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A lightweight, high performance nuclear reactor power system can offer significant advantages for many space missions. Conceptual design has been completed for the SP-100, a system which utilizes many thermoelectric converters and is capable of delivering 100 kilowatts of electrical power. A reference design, using thermoelectric materials with an average figure of merit of 0.001/K and a reactor heat pipe temperature of 1500 K, is presented which has a mass of 2280 kg not including contingency. The sensitivity of system mass to changes in the configuration and thermoelectric material properties are presented

The successful utilization of a radioisotope thermoelectric generator (RTG) as the power source for spaceflight missions requires that the performance of such an RTG be predictable throughout the mission. Several mechanisms occur within the generator which tend to degrade the performance as a function of operating time. The impact which these mechanisms have on the available output power of an RTG depends primarily on such factors as time, temperature and self-limiting effects. The relative magnitudes, rates and temperature dependency of these various degradation mechanisms have been investigated separately by coupon experiments as well as 4-couple and 18-couple module experiments. This paper discusses the different individual mechanisms and summarizes their combined influence on the performance of an RTG. Also presented as part of the RTG long-term performance characteristics is the sensitivity of the available RTG output power to variations of the individual degradation mechanisms thus identifying the areas of greatest concern for a successful long-term mission.

A large number of planetary missions under consideration for the next two decades will be using radioisotope thermoelectric generators to provide the necessary spacecraft electrical power. Performance, reliability, lifetime and cost of these RTGs are expected to be more demanding and will require the development of advanced thermoelectric materials. An advanced thermoelectric materials development program is being conducted by JPL and is the subject of this paper. Under this program, alloys of rare earth chalcogenides have been synthesized and their thermoelectric properties evaluated. The selection of specific compounds on which the program is currently concentrating is influenced by criteria such as low thermal conductivity, reasonably high operating temperatures (not less than 1000 C) and stability of the compounds. The paper discusses the rationale, approach and current status of the program. In addition, the thermoelectric and thermophysical property measurement used to evaluate the materials are described. The preliminary results and analysis of these experimental data are also given.

A low specific weight (20 to 40 Kg/Kwe), long life (6 to 10 year), 100 to 400 KWe nuclear electric propulsion (NEP) system can deliver 2,000 to 10,000 Kg payloads for intensive study of our solar system. The nuclear power source is about 80% of the NEP system mass, thus the NASA program is focused on developing the power source technology. Because of the long life requirements, direct thermal to electric energy conversion technology (thermionic and thermoelectric) is being pursued. In order to meet the low specific weight it is necessary to develop a 10 to 15% conversion module with a 875 K minimum heat rejection temperature and a 1675 K maximum input temperature. The thermoionic converter is about 9% efficient at these temperatures. The thermoelectric converter is limited to a 1300 K input temperature. So, the thermionic program is focused on improving the efficiency while the thermoelectric program is focused on increasing the temperature.

Several thermoelectric generators ranging in output power from 0.5 to 155 W have been completed or are undergoing testing at JPL. These generators represent a wide range of technologies, using Bi2Te3, PbTe and SiGe thermoelectric materials. Several of these generators are of a developmental type, such as HPG S/N2, and others are representative of Transit and Multi-Hundred Watt (MHW) Technology. Representative flight performance data of LES 8/9 and Voyager RTG's are presented and compared with the DEGRA computer program based on the data observed from tests of SiGe couples, modules and MHW generators.

Results are presented for experimental and analytical investigations of the overall performance of a selenide radioisotope thermoelectric generator intended to provide the electrical power for interplanetary spaceprobes such as the Galileo mission to Jupiter. The discussion focuses on technology areas of concern, electrical properties of the selenide thermoelectric materials used, and thermal conductivity of these materials for superior performance. It is shown that the selenide thermoelectric materials offer the advantage of high conversion efficiency. The long-life requirement on the power system for the Galileo mission necessitates proper design, known fabrication techniques, and reproducible assembly techniques in order to ensure stability of the thermoelectric properties. However, the thermophysical properties - sublimation and creep - of the p-material remains an area of considerable concern.

The paper is concerned with the evaluation of the materials used in a selenide radioisotope thermoelectric generator (RTG). These materials are composed of n-type gadolinium selenide and n-type copper selenide. A three-fold evaluation approach is being used: (1) the study of the rate of change of the thermal conductivity of the material, (2) the investigation of the long-term stability of the material's Seebeck voltage and electrical resistivity under current and temperature gradient conditions, and (3) determination of the physical behavior and compatibility of the material with surrounding insulation at elevated temperatures. Programmatically, the third category of characteristic evaluation is being emphasized.