Roger G. Miller

In situ strain and temperature measurement and modelling during arc welding

Abstract Experiments and numerical models were applied to investigate the thermal and mechanical behaviours of materials adjacent to the weld pool during arc welding. In the experiment, a new high temperature strain measurement technique based on digital image correlation (DIC) was developed and applied to measure the in situ strain evolution. In contrast to the conventional DIC method that is vulnerable to the high temperature and intense arc light involved in fusion welding processes, the new technique utilised a special surface preparation method to produce high temperature sustaining speckle patterns required by the DIC algorithm as well as a unique optical illumination and filtering system to suppress the influence of the intense arc light. These efforts made it possible for the first time to measure in situ the strain field 1 mm away from the fusion line. The temperature evolution in the weld and the adjacent regions was simultaneously monitored by an infrared camera. Additionally, a thermal–mechanical finite element model was applied to substantiate the experimental measurement.

Considerations in the fabrication, assembly, and testing of radioisotopic thermo‐photovoltaic (RTPV) generators for future space missions

To increase energy output with a smaller size and mass than the radioisotopic thermoelectric generators (RTGs) that were previously used on deep space missions, a radioisotopic thermophotovoltaic (RTPV) system is being developed for the ‘‘Pluto Express’’ flyby mission. To minimize cost and development time, some facilities and components currently used for RTG production can be used to produce RTPVs. Production options also include out‐sourcing and use of off‐the‐shelf hardware. Necessary modifications to tooling, production equipment, testing and shipping methods can be achieved in a timely manner so that the RTPV will be ready well before the planned launch of ‘‘Pluto Express.’’

Comparison of Measurement Techniques for Determining the Thermal Emittance of Coupons at Elevated Temperatures

The development of a highly efficient nuclear space power system requires that all of the available thermal energy emitted from the General Purpose Heat Source (GPHS) modules (∼250 thermal watts per module) be utilized in the most efficient manner. This includes defining the heat transfer/thermal gradient profile between the surface of a GPHS module and the surface of the selected converter’s hot end. Control of the radiant heat transfer between the two surfaces can be achieved by regulating how efficiently the converter’s hot end surface transfers heat compared to a perfect blackbody (i.e. its infrared emittance). By oxidizing and/or grit blasting the surface of candidate converter materials it is possible to increase their emittance. L‐605 test specimens were WC grit blasted and heat treated at 1023K for 72hours in air and their emittance values at elevated temperatures up to ∼1000K were determined using three different measurement techniques (Infrared Camera, Infrared Reflectometer, and a Calorimetric ...

Welding of unique and advanced alloys for space and high-temperature applications: welding and weldability of iridium and platinum alloys†

In the last five decades, significant advances have been made in developing alloys for space power systems for spacecraft that travel long distances to various planets. The spacecraft are powered by radioisotope thermoelectric generators (RTGs). The fuel element in RTGs is plutonia. For safety and containment of the radioactive fuel element, the heat source is encapsulated in iridium or platinum alloys. Ir and Pt alloys are the alloys of choice for encapsulating radioisotope fuel pellets. Ir and Pt alloys were chosen because of their high-temperature properties and compatibility with the oxide fuel element and the graphite impact shells. This review addresses the alloy design and welding and weldability of Ir and Pt alloys for use in RTGs.

Thermodynamic Prediction of Compositional Phases Confirmed by Transmission Electron Microscopy on Tantalum‐Based Alloy Weldments

Tantalum alloys have been used by the U.S. Department of Energy as structural alloys for radioisotope based thermal to electrical power systems since the 1960s. Tantalum alloys are attractive for high temperature structural applications due to their high melting point, excellent formability, good thermal conductivity, good ductility (even at low temperatures), corrosion resistance, and weldability. Tantalum alloys have demonstrated sufficient high‐temperature toughness to survive prolonged exposure to the radioisotope power‐system working environment. Typically, the fabrication of power systems requires the welding of various components including the structural members made of tantalum alloys. Issues such as thermodynamics, lattice structure, weld pool dynamics, material purity and contamination, and welding atmosphere purity all potentially confound the understanding of the differences between the weldment properties of the different tantalum‐based alloys. The objective of this paper is to outline the th...

Thermal emittance measurements on candidate refractory materials for application in nuclear space power systems

The development of a highly efficient General Purpose Heat Source (GPHS) space power system requires that all of the available thermal energy from the GPHS modules be utilized in the most thermally efficient manner. This includes defining the heat transfer/thermal gradient profile between the surface of the GPHS’s and the surface of the energy converter’s hot end whose geometry is dependent on the converter technology selected. Control of the radiant heat transfer between these two surfaces is done by regulating how efficiently the selected converter’s hot end surface can reject heat compared to a perfect blackbody, i.e. its infrared emittance. Several refractory materials of interest including niobium-1% zirconium, molybdenum-44.5% rhenium and L-605 (a cobalt-based alloy) were subjected to various surface treatments (grit blasting with either SiC or WC particulates) and heat treatments (up to 1198 K for up to 3000 hours). Room temperature infrared emittance values were then obtained using two different i...

Compatibility issues of potential payloads for the USA/9904/B(U)F-85 RTG transportation system (RTGTS) for the “Pluto Express” mission

The specific electric power system for the “Pluto Express” mission has yet to be specified. However, electric power will be provided by either radioisotopic thermoelectric generators (RTG), radioisotope thermophotovoltaic systems (RTPV), alkali metal thermal to electrical conversion (AMTEC) systems, radioisotope Stirling systems, or a combination of these. The selected radioisotopic power system will also be transported using the USA/9904/B(U)F-85, Radioisotope Thermoelectric Generator (RTG) Transportation System (RTGTS). As a result, all of the potential payloads present uniquely different environmental and physical configuration requirements. This paper presents the major compatibility issues of the potential payloads for the USA/9904/B(U)F-85 RTG Transportation System for the “Pluto Express” mission.

Logistical concepts associated with international shipments using the USA/9904/B(U)F RTG Transportation System (RTGTS)

Over the last 30 years, radioisotopes have provided heat from which electrical power is generated. For space missions, the isotope of choice has generally been 238PuO2, its long half-life making it ideal for supplying power to remote satellites and spacecraft like the Voyager, Pioneer, and Viking missions, as well as the recently launched Galileo and Ulysses missions, and the presently planned Cassini mission. Electric power for future space missions will be provided by either radioisotopic thermoelectric generators (RTG), radioisotope thermophotovoltaic systems (RTPV), alkali metal thermal to electrical conversion (AMTEC) systems, radioisotope Stirling systems, or a combination of these. The type of electrical power system has yet to be specified for the “Pluto Express” mission. However, the current plan does incorporate the use of Russian launch platforms for the spacecraft. The implied tasks associated with this plan require obtaining international certification for the transport of the radioisotopic p...

Pelletization and encapsulation of general purpose heat source (GPHS) fueled clads for future space missions

Mankind must continue to explore the universe in order to gain a better understanding of how we relate to it and how we can best use its resources to our benefit. Because of the significant costs of this type of exploration, it can more effectively be accomplished through an international team effort. This unified effort must include the design, planning, and execution phases of future space missions, extending down to such activities as the processing, pelletization, and encapsulation of the fuel that will be used to support the spacecraft electrical power generation systems. Over the last 30 years, radioisotopes have provided heat from which electrical power is generated. For space missions, the isotope of choice has generally been 238PuO2, its long half‐life making it ideal for supplying power to remote satellites and spacecraft like the Voyager, Pioneer, and Viking missions, as well as the recently launched Galileo and Ulysses missions, and the presently planned Cassini mission. Electric power for future space missions will be provided by either radioisotopic thermoelectric generators (RTG), radioisotope thermophotovoltaic systems (RTPV), radioisotope Stirling systems or a combination of these. However, all of the aforementioned systems will be thermally driven by General‐Purpose Heat Source (GPHS) fueled clads in some configuration. Each GPHS fueled clad contains a 150‐gram pellet of 238PuO2, and each pellet is encapsulated within an iridium‐alloy shell. Historically, the fabrication of the iridium‐alloy shells has been performed at EG&G Mound, and Oak Ridge National Laboratory, and the girth welding of the GPHS capsules has been performed at Westinghouse Savannah River Corporation, and Los Alamos National Laboratory. This paper describes a cost effective alternative method for the production of GPHS capsules. Fundamental considerations such as the potential production options, the associated support activities, and the methodology to transport the welded fueled clads are discussed.