1st quarter 2001 TheLos Actinide Alamos National Research Laboratory N u c l e a r M Quarterlya t e r i a l s R e s e a r c h a n d T e c h n o l o g y Researcher Provides a Historical Perspective for Plutonium Heat Sources In This Issue We begin the seventh year of Actinide Research Quarterly For more than 30 years, by focusing on Los Alamos has designed, major programs in 4 developed, manufactured, NMT Division. The Pit Manufacturing and tested heat sources for publication team Project Presents Many radioisotope thermoelectric also welcomes its Challenges generators (RTGs). These new editor, Meredith powerful little “nuclear “Suki” Coonley, who is 6 batteries” produce heat assuming the position Can Los Alamos from the decay of radioac- while Ann Mauzy Meet Its Future Nuclear tive isotopes—usually takes on acting Challenges? plutonium-238—and can management provide electrical power duties in IM-1. 9 and heat for years in Detecting and satellites, instruments, K.C. Kim Predicting Plutonium and computers. Aging are Crucial to continued on page 2 Stockpile Stewardship 12 Pit Disassembly and Conversion Address a ‘Clear and Present Danger’ 14 Publications and Invited Talks Newsmakers 15 Energy Secretary Spencer Abraham Addresses Employees artist rendering of Rover Pathfinder on Mars from NASA/JPL Nuclear Materials Technology/Los Alamos National Laboratory 1 Actinide Research Quarterly This article was contributed by Gary Rinehart (NMT-9) Early development efforts from the Each Multihundred Watt RTG provided about 157 mid-1960s through the early 1970s focused watts of power at the beginning of the mission. on fuel forms for space applications. The first, To meet the larger power requirements of plutonia-molybdenum-cermet (PMC), was the Galileo, Ulysses, and Cassini missions in used on the Pioneer 10 and 11 missions to the late 1970s, Los Alamos developed the Jupiter and Saturn and the Viking Lander General Purpose Heat Source (GPHS). This missions. PMC was fabricated by hot-pressing RTG contained 572 silicon germanium ther- microspheres of molybdenum-coated plutonia moelectric couples into hockey-puck-shaped discs, which were inside a thermoelectric then stacked and encapsulated in a converter and pro- refractory alloy. vided 285 watts of During the same period, Los Alamos electrical power at developed a medical-grade fuel for use in the beginning. cardiac pacemakers and early artificial hearts. The heat source The fuel in the artificial hearts was made of 90 for these RTGs con- percent enriched plutonium-238 and provided sisted of 18 GPHS up to 50 watts of power. Some of the early modules. Each module pacemakers are still in use; some have been had four fueled clads returned to Los Alamos’ Plutonium Facility and each cylindrical for recovery. fueled clad contained a hot-pressed 150- A General Purpose Heat gram pellet of Source (GPHS) fueled plutonium-238 dioxide clad. encapsulated in an iridium-tungsten container. Los Alamos fabricated the 216 GPHS fueled clads used on the Cassini mission. Each iridium clad contained a sintered iridium powder frit vent designed to release the helium generated by the alpha particle decay of the fuel. The iridium is compatible with Los Alamos developed As electrical power requirements for plutonium dioxide at temperatures greater a General Purpose spacecraft increased during the 1970s, so did than 1,773 K and melts at 2,698 K. Heat Source (GPHS) in the need to increase the power density of the In addition to developing RTGs to provide the late 1970s to meet heat source. Using “inert” materials such as electrical power to operate instruments, Los the larger power molybdenum proved unattractive. Los Alamos Alamos also designed and fabricated heater requirements of the began investigating fuel forms that used pure units to keep equipment operating in the Galileo, Ulysses, plutonia, which can generate more power in deep-freeze of space. The Light Weight and Cassini a smaller, lighter package. Radioisotope Heater Unit (LWRHU) provided space missions. The first pure plutonia fuel form one thermal watt of heat and was used on the developed at Los Alamos was the Galileo and Cassini spacecraft and Mars Multihundred Watt (MHW) for the RTGs used Pathfinder Rover. At the heart of this heater on the Voyager 1 and 2 missions to Jupiter, unit was a platinum-30 percent rhodium Saturn, Uranus, and Neptune. The heat source fueled clad containing a hot-pressed was a 100-watt hot-pressed sphere of plutonia 2.67-gram pellet of plutonium-238 dioxide. encapsulated in iridium. Twenty-four heat Los Alamos has also fabricated a large sources were contained within the RTG. Heat number of heat sources for the weapons was converted to electrical power by 312 program. From 1981 to 1990 more than 3,000 silicon-germanium thermoelectric couples. Milliwatt Generator (MWG) heat sources were 2 Nuclear Materials Technology/Los Alamos National Laboratory 1st quarter 2001 fabricated and shipped to the Pinellas, Fla., A significant amount of the heat source plant for assembly into the MWG-RTG. These fuel in the Los Alamos inventory, including the RTGs were used in devices that reduce the Russian material and fuel from disassembled possibility of accidental detonation from a terrestrial heat sources, must be purified nuclear warhead. Each of the Milliwatt before it can be fabricated into new heat Generator heat sources had a nominal power sources. Heat source fuel was previously of 4.0 to 4.5 watts. Los Alamos is currently recycled and purified at Savannah River. recovering fuel from excess MWG-RTGs and Los Alamos is qualifying a plutonium-238 performing shelf-life and stockpile surveil- aqueous recovery lance on both the MWG heat source and the process in its pluto- MWG-RTG. See Actinide Research Quarterly, nium facility to Winter 1994, “Milliwatt Surveillance Program provide feed for the Ensures RTG Safety and Reliability.” heat-source fabrica- Historically, reactors at Savannah River tion process. produced all of the United States’ plutonium- A new glove-box 238 until the late 1980s. With the shutdown of line is being installed the Savannah River reactors, the United States for this work and the has had to purchase plutonium-238 dioxide full-scale aqueous from Russia to supplement U.S. inventories recovery process is for future NASA space missions. Recently, the expected to become DOE has proposed that plutonium-238 be pro- operational this fiscal duced at the Advanced Test Reactor (ATR) in year, which will have the capacity to purify A Light Weight Idaho and the High Flux Isotope Reactor 5 kilograms of plutonium-238 per year. In Radioisotope Heater (HFIR) at Oak Ridge National Laboratory. addition, Los Alamos has the capability of Unit (LWRHU) before fabricating more than 5 kilograms of final assembly. The heat source consists of a plutonium-238 into heat sources per year. graphite aeroshell, These improvements to the plutonium pyrolytic graphite facility will ensure that Los Alamos can fully thermal insulators, and support future NASA requirements. The a platinum-rhodium Europa Orbiter mission, scheduled for launch fueled clad. A sintered in 2006, and the Solar Probe mission, scheduled platinum powder frit vent for launch in 2007, will each require approxi- is electron-beam-welded mately 3 kilograms of plutonium-238 if a into the end cap of the Stirling radioisotope power system were fueled clad to release used. If RTGs are used, approximately helium generated by the 8 kilograms of plutonium-238 will be alpha decay of the fuel. required for each mission. Over the next decade, several Mars exploration missions will carry LWRHUs, which will require approximately 0.3 kilogram of plutonium-238 per mission. In the long term—the next 20 to 35 years—NASA is expected to need 2 to 5 kilograms of plutonium-238 per year for its space missions.I The Mars Pathfinder Rover contained three Light Weight Radioisotope Heater Units (LWRHUs). Nuclear Materials Technology/Los Alamos National Laboratory 3 Actinide Research Quarterly Pit Manufacturing Project Presents Many Challenges This article was contributed by David Mann and Sean McDonald, Central to the function of a thermonuclear final assembly of the nuclear and nonnuclear NMT-5 weapon is a plutonium pit, the trigger that ini- components. Complicating matters are the tiates the sustained nuclear reaction. Los changes that have had to be made in a substan- Alamos’ experience in making pits dates back tial number of these processes because of the more than 50 years, to the Manhattan Project. unavailability of the original equipment, Since that time, the Lab has produced a small the need to comply with new environmental number of pits—about a dozen a year—for regulations, or the ability to take advantage research purposes and underground testing of improved equipment. in Nevada. By comparison, the number of pits NMT has faced a number of challenges produced yearly at the now-closed Rocky in setting up these capabilities. The first and Flats defense plant near Denver was in probably most significant challenge has been the thousands. to set up a formal manufacturing operation— In 1993, the Department of Energy (DOE) with all its controls, infrastructure, and requested that Los Alamos develop the manu- restrictions—in an existing research and facturing capabilities to produce war-reserve development facility, and to do so without pits as part of the stockpile stewardship pro- completely disrupting the existing programs. gram. The designation “war reserve” means To manufacture pits to the same specifications that the components meet the specifications as Rocky Flats, each of the more than 100 pro- required for use in a stockpiled nuclear cesses involved has had to be installed, tested, weapon. Los Alamos was chosen in part and proven to function as it did at Rocky Flats.