Radiation-induced Electrical Degradation Research Articles

Japanese activities of the R&D on silicon carbide composites in the broader approach period and beyond

The R&D on SiC/SiC composites under the broader approach (BA) activities between Japan and the EU for fusion DEMO developed a fundamental database of mechanical (Task-1) and physical/chemical (Task-2) properties, with a primary target of the application of SiC/SiC composites as functional structure to be used in the dual coolant breeding blanket concept. This paper aims to summarize previous 10-years activities of the R&D of Japan and to provide the key deliverables toward the DEMO design. In Task-1, good creep and fatigue durability were first demonstrated. Besides, in-plane and inter-laminar strength anisotropy maps at elevated temperatures were comprehensively identified. In parallel, the irradiation effects of SiC materials were specifically determined as input parameters of the analytical model to provide for the irradiation-induced residual stresses. In Task-2, the apparent dose-dependence of the radiation-induced electrical conductivity and the indicative radiation-induced electrical degradation was identified by various irradiation sources. In addition, good gas confinement was identified. Furthermore, no accelerated corrosion for duration of 3000 h at below 1173 K was first demonstrated. With these achievements, it is suggested that the in-vessel component technology, e.g., material corrosion database development, activated corrosion product evaluation code development, compact module tests for validation of the key functions of the components, technology integration assessment for fusion nuclear tests, etc., should be further developed toward DEMO in near-term.

Functional materials for ITER diagnostic systems – Radiation aspects

ITER will be the first fusion device able to maintain fusion for long periods of time and therefore suitable to test integrated technologies, materials, and physics regimes necessary for the commercial production of fusion-based electricity. Therefore, ITER provides a harsh radiation environment that represents an important challenge for diagnostic systems located in various regions inside the tokamak, in particular the first wall, divertor areas, and ports, but as well ex-vessel areas, where radiation levels are also significantly high for some electronic components. The radiation induced effects in functional materials have been identified and studied in the past in the fusion and fission community. These effects comprise nuclear heating, changes in physical and mechanical properties due to transmutation and displacement damage, radiation induced conductivity (RIC) or radiation induced electrical degradation (RIED) effects on insulators, radiation induced absorption (RIA) for optical materials, radio luminescence observed in fibers and windows, etc. This paper summarizes the current status of the functional material selection for ITER diagnostic systems and the expected radiation conditions in the areas where they are located.Additional testing will be required for some materials and assemblies, such as bolometers and magnetic coils. Special attention is paid to the tests required for qualification of safety important components, such as diagnostic window assemblies and vacuum feedthroughs.Finally, as one of the best ways to reduce the impact of radiation in materials is to develop a proper shielding strategy, some of the shielding options envisaged for ITER are presented and discussed in this paper.

Dielectric properties of alumina ceramics for fusion applications

The insulating materials for radio frequency (RF) windows and different antenna supports for heating systems and some diagnostics are still an unresolved issue in future fusion machines such as DEMO. Alumina ceramic (Al2O3) is one of the main candidate materials for these systems. The dielectric properties, such as electrical permittivity (ε) and loss tangent (tanδ), determine its power losses. Therefore, tanδ values need to be low, between 10–6 and 10–3 depending on the application. However, due to the crucial role of the manufacturing process in determining its final dielectric properties, there is the need to undertake a joint effort with the industry in order to validate a standard manufacturing route that ensures a supply of ceramic material with homogeneous and standardized dielectric properties for future fusion machines. In DEMO, these ceramic materials will be operated under extreme conditions. They will be exposed to considerable levels of neutron irradiation. Radiation Induced Conductivity (RIC) and Radiation Induced Electrical Degradation (RIED) are phenomena that influence notoriously on the dielectric performance of these materials. Therefore, their optimum properties must be tested under relevant irradiation conditions. Hence, it is important to accurately characterize their dielectric properties before irradiation to be able to compare them with the future post neutron irradiation results. Furthermore, other types of radiation such as beta and gamma are also used in the irradiation scheme to provide more information on the variation of dielectric properties of ceramics due to radiation effects. In this work, loss tangent and permittivity values of some pre-irradiated candidate materials will be presented as well as the measuring techniques used at CIEMAT. These techniques have the advantage of covering a very broad range of frequencies required for alumina applications in fusion (from kHz to GHz) and giving the best accuracy for very low losses.

Dynamic measurements of radiation-induced electrical-property modifications in CVD-SiC under fast-neutron irradiation

In situ measurements of the volume electrical conductivities of chemical-vapor-deposited silicon carbide (CVD-SiC) samples were carried out under irradiation by 2.5- and 14-MeV fast-neutron beams in air at room temperature. A slight radiation-induced conductivity (RIC) was detected under fast-neutron irradiation. A radiation-induced electrical degradation (RIED)-like behavior was observed in the form of degradation of the base conductivity in the absence of irradiation with increase in the fast-neutron fluence. The SEM micrograph images and XPS analysis of the surface of the fast-neutron-irradiated CVD-SiC samples revealed that the dissociation of carbon from existing SiOxC3−x compositions did not only occur via displacement damage, but also via ionizing effects (radiolysis), thereby leading to the observed RIED-like behavior as well as RIC in the electrical properties of the CVD-SiC samples.

Radiation induced changes in electrical conductivity of chemical vapor deposited silicon carbides under fast neutron and gamma-ray irradiations

The radiation-induced changes in the volume electrical conductivities of chemical vapor deposited silicon carbides (CVD-SiCs) were in-site investigated by performing irradiation using 1.17 and 1.33-MeV gamma-ray and 14-MeV fast neutron beams in air and vacuum. Under gamma-ray irradiation at ionization dose rates of 3.6 and 5.9 Gy/s and irradiation temperature of approximately 300 K, the initial rapid increase in electrical conductivity; this is indicative of radiation-induced conductivity (RIC), occurred due to electronic excitation, and a more gradual increase followed up to a dose of approximately 10–50 kGy corresponding to the results in base conductivity without radiation; this is indicative of radiation-induced electrical degradation (RIED). However, the radiation-induced phenomena were not observed at irradiation temperatures above 373 K. Under neutron irradiation at a further low dose rate below approximately 2.1 Gy/s, a fast neutron flux of 9.2 × 10 14 n/m 2 s, and 300 K, the RIED-like behavior according to radiation-induced modification of the electrical property occurred with essentially no displacement damage, but ionizing effects (radiolysis).

Radiation-induced changes in electrical and proton conductivities of BaCe 0.9Y 0.1O 3 − δ upon irradiation by electron beams

The radiation-induced changes in the electrical and proton conductivities of hydrogen (H or D)-doped BaCe 0.9Y 0.1O 3 − δ were investigated by performing irradiation using 1.8-MeV electron beams at ionization dose rates of 10–1000 Gy/s and an irradiation temperature of 473 K. For dose rates below 300 Gy/s, radiation-induced conductivity (RIC) occurred due not only to electronic excitation but also to the increased hydrogen diffusion caused by the ionizing effects of radiation. For dose rates above 300 Gy/s, the base conductivity and RIC were gradually reduced as the irradiation dose increased up to a dose of approximately 8.0 MGy; this is indicative of radiation-induced electrical degradation (RIED). The dependence of RIC and RIED on the dose is not affected by the presence of doped hydrogen. Radiation-enhanced diffusion of the constituent oxygen in BaCe 0.9Y 0.1O 3 − δ not only leads to the occurrence of RIC and RIED but also suppresses the migration of protons activated by the ionizing effects of radiation. This suppression occurs because of the formation of Ba and Ce hydroxides and/or hydrides on the surface of the irradiated samples; the formation of these compounds is revealed by XPS and optical absorption spectra.

Effects of ionization on migration of hydrogen doped into BaCe 0.9Y 0.1O 3 − δ

The effects of radiation on the electrical and proton conductivities of hydrogen (H)-doped BaCe 0.9Y 0.1O 3 − δ were studied by performing irradiation using 1.8-MeV electron beams at ionization dose rates of either 10 or 1000 Gy/s. The base conductivity ( σ BC) of BaCe 0.9Y 0.1O 3 − δ became higher under electron irradiation that is indicative of radiation-induced conductivity (RIC; σ RIC). The difference between RIC and the base conductivity, ΔRIC ( σ RIC − σ BC), at an ionization dose rate of 10 Gy/s increased as the irradiation temperature increased from 298 to 623 K. It was found that the ΔRIC value for H-doped BaCe 0.9Y 0.1O 3 − δ only at 10 Gy/s and 473 K was approximately one order of magnitude higher than that of nondoped BaCe 0.9Y 0.1O 3 − δ . The results indicate that the radiation-induced conduction occurs not only due to electronic excitation but also due to hydrogen diffusion increased by the ionizing effects of radiation. The RIC, ΔRIC, and base conductivity of samples irradiated at 10 Gy/s and 623 K and at 1000 Gy/s and 473 K gradually reduced as the irradiation time increased; this is indicative of radiation-induced electrical degradation (RIED). The dependence of the conductivities on the irradiation time is not affected by the presence of doped hydrogen. Radiation-enhanced diffusion of the constituent oxygen not only leads to the occurrence of RIED but also suppresses the migration of protons activated by the ionizing effects of radiation.

05/01771 Performance of functional materials and components in a fusion reactor: The issue of radiation effects in ceramics and glass materials for diagnostics

It is envisaged that early in this new century International Thermonuclear Experimental Reactor (ITER) will come into operation to bridge the gap between the present day large “physics” machines and the pre-commercial DEMO fusion reactor. Although ITER will undoubtedly help to solve many of the problems which still remain in the field of plasma physics, it will also present additional operational and experimental problems due to radiation damage effects as a result of the intense radiation field from the “burning” plasma. For structural metallic materials the problem of radiation damage is expected to be severe, although tolerable, only near to the first wall, however the problem facing the numerous insulating components is far more serious due to the necessity to maintain not only the mechanical, but also the far more sensitive physical properties intact. Insulating materials will be required in a number of key systems ranging from heating and current drive, to diagnostics, and remote handling which affect not only the operation, but also the safety and control of the machine, as well as maintenance and repair. The effects of radiation on the electrical and optical properties will be discussed, in particular radiation induced conductivity (RIC), radiation induced electrical degradation (RIED), radiation induced electromotive force (RIEMF) and radioluminescence.

Electrical Property of Calcium-Zirconium-Indium Oxide by Irradiation

Radiation induced changes in electrical properties of calcium-zirconium-indium oxide ceramics (CaZr0.9In0.1O3-d) were investigated using a fission reactor of Japan Materials Testing Reactor (JMTR). Electrical conductivity of CaZr0.9In0.1O3-d under the reactor irradiation increased by the Radiation Induced Conductivity (RIC), Radiation Induced Electromotive Force (RIEMIF)and Radiation Induced Electrical Degradation (RIED), caused by the electronic excitation. The RIC and the RIEMF decreased gradually for the constant value of the reactor power with the increase of irradiation time. Moreover, it was confirmed by Post-irradiation Examination (PIE) experiments that the initial electrical and thermal properties were completely degraded by the irradiation. These radiation effects are attributed to point defects or their clusters produced by atomic displacements caused by neutron collisions and also by the radiolysis.

Radiation Effects on Electrical Conductivity of Proton Conducting Oxide Ceramics

Radiation Induced Conductivity (RIC) of proton conducting oxide ceramics (SrCe0.95Yb0.05O3-δ) was studied using 1.8 MeV electrons at an ionization dose rate of 10 Gy/s. The observed RIC was enhanced by an increase of the base conductivity without radiation as the irradiation temperature was increased from 298 to 673 K, due to Radiation Induced Electrical Degradation (RIED). It was found that the degradation rate of the base conductivity was strongly dependent on irradiation temperature and fluence.It was also found that the base conductivity at temperatures above about 433 K did not change due to electron irradiation at the total ionization dose of 2.85 MGy. The result indicates that the radiation effect by the electrons has no influence on protonic or ionic conduction.

Challenges for insulating materials in fusion applications

It is envisaged that early in this new century International Thermonuclear Experimental Reactor (ITER) will come into operation to bridge the gap between the present day large “physics” machines and the pre-commercial DEMO fusion reactor. Although ITER will undoubtedly help to solve many of the problems which still remain in the field of plasma physics, it will also present additional operational and experimental problems due to radiation damage effects as a result of the intense radiation field from the “burning” plasma. For structural metallic materials the problem of radiation damage is expected to be severe, although tolerable, only near to the first wall, however the problem facing the numerous insulating components is far more serious due to the necessity to maintain not only the mechanical, but also the far more sensitive physical properties intact. Insulating materials will be required in a number of key systems ranging from heating and current drive, to diagnostics, and remote handling which affect not only the operation, but also the safety and control of the machine, as well as maintenance and repair. The effects of radiation on the electrical and optical properties will be discussed, in particular radiation induced conductivity (RIC), radiation induced electrical degradation (RIED), radiation induced electromotive force (RIEMF) and radioluminescence.

Radiation-induced electronic and ionic charge storage and release in sapphire

Radiation-induced thermally stimulated relaxation (TSR) processes in the reduced f -Al 2 O 3 (sapphire) crystal were investigated at 290-650 v K by means of the TS current (TSC), ionic depolarisation current (TSDC) and electron emission (TSEE) techniques. After thermal (ionic) polarisation of sapphire wide (∼75 v K) and asymmetric ionic dipolar TSDC peak at T max , 590 v K (disorientation of the anion vacancy-related dipoles) was detected. This peak correlates with the wide TSEE peak at T max , 615 v K, the radiation-induced electrical degradation (RIED) yield rise above 550 v K ( T max , 745 v K) and the chromium emission line broadening in ruby. Above 450-500 v K the anion vacancy hopping (migration) starts. This can lead to lattice dynamic disordering and anion vacancy diffusion-controlled processes in sapphire (especially in vacuo near the sample surface, grain boundaries, dislocations) in various TSR (TSC, TSDC, TS heat release and bleaching) and RIED phenomena. Surface structure and impurity content, surrounding atmosphere (vacuum or air) and electric fields determine these phenomena.

Electrical conductivity of alumina after heavy irradiation in a high-flux fission reactor

Abstract The electrical conductivities of seven different alumina specimens were measured after they were irradiated in the High-Flux Isotope Reactor (HFIR), Oak Ridge National Laboratory. Results indicated a moderate increase in the electrical conductivity due to the irradiation in HFIR up to 2.8 displacements per atom at 450–500°C. A large offset current was observed in the irradiated specimens after they were subjected to an applied voltage, which indicated the existence of a large number of trapped electrical charges. An abrupt release of the trapped electrical charges may be a trigger to cause the previously reported electrical breakdown under irradiation (called radiation-induced electrical degradation).

An initial model for the RIED effect

A simple model based on electron acceleration in the conduction band giving rise to an increased F + oxygen vacancy lifetime provides an explanation for several radiation induced electrical degradation (RIED) associated observations in Al 2O 3. The increased F + radioluminescence noted during RIED is a direct consequence of the lifetime increase. The model predicts the observed electric field threshold for RIED, and an increase in the field threshold with increasing impurity content. RIED for RF electric fields is also explained. In addition the lifetime increase provides an explanation for the enhanced oxygen vacancy aggregation including colloid and gamma alumina production observed under RIED conditions.

Current–voltage characteristic of alumina and aluminum nitride with or without electron irradiation

The current–voltage ( I– V) characteristics of single- and poly-crystal alumina and aluminum nitride (AlN) were measured at temperatures ranging from room temperature to 723 K with or without 1 MeV electron irradiation in a high voltage electron microscope (HVEM). Both alumina and AlN specimens exhibit non-ohmic I– V characteristics without irradiation. The I– V characteristics in alumina, however, change from non-ohmic to almost ohmic under electron irradiation. But the I– V characteristics in AlN is still non-ohmic under irradiation. There are remarkable differences in I– V characteristics between the alumina and AlN specimens. The non-ohmic behavior is due to the electronic barrier formed near the interface between the titanium electrode and the alumina or AlN specimen. No bulk and surface radiation induced electrical degradation (RIED) was found in AlN up to 1.5×10 −5 dpa.

Difficulties in measuring electrical conductivities in highly insulating materials: Radiation-induced electrical degradation is an artifact

Measurement of the electrical conductivity in high-resistance insulators is made difficult by previously unrecognized limits in the electric guarding technique. Electron irradiation experiments on single-crystalline Al2O3, performed for studying the effects of irradiation on the electrical conductivity, revealed that radiation-induced electrical degradation effects in ceramic insulators, previously reported to occur after electron, ion, and neutron irradiation, are an artifact.

Search for radiation-induced electrical degradation in ion irradiated sapphire and polycrystalline Al2O3

An international discussion is being carried out on whether radiation-induced electrical degradation in ceramic insulators does or does not exist. In the present experiments on radiation-induced conductivity and radiation-induced electrical degradation in high purity polycrystalline Al2O3 and sapphire all interfering effects resulting from surface conductances have been eliminated. The results have not confirmed a permanent degradation of the volume conductivity. Radiation-induced conductivity values were observed to decrease with dose. A transient in the electrical conductivity after ion beam off was discovered.

Long term degradation of electrical insulation of Al 2O 3 under high flux fission reactor irradiation

Long term increase of electrical conductivity under irradiation was observed in some Al 2O 3 (polycrystal and single crystal alumina) specimens in in situ electrical measurements in the High Flux Isotope Reactor in the framework of the Japan/USA collaboration called JUPITER project. Especially, chromium doped single crystal alumina showed substantial increase of electrical conductivity at moderate doses of ∼0.1 dpa, which had similarity to the phenomenon called radiation induced electrical degradation (RIED). Irradiation to doses above 0.1 dpa did not cause a further degradation of electrical conductivity, but instead resulted in an improvement in insulating ability. Behavior of the observed increase of electrical conductivity is described in conjunction with the reported phenomena in RIED and some mechanisms responsible for the observed increase are speculated from the results obtained.

Characterization of Y 2O 3 coating under neutron irradiation

Ceramic coatings on the surface of structural materials such as 316SS have been considered for electrical insulators and tritium permeation barriers in fusion reactor designs. Y 2O 3 is one of the most promising materials as a coating from a point of high electrical resistivity, etc. In this report, the electrical conductivity of a Y 2O 3 coating was investigated under neutron irradiation with the Japan Materials Testing Reactor (JMTR). The specimen was 316SS with a Y 2O 3 coating, and was irradiated at 300°C in a He atmosphere. From the results of in situ measurements on the electrical conductivity of a Y 2O 3 coating, the Radiation Induced Conductivity (RIC) was observed at JMTR power-up. The electrical conductivity of the coating before neutron irradiation and under neutron irradiation were about 3 × 10 −13 and about 1 × 10 −9 1/Ω/cm, respectively. The electrical conductivity of the coating was constant under neutron irradiation but after neutron irradiation recovered up to that before neutron irradiation. The Radiation Induced Electrical Degradation (RIED) in the Y 2O 3 coating was not recognized up to a fluence of about 5 × 10 20 n/cm 2 (>1 MeV). It was clear that the coating had good electrical resistivity under neutron irradiation corresponding to about 4 × 10 10 Gy.

Study of dynamic radiation effects in ceramics in fission reactor

A system for studying dynamic irradiation effects in a fission reactor is developed to measure electrical and optical properties of ceramics. The dynamic irradiation would affect some functional properties of ceramics, such as electrical conductivity and optical transmissibility, which should be studied from a practical point of view. At the same time, the study of dynamic irradiation effects would yield valuable information concerning primary processes of the radiation effects. Interaction of the system with radiation was found to cause physical phenomena which are interesting but disturbing for the measurements. Some countermeasures were taken to avert the disturbing phenomena. The electrical conductivity of the ceramic insulators was measured, and the phenomena called radiation induced conductivity (RIC) and radiation induced electrical degradation (RIED) were studied. Also, radiation induced electromotive force (RIEMF) was examined. Optical absorption and radioluminescence behavior of the ceramics was studied under the reactor irradiation.