High efficiency thermionic converter studies

The objective is to improve thermionic converter performance by means of reduced interelectrode losses, greater emitter capabilities, and lower collector work functions until the converter performance level is suitable for out-of-core space reactors and radioisotope generators. Electrode screening experiments have identified several promising collector materials. Back emission work function measurements of a ZnO collector in a thermionic diode have given values less than 1.3 eV. Diode tests were conducted over the range of temperatures of interest for space power applications. Enhanced mode converter experiments have included triodes operated in both the surface ionization and plasmatron modes. Pulsed triodes were studied as a function of pulse length, pulse potential, inert gas fill pressure, cesium pressure, spacing, emitter temperature and collector temperature. Current amplifications (i.e., mean output current/mean grid current) of several hundred were observed up to output current densities of one amp/sq cm. These data correspond to an equivalent arc drop less than 0.1 eV

Clathrate Structure for Electronic and Electro-Optic Applications

A method including the steps of (a) depositing a metal layer on a selected portion of a silicon substrate under a first set of predetermined conditions to form an metal silicide layer and an intermediate n-type silicon layer; and (b) exposing the metal silicide layer and the n-type silicon layer to a second set of predetermined conditions to form a silicon clathrate film on the selected portion of the silicon substrate, where the intermediate n-type silicon layer acts to bond the silicon clathrate to the silicon substrate to form a silicon clathrate structure

Gas-turbine critical research and advanced technology support project

The technical progress made during the first 15 months of a planned 40-month project to provide a critical-technology data base for utility gas-turbine systems capable of burning coal-derived fuels is summarized. Tasks were included in the following areas: (1) combustion, to study the combustion of coal-derived fuels and conversion of fuel-bound nitrogen to NOx; (2) materials, to understand and prevent hot corrosion; and (3) system studies, to integrate and guide the other technologies. Significant progress was made

Characterization of Quantum Efficiency and Robustness of Cesium-Based Photocathodes

High quantum efficiency, robust photocathodes produce picosecond-pulsed, high-current electron beams for photoinjection applications like free electron lasers. In photoinjectors, a pulsed drive laser incident on the photocathode causes photoemission of short, dense bunches of electrons, which are then accelerated into a relativistic, high quality beam. Future free electron lasers demand reliable photocathodes with long-lived quantum efficiency at suitable drive laser wavelengths to maintain high current density. But faced with contamination, heating, and ion back-bombardment, the highest efficiency photocathodes find their delicate cesium-based coatings inexorably lost. In answer, the work herein presents careful, focused studies on cesium-based photocathodes, particularly motivated by the cesium dispenser photocathode. This is a novel device comprised of an efficiently photoemissive, cesium-based coating deposited onto a porous sintered tungsten substrate, beneath which is a reservoir of elemental cesium. Under controlled heating cesium diffuses from the reservoir through the porous substrate and across the surface to replace cesium lost to harsh conditions -- recently shown to significantly extend the lifetime of cesium-coated metal cathodes. This work first reports experiments on coated metals to validate and refine an advanced theory of photoemission already finding application in beam simulation codes. Second, it describes a new theory of photoemission from much higher quantum efficiency cesium-based semiconductors and verifies its predictions with independent experiment. Third, it investigates causes of cesium loss from both coated metal and semiconductor photocathodes and reports remarkable rejuvenation of full quantum efficiency for contaminated cesium-coated surfaces, affirming the dispenser prescription of cesium resupply. And fourth, it details continued advances in cesium dispenser design with much-improved operating characteristics: lower temperature and cleaner operation. Motivated by dispenser integration with semiconductor coatings, initial fabrication of those coatings are reported on dispenser-type substrates with measurement of quantum efficiency and analysis of thermal stability. Detailed investigations are performed on dispenser substrate preparation by ion beam cleaning and on dispenser pore structure by electron microscopy and focused ion beam milling. The dissertation concludes by discussing implications of all results for the demonstration and optimization of the future high quantum efficiency cesium dispenser photocathode

Carbon Film in Radio Frequency Surface Plasma Source with Cesiation

It is assumed that persistent cesiation in the SNS RF SPS is related to deposition of carbon film on the collar converter. The work function dependence for graphite with alkali deposition has no minimum typical for metals and semiconductors and the final work function is higher. For this reason, the probability of H- secondary emission from cesiated metal and semiconductors can be higher than from cesiated carbon films but the carbon film maintains cesiation longer and can operate with low cesium consumption

Mission and spacecraft support functions of the Materials Engineering Branch: A space oriented technology resource

The capabilities of the Materials Engineering Branch (MEB) of the Goddard Space Flight Center, Greenbelt, Maryland, are surveyed. The specific functions of spacecraft materials review, materials processing and information dissemination, and laboratory support, are outlined in the Activity Report. Further detail is provided by case histories of laboratory satellite support and equipment. Project support statistics are shown, and complete listings of MEB publications, patents, and tech briefs are included. MEB staff, and their respective discipline areas and spacecraft liaison associations, are listed

Tellurium Behavior and Management in the Liquid Phases in the Containment During a Severe Nuclear Reactor Accident

No industry is immune to accidents; however, the consequences and the probability are the parameters to consider when assessing the risks. When considering nuclear power, two of the highest-level accidents have occurred during the course of the commercial use of nuclear energy. The consequences of these events were the release of radioactive material to the environment and increased radiation dose to the people. Severe nuclear accident research is therefore crucial in both minimizing the consequences and assessing the effects of the potential releases. The lessons learned from previous nuclear reactor accidents have resulted in higher safety standards, more accurate source term assessment, and improvements in accident management actions. Yet, there are still uncertainties about the behavior of radionuclides during a severe nuclear reactor accident that need to be addressed.\ua0One of the elements released in a severe reactor accident is tellurium. It has several radioactive isotopes that can potentially cause an increased dose in the population if released. Moreover, many of the tellurium isotopes decay to iodine and therefore contribute to the iodine source term. The behavior and release of tellurium have been investigated in the fuel and the reactor system during the past decades. However, the released species, including tellurium, are subjected to different management actions after entering the containment including the containment spray system. The removal efficiency of the spray system towards tellurium species formed under various conditions has been unclear. In this work, the effectiveness was investigated in relation to tellurium species under various atmospheres and in the presence of cesium iodide. In addition, the effect of the chemical composition of the spray was also examined. The spray system was found to be relatively effective in all conditions tested. Moreover, the increase in chemical content of the spray solution increased the removal efficiency. \ua0After being removed from the containment atmosphere, the species, including various tellurium compounds, may enter the containment sump. Due to the complex chemistry of tellurium, it is difficult to predict the behavior under different redox conditions and especially under irradiation. This work therefore investigated the behavior of tellurium dioxide was investigated in simplified containment sump conditions in relation to dissolution, redox reactions and interactions with water radiolysis products. The results indicate that radiolysis products have a significant effect on tellurium chemistry in both reducing and oxidizing manner depending on the solution composition. The redox reactions also affect the solubility of tellurium both by increasing and decreasing it depending on the prevailing conditions. The results show that the current information used to assess tellurium source term needs to be re-evaluated for both severe accident management as well as for severe accident code validation purposes