Ceramic BeO Research Articles

Multiphysics modeling of novel UO2-BeO sandwich fuel performance in a light water reactor

The relatively poor thermal conductivity of the UO2 fuel is a major challenge for optimizing reactor operation and safety performance despite its widespread use in the majority of power reactors. The already low thermal conductivity further degrades with burnup due to accumulation of defects, fission product precipitates and fission gas bubbles. The high thermal stresses cause significant pellet cracking, leading to more pellet expansion and causing pellet cladding interaction and the release of fission product gases. Mitigation of these phenomena is currently accomplished by limiting operating power and ramp rates at high burnup. This study presents a novel design of the fuel pellet. The pellet was a combination UO2 fuel and ceramic BeO, and three types of geometry were proposed to mitigate the thermal stress. The corresponding fuel performances were compared based on the modified version of our previously developed CAMPUS code. The fuel pellets with BeO inner or middle layers in the radial direction were found to greatly decrease fuel centerline temperature, and mitigate the fuel and cladding mechanical interaction by delaying the gap closure time. So, the corresponding fuel performance and reactor safety would be improved. The fabrication cost of these two types of fuel pellets would also be decreased significantly compared to the UO2-BeO composite fuel due to the simplicity of the pellet design.

Investigation of Discharge Channel Elongation under Thermal Dielectric Breakdown

A mathematical model of the expansion of a discharge channel due to a growing electrothermal instability in a solid dielectric material under constant voltage is considered. The model describes temperature dynamics, electric field strength, discharge density, and melting of the dielectric. The paper reports the results of a numerical investigation of the discharge-channel expansion during the breakdown of ceramic BeO in a point-to-plane electrode configuration.

A miniature metal-ceramic x-ray source for spacecraft instrumentation.

Definitive mineralogical identification of materials with x-ray diffraction and fluorescence on remote planetary probes requires the development of a rugged miniature x-ray source that complies with the mass, power, thermal, and electrical management constraints imposed by space missions. Conventional x-ray tubes are generally fragile, glass-envelope designs with heat-sensitive seals. They are too brittle and bulky for planetary missions, and usually require cumbersome and power-consuming cooling systems. Here we describe the development of a novel, rugged miniature x-ray source employing a ceramic BeO substrate upon which a metal target material is deposited. Conventional thermionic emission and high-voltage acceleration of electrons to strike the metal target material produce an x-ray yield comparable to conventional x-ray tubes. Thermal management of the x-ray source is achieved with the excellent heat transport properties of the BeO target substrate coupled with a passive heatpipe.

The Use of Passive Detectors to Monitor Tritium on Surfaces

Commercially available BeO exoelectron dosemeters and electret ion chambers (EICs) are being adapted and applied to in situ field monitoring of tritium on surfaces. Thin-layer BeO on a conductive graphite substrate is of the order of 50 times more sensitive to tritium than the EIC. At the US Department of Energy release limit for fixed surface tritium of 5000 dpm per 100 cm 2 , the exposure time for quantification with the exoelectron dosemeter is of the order of one hour. A multipoint Geiger counter was used for reading exoelectron emission. An alternative ceramic BeO dosemeter (Thermalox 995) has low electrical conductivity and will require a different reader to overcome problems of surface charging during exoemission. The electret is very easy to use and read. Its practical use will be for surfaces with relatively high levels of tritium contamination.

Selective suppression of thermal emission from radomes and materials selection therefor

The possibility of using selectively low emission to reduce the thermal signature of radomes is identified. A material can simultaneously have low radiance in the working range of a detector and cool radiatively to the surrounding atmosphere. If the band of low emission is based on lattice excitation, the signature reduction is compatible with electrically insulating properties and radar transmittance. High-density, polycrystalline beryllium oxide is identified as a material with low emittance in the upper atmospheric window at 8 to 13 μm, owing to a strong reststrahlen band. Radiometer measurements and bulk reflectance spectra are reported for ceramic BeO and are used to calculate the thermal infrared emission factors as well as the radiance for a 50°C BeO surface. The results depend heavily upon the short wavelength threshold of the detector. The calculated values have been compared with radiometer measurements in the 3- to 5-μm and 8- to 13-μm ranges. The possibility of reducing the emittance even further with a second material has been investigated with Fresnel calculations. Very favorable calculated and measured results for a 0.8-μm silicon overlayer on BeO are reported.

Measurement of Skin Dose with TSEE From BeO

Abstract A commercial ceramic BeO disc (Thermalox 995) and a ZnO thin film were used as TSEE specimens. The ß ray sensitive surface layer of a TSEE specimen acts as a very thin absorbed dosemeter. The absorbed dose of ß rays was measured using an extrapolation ionisation chamber. The experimental value of the conversion factor between the number of exoelectrons emitted from a Thermalox 995 TSEE specimen and the absorbed dose in the thin surface layer of the same specimen was about 84 pGy per exoelectron, and the value for a ZnO thin film was about 96 µGy per exoelectron. By covering a TSEE specimen with a tissue-equivalent material layer of 7 mg.cm-2 thickness, an easy measurement of the individual dose equivalent, superficial, Hs(0.07), is possible without any information on ß ray irradiation energy.

Gain of a waveguide CO laser

An experimental investigation was made of the gain of a water-cooled waveguide CO laser with a static active mixture. Illumination of the waveguide with radiation from a probe laser and application of a balance method made it possible to determine the gain for different cross sections of waveguides made of ceramic BeO and of quartz. The maximum gain of 0.7 m−1 was obtained for the P10–9(17) transition in a waveguide made of ceramic BeO and of 1×1 mm cross section.

Present State of Art of TSEE Dosimetry

Abstract The principles and application potentials of the TSEE method for dosimetry are reviewed. The physical and dosimetric properties of BeO thin film detectors, the most advanced TSEE dosemeters at present, are described in detail. These detectors differ in their TSEE behaviour from ceramic BeO because of their smaller grain size, which reduces spurious effects caused by the pyroelectric properties of BeO.

Relationship Between Thermoluminescence and Thermally Stimulated Exoelectron Emission of BeO Thin Film Dosemeters

Abstract BeO thin films have been suggested as TSEE dosemeters. Apart from TSEE they also show thermoluminescence. Their TL properties are very similar to those of BeO ceramics with respect to temperature positions of the maxima, spectral distribution of the emitted light, optical fading and mass specific TL sensitivity. For both TSEE and TL optical fading is more pronounced at the low temperature glow peaks. However, TSEE and TL originate from different centres whose relationship to the corresponding centres in ceramic BeO is discussed. Previously emptied emission centres are refilled by phototransfer during UV irradiation. Thus dose reassessment is feasible. The TSEE signal can still be re-established after a dose readout heating cycle up to 600 oC, whereas TL is annealed completely already below 300 oC.

The influence of the ambient atmosphere on the TSEE of ceramic BeO

In view of the application of the thermally stimulated exoelectron emission (TSEE) of ceramic BeO for radiation dosimetry, an encapsulation with different atmospheres is simulated for the purpose of studying its influence on TSEE. In particular, it is shown that the exoelectron yield at the same exposure condition strongly depends on the water admission to the dried atmosphere. Moreover it is stated that there is a distinction whether the break-in of air humidity took place after or before the γ-irradiation. The detailed surface processes leading to these changes in TSEE are discussed. Im Hinblick auf die Anwendung der thermisch stimulierten Exoelektronen-Emission (TSEE) von BeO-Keramiken für die Dosimetrie ionisierender Strahlung wird eine Kapselung mit verschiedenen Atmosphären simuliert, um deren Einfluß auf die TSEE zu untersuchen. Es wird insbesondere gezeigt, daß die Exoelektronen-Ausbeute bei gleichen Bestrahlungsbedingungen entscheidend von einer Wasser-Zugabe zur trockenen Atmosphäre abhängt. Darüber hinaus wird festgestellt, daß ein Unterschied besteht, ob die Luftfeuchtigkeit vor oder nach der γ-Bestrahlung auf die Probe einwirkt. Die einzelnen Oberflächen-Prozesse, die zu den Änderungen der TSEE führen, werden diskutiert.

An EPR-ENDOR study of the [Li]° center in crystalline and ceramic BeO

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Improvements in the Use of Ceramic BeO for TLD

Improvements in the Use of Ceramic BeO for TLD

Measuring tritium with exoelectron dosimeters

Small disks of ceramic BeO, called Thermalox 995, are sensitive detectors of tritium when read for thermally stimulated exoelectron emission (TSEE). An additional reading of the thermoluminescence provides a means for estimating the exposure to tritium in the presence of other penetrating radiations. Weakly contaminated surfaces, <1 nCi/cm 2, which are small, curved, or difficult to reach, can be monitored accurately. While the detector sensitivity is sufficient to measure tritium gas or tritiated water vapor at allowable, occupational exposure levels, the sensitivity of this particular detector is insufficient for monitoring members of the public exposed continuously to HTO vapor at the MPC. The TSEE peak location and shape are useful features for identifying an exposure to tritium. Prior to reading, rinsing the detector in alcohol is desirable to prevent spurious electron emissions.

Fast-neutron response characteristics of TSEE dosimeters

Further applications are described of the previously reported technique of fast neutron dosimetry with combinations of hydrogenous radiators and ceramic BeO, to be evaluated by thermally stimulated exoelectron emission (TSEE). For example, a pronounced directional response has been minimized with a dosimeter-radiator-dosimeter sandwich; and information on the effective neutron energy can be obtained by varying either the radiator thickness, or the thickness of an absorber layer between radiator and TSEE dosimeter.

Adsorbed gases, atmospheric, and temperature effects on TSEE from BeO

Abstract Oxygen, water vapor and/or carbon dioxide adsorbed on ceramic BeO before or after irradiation profoundly affect the thermally stimulated exoelectron emission (TSEE). In their presence the TSEE is dominated by a single peak at a surface temperature of ∼275°C. Removal of these adsorbates by outgassing during read-out causes the exoelectron emission to intensify with the TSEE curve becoming more complex. Apparently, sorption layers render inoperative certain traps effective for TSEE. Increased temperature during irradiation reduces the response (20% between 22° to 150°C). The influence of the gas surrounding the sample during irradiation was tested for the inert gases helium, nitrogen, neon, argon and krypton using 5.6 keV, 22 keV and 0.66 MeV photons. The enhancement in sensitivity for argon and krypton was much less than expected, which indicates that absorption of radiation in the volume of the solid is more important for excitation of the dosimeter than hitherto assumed.