The Space Congress® Proceedings 1967 (4th) Space Congress Proceedings Apr 3rd, 12:00 AM An Undersea Radioisotope Power Supply H. C. Carney Aerojet-General Corporation Follow this and additional works at: https://commons.erau.edu/space-congress-proceedings Scholarly Commons Citation Carney, H. C., "An Undersea Radioisotope Power Supply" (1967). The Space Congress® Proceedings. 4. https://commons.erau.edu/space-congress-proceedings/proceedings-1967-4th/session-18/4 This Event is brought to you for free and open access by the Conferences at Scholarly Commons. It has been accepted for inclusion in The Space Congress® Proceedings by an authorized administrator of Scholarly Commons. For more information, please contact [email protected]. AN UNDERSEA RADIOISOTOPE POWER SUPPLY H. C . Carney Aerojet-General Corporation San Ramon, California St.mnnary The results and current status of an Aerojet­ cable repeaters, etc to support expanding ocean sponsored program to develop a product line of low­ engineering activities . The first lead shield, power radioisotope thermoelectric generators for radioisotope-fueled URIPS has been fabricated and marine applications are discussed . A 1-watt( e) tested, and was publicly exhibited at the Off­ Undersea Radioisotope Power Supply ( URIPS) has been shore Exploration Conference (OECON ) at Long designed, fabricated, and tested . Ex tensive para­ Beach, California in February of this year . A metric studies were performed to select optimum de­ second lead shield URIPS has been purchased by sign characteristics; typical parameters investi­ the Navy and, following qualification testing, gated wer e : r adioisotope and chemical fuel form, will be delivered to the Naval Civil Engineering thermoel ectr ic material, fuel capsule L/D ratio, Laboratory at Port Hueneme, California this s h iel d materia l and geometry , and operating condi­ sununer. A compact, lightweight uranium-shield tions . The design emphasizes high reliability and URIPS is currently in the final fabrication stage, low cost, but considerable emphasis was placed on and will be subjected to long term endurance test­ adaptability to meet a wide spectrum of user re­ ing at Aeroj et. quirements . URIPS is designed to provide a steady power output for a minimum of five years at ocean Design Considerations depths up to 20 , 000 feet . An RTG consists, basically , of an encapsu­ Introduction lated radioisotope heat source, thermal insulation, biological shielding, a thermoelectric converter, In 1965 , Aerojet-General Corporation initiated a power conditioner, and various structural com­ a company-sponsored program to develop a product ponents. line of low -power radioisotope thermoelectric gener­ a tors (RTG) for marine applications . The initial Thermal energy from the radioi sotope heat objective of this Undersea Radioisotope Power Supply source is converted into low voltage d -c electri­ Program was to design , fabricate, and test a highly cal power by the thermoelectric module . Thermal reliable power supply capable of providing a continu­ insulation is required to minimize parasitic heat ous output of 1 watt(e) for a minimum of 5 years in loss. Radiation originating from the radioactive ocean depths up to 20,000 feet . decay process requires the provision of a biologi­ cal shield. A power conditioning sub-system is Conventional primary chemical batteries, al­ required to transform the low voltage, low current though less costly than an RTG have an endurance output of the thermoelectric module to the higher limit of sev eral y ears; RTG systems have demonstra­ potential required by the load . Other functions ted the capability for 5 years of operating life of the power conditioner are to regulate the out­ and show promise of satisfying 10 to 20 year re­ put voltage and power, by compensating for the quirements . Optimum use of an RTG is in missions fluctuation over the mission lifetime associated requiring a long-term unattended electrical power with the decay of the radioisotope and with the supply . For example , the longer life of the RTG degradation in performance of the thermoelectric more than offsets the lower capital cost of con­ module. The electrical power can be used to ventional bat teries in cases where: charge a secondary battery system , or, alternate­ ly, an energy storage capacitor can be used , if 1 . Costs associated with battery replace- required, to provide a low impedance source for a ment are high, as in remote areas where access is pulsing lamp or transducer load. difficult . The interrelationship between the design 2 . A complete battery-powered system must variables associated with each of the components be periodically replaced because the system is not discussed above must be defined and systematical­ recoverable, resulting in high capital costs . ly evaluated within the framework of established design goals and criteria to arrive at an optimum 3 . Power interruptions or physical dis- RTG design. The major URIPS design goals were low turbance during periodic replacement operations cost, and high reliability . The design criteria, are unacceptable . while of similar importance but which are in some cases subject to overriding constraints. imposed URIPS was developed to satisfy the indicated by the design goals, were: requirement for small , economical radioisotope t hermoelectric generators suitable for powering scientific instrumentation, acoustic transponders, 18-33 60 1. Minimum size and weight. shield thickness required for Co is approximate­ ly twice that for sr90 for this low power system, 2. Optimum design for a power range capa- which results in a considerably heavier shield and bility from 150 milliwatts to 5 watts. more costly system. The titanate fuel form of strontium (SrTi03) was selected because of its ad­ 3. Adaptable to a wide variety of appli- vanced development status, availability in suffi­ cations with minimum design modifications. cient quantity, and relatively low solubility in water, which is advantageous from the hazards 4. Minimum unattended lifetime of 5 years standpoint. and potential capability of 10 to 20 years. Thermoelectric Material 5. Capable of immersion in sea water at depths up to 20,000 feet. A large number of thermoelectric materials were evaluated for DRIPS application (see Table 1). 6. Capable of withstanding all forseeable System electrical efficiency, ~E' which includes environmental conditions during storage, transpor­ both the thermoelectric module and power condition­ tation, handling, and operation without mechanical er efficiency, was approximated to provide a tem­ or electrical damage or degradation in rated per­ perature-dependent analytic function for the TEHE- formance. 3 code by using an effective (psuedo) thermo­ electric figure of merit, ZE, in the basic thermo­ 7. Capable of complying with all safety electric efficiency equation for matched load con­ regulations specified in applicable local, state ditions: and federal ordinances, laws, and codes concerning manufacture, transportation, handling, testing, operation, and disposal. ~E T -T 2T + ~ - H c Design Selection H ZE -2- An extensive parametric analysis of RTG de­ where : sign characteristics and operating parameters was performed during the conceptual design phase of hot junction temperature (°K) . the DRIPS program, to select optimum design cold junction temperature (°K) features consistent with the previously described 0 1 goals and criteria. The results of the studies effective figure of merit ( c- ) concerned with fuel, shielding, thermoelectric selection, and materials, and thermal insulation The effective figure of merit characteristic of are sununarized be­ overall RTG system optimization each thermoelectric material ( see Table 2) was were performed using low. The parametric analyses determined by computing detailed performance maps by Aerojet an Im-1 7094 computer program developed relating electrical efficiency to thermoelectric Large Milliwatt under the AEC sponsored Advanced operating parameters and design characteristics. Program.l This Thermoelectric Heater Generator These computer programs are also described in (TEHE-3) code optimiz;s the pe;formance ~aluation Reference 1. Typical results, showing the system of an RTG and calculates the corresponding physi­ diameter, weight, and efficiency as a function of and associated costs. cal characteristics ZE for a 1-watt(e) and 150 milliwatt(e) RTG are shown in Figures 1 and 2. Radioisotope Fuel TABLE 1 All radioisotopes currently under develop­ ment by the AEC for heat source applications were CANDIDATE THERMOELECTRIC MATERIALS evaluated. In consideration of the design cri­ FOR DRIPS APPLICATIONS a minimum unattended lifetime of teria requiring Bismuth Telluride, N/P-Type (Asarco) 5 years (and the capability for as long as 20 years), the candidate radioisotopes were restric ­ Lead Telluride, N-Type (MMM -T EGS-3N) with half-lives in excess of 5 years; ted to those Lead-Tin-Telluride, P-Type (MMM-TEGS-3P) these included Co60, sr90, csl37, Pu238, and Cm244 Cobalt-60 and strontium-90 proved to be Lead Telluride (MMM-TEGS-2N/2P) for DRIPS application on the the most competitive MCC 40, N/P-Type (Monsanto) basis of fuel cost and availability. Strontium-90 was selected since it results in the lowest over­ Silicon Germanilllll Alloy, N/P-Type (RCA) weight, and size. Most of the all system cost, Cupron Special (Wilbur B. Driver) Co-60 decay energy is released as gamma photons, and since these are penetrating radiations, much Tophel Special (Wilbur B.