW) provides insight into the mechanism of initiation of detonation in low-density PETN by the plasma source. A novel diagnostic technique to determine the run-distance to detonation also known as the apparent Center-of-Initiation (COI) will also be discussed.

Instrumented Floret Tests of Detonation Spreading

The floret test was originally devised to permit comparison of detonation‐spreading performance of various insensitive explosive materials, using only the dent in a copper witness plate as a metric. Dent depth in the copper plate is directly related to the fraction of a thin acceptor pellet that was detonated by impact of a small explosive‐driven flyer plate. We have now added instrumentation to quantitatively measure the detonation corner‐turning behavior of IHEs. Results of multi‐fiber optical probe measurements are shown for LLM‐105 and UF‐TATB explosive materials. Results are interpreted and compared with predictions from one reaction‐rate model used to describe detonation spreading, and may be advantageous for comparison with other reactive‐flow wave‐code models. Detonation spreading in UF‐TATB occurred with formation of a non‐detonating region surrounding a detonating core, and re‐establishment of detonation in a “lateral” direction beyond that region.

Promising copper alloys for high heat load applications in neutron environments

Four candidate copper alloys and two elemental coppers were irradiated in the Experimental Breeder Reactor to damage levels of about 3 and 15 dpa at 385°C. The irradiated and control samples were evaluated to determine swelling, room temperature tensile properties, transmutation product levels, and electrical resistivity. Transmission electron microscopy was used to characterize the damaged microstructures. Among the six materials studied, the alumina-dispersed copper alloys (Glidcop Al-20 and Al-60 sheet material) exhibited the best, overall resistance to fast neutron damage as they had minimal swelling and retained their original values of yield strength and electrical resistivity (corrected for transmuted elements) after irradiation. The two cold-worked and age-hardened alloys (AMZIRC and MZC) suffered loss of yield strength after irradiation at 385°C. The two elemental coppers and the AMZIRC alloy showed large swelling (3.6–6.8 vol%) due to void formation after the high neutron fluence. Further study is needed to understand microstructural alterations caused by neutron damage of the alumina-dispersed copper alloy.

Radiation-induced conductivity in alumina from 100 Hz to 10 MHz during proton irradiation*

Changes in the dielectric constant and loss tangent of alumina have been measured in situ during proton irradiation. In these experiments, single crystal sapphire specimens were irradiated with 3 MeV protons which passed through the sample and were stopped in a copper-block heat sink. Dielectric properties were measured between 100 Hz and 10 MHz using a guard-ring capacitor configuration. The proton irradiation caused an immediate increase in loss tangent from about 10 −4 to more than 1.0. We have evaluated these changes at 300 and 373 K, vs irradiation time, flux and frequency. While the in situ radiation-induced conductivity (RIC) depends on these variables as well as on the history of previous irradiation, we believe that it is caused by a balance between the generation rate of electrons and holes, and their trapping and annihilation at displacement-type defects.

Mechanical Properties of Three Candidate Organic Insulator Materials for Fusion Reactors

Insulators in superconducting magnets for fusion reactors will be exposed to radiation doses of 10 to 100 Mgy with more than half the dose coming from fast neutrons. One polyimide- and two epoxy-based laminates* were neutron irradiated to total doses of 2.6 and 4.1 × 1021 n/m2, E>0.1 MeV at 4.2 K in the Intense Pulsed Neutron Source at Argonne National Laboratory. Flexural properties were determined at 75 K and compared to results from previous studies of gamma irradiation effects at Oak Ridge National Laboratory.