Ceramics for applications in fusion systems

Six critical applications for ceramics in fusion systems are reviewed, and structural and electrical problem areas discussed. Fusion neutron radiation effects in ceramics are considered in relation to fission neutron studies. A number of candidate materials are proposed for further evaluation

Alpha decay self-damage in cubic and monoclinic zirconolite

Samples of primarily-monoclinic /sup 238/Pu-doped zirconolite were stored at ambient temperature to allow accumulation of alpha decay self-damage to a dose of 1 x 10/sup 24/ ..cap alpha../m/sup 3/ (equivalent to a SYNROC age of approx. 10/sup 3/y). Bulk swelling reached 2.3 vol% with no tendency toward saturation, a damage response similar to that observed for cubic Pu-doped zirconolite. X-ray volumetric swelling at 4 x 10/sup 24/ ..cap alpha../m/sup 3/ was 1 vol%, considerably less than that for the cubic material. Changes in cell dimensions differed significantly from those reported by others for a monoclinic natural mineral. Extensive microcracking was observed, and is attributed at least partially to swelling differences between the matrix and minor phases

Effect of irradiation-induced defects on fusion reactor ceramics

Structural, thermal, and electrical properties critical to performance of ceramics in a fusion environment can be profoundly altered by irradiation effects. Neutron damage may cause swelling, reduction of thermal conductivity, increase in dielectric loss, and either reduction or enhancement of strength depending on the crystal structure and defect content of the material. Absorption of ionizing energy inevitably leads to degradation of insulating properties, but these changes can be reduced by alterations in structural or compositional makeup. Assessment of the irradiation response of candidate ceramics Al/sub 2/O/sub 3/, MgAl/sub 2/O/sub 4/, SiC and Si/sub 3/N/sub 4/ shows that each may find use in advanced fusion devices. The present understanding of irradiation-induced defects in ceramics, while far from complete, nevertheless points the way to methods for developing improved materials for fusion applications

Ceramics for fusion applications

Ceramics are required for a variety of uses in both near-term fusion devices and in commercial powerplants. These materials must retain adequate structural and electrical properties under conditions of neutron, particle, and ionizing irradiation; thermal and applied stresses; and physical and chemical sputtering. Ceramics such as Al/sub 2/O/sub 3/, MgAl/sub 2/O/sub 4/, BeO, Si/sub 3/N/sub 4/ and SiC are currently under study for fusion applications, and results to date show widely-varying response to the fusion environment. Materials can be identified today which will meet initial operating requirements, but improvements in physical properties are needed to achieve satisfactory lifetimes for critical applications

Overview of irradiation effects research on insulating ceramics

Insulating ceramics have many uses in fusion reactors. Although various environmental conditions can degrade these materials, the most severe problems lie in the area of irradiation damage. Such damage can be detrimental to dimensional stability, strength, thermal conductivity, and various electrical properties; failure of an insulator can result from deleterious changes in any of these areas. Degradation of electrical properties falls into two categories: permanent effects, and transient effects that occur only when irradiation fluxes are actually impinging on the material. Consequences of irradiation damage can be severe not only for power reactors but for nearer-term machines such as ITER. Fusion reactors will require electrical ceramics to serve as insulators in rf heating systems and neutral beam injectors, diagnostic components, lightly-shielded magnetic coils, and current breaks between metallic structural elements. When these insulators are subjected to radiation fields (neutron, gamma, or ion bombardment) various physical properties can be degraded; these are discussed

Inorganic insulator program at LASL

Experiments are conducted to determine electrical and structural changes resulting from neutron and ionizing radiation. Calculations were made of damage effects in compounds and the dependence of this damage on neutron energy was studied. These activities are outlined. (MOW

Differential scanning calorimetry of metamict Pu-substituted zirconolite

Samples of CaPuTi/sub 2/O/sub 7/ were prepared by cold pressing and sintering. Plutonium was substituted for zirconium in order to characterize radiation damage effects. The energy stored in a sample which had reached saturation in swelling after storage at ambient temperature was measured with a differential scanning calorimeter. The total energy of 6.6 +- 0.1 cal/g is released over the range 485 to 715/sup 0/C. The activation energy of annealing of the damage is 1.22 +- 0.05 eV. The temperature dependence of the rate constant is described by k/sub T/ = 5.96E4 exp(-1.22/k/sub B/T) s/sup -1/ where kB and T are the Boltzmann's constant and temperature (K) respectively. A sample stored at 600/sup 0/C was similarly evaluated and showed no release of stored energy to the precision of the apparatus (+- 0.1 cal/g). These results are applied to analysis of waste incorporation in SYNROC and are correlated with analogous parameters for other materials

Dielectric changes in neutron-irradiated rf window materials

Ceramics used for windows in ECRH heating systems for magnetically-confined fusion reactors must retain adequate properties during and after intense neutron irradiation. Of particular concern is a decrease in transmissivity, a parameter inversely related to the product of dielectric constant K and loss tangent tandelta. Samples of polycrystalline Al/sub 2/O/sub 3/ and BeO were irradiated to 1 x 10/sup 26/ n/m/sup 2/ at 660K in the EBR-II fission reactor, and the above properties subsequently measured at 95 GHz. It was found that ktandelta for both materials doubled, implying a doubling of thermal stresses and a consequent reduction of time-to-failure from an assumed one year to 20 min for beryllia and 2 s for alumina. In the case of BeO, a large increase in reflectance of the incident millimeter-wave power results from dielectrically uncompensated swelling. This phenomenon could significantly degrade source performance