To Mars and Beyond

Los Alamos expertise in plutonium-238 heat-source production will play an important role in a new NASA initiative

NASA has announced a new research program that will propel deep-space exploration through the coming decades. Plutonium-238 and Los Alamos will play an important role in the initiative, which will rely on radioisotope heat sources fabricated at Los Alamos to supply power and heat for missions to Mars and beyond.

NASA said the initiative will enable sophisticated mobile laboratories to travel over the surface of Mars, drilling deep underground at promising sites where signs of life can be sought, and conduct a large variety of other experiments day and night, around the clock.

Radioisotope Thermoelectric Generators (RTGs) fueled by plutonium-238 have been used to provide electrical power for spacecraft since 1961. In addition, low-power Light Weight Radioisotope Heater Units (LWRHUs) have been used to maintain spacecraft equipment within their normal operating temperatures. These RTGs and LWRHUs are in two dozen spacecraft, including Pioneer, Voyager, Galileo, Cassini, and Mars Pathfinder. (For more information on the historical perspective on plutonium-238 heat sources, see ARQ, 1st Quarter, 2001.)

Los Alamos has the only plutonium-238 scrap recovery, fuel processing, and analysis capabilities in the United States. Until the late 1980s, plutonium-238 was produced and purified in reactors at Savannah River. Those reactors have been shut down, so the aqueous scrap recovery process of heat-source materials is now performed at Los Alamos' Plutonium Facility at TA-55. For the next two decades, it is estimated that Los Alamos will produce two to eight kilograms of plutonium-238 fuel per year to meet the needs of NASA's space applications. Currently, the aqueous scrap recovery process is being done in a bench-scale operation. A full-scale aqueous scrap recovery glove-box line is expected to become operational later this year. The Plutonium-238 Science and Engineering Group (NMT-9) oversees the plutonium-238 recovery and heat-source fabrication operation.

To make a general-purpose heat source (GPHS) unit or LWRHU, researchers at TA-55 recover plutonium-238 fuel from old, disassembled heat sources. The scrap is purified to remove decay products (mainly uranium-234) and other impurities from the plutonium dioxide. The resulting purified oxides are formed into fuel pellets and encapsulated to form fueled clads.

Before these fueled clads can be used in GPHSs or LWRHUs, a set of chemical and physical parameters must be met during purification and fabrication steps. Los Alamos has full capabilities to determine these parameters for heat-source production and provide analysis for safety impact testing. These capabilities include chemical analyses, neutron emission-rate measurements, particle size determination, calorimetric measurements, helium leak tests, metallography and ceramography, ultrasonic weld examinations, and radiography.

After a fuel pellet is hot-pressed, it is encapsulated in an iridium alloy container and subjected to ultrasonic immersion testing to examine the integrity of the weld. These images show the test results of conforming (top) and nonconforming (middle) welds of fueled clads. The green area in the nonconforming weld's data indicates a possible defect. The bottom table shows the magnitude of the signal reflected in the test. The black line is the trace of the magnitude illustrated by color in the middle image, and the blue line is the trace of the magnitude in the top image. If a fueled clad has ultrasonic reflectors in excess of a set level, it is inspected with radiography to determine whether the heat source is acceptable for flight use. Rejected heat sources are recycled, and the plutonium oxide recovered from them is used to fabricate new fuel pellets.

The full-scale aqueous scrap recovery operation will also include gamma-based measurement equipment (a plutonium process monitoring-or PPM-system) and a solution in-line alpha counter (SILAC). These technologies will be used to monitor the ion-exchange process and alpha concentration of solutions inside the glove-box line.

Chemical analysis capabilities

Researchers need chemical data on plutonium-238 samples (feed oxides, purified oxides, granular plutonium-238, and process solutions) to establish the necessary baseline parameters and measurements for process control, material control and accountability, waste disposal, and product certification. To acquire the data, members of the Actinide Analytical Chemistry Group (C-AAC) perform chemical analyses at laboratories located in the Chemistry Materials and Research (CMR) Building.

The chemical analysis begins by dissolving plutonium material in concentrated hydrochloric and hydrofluoric acids using the sealed- reflux procedure. The sealed-reflux dissolution method allows researchers to dissolve high-fired fuel at a temperature of 150 to 200 degrees Celsius and a pressure of 50 to 115 pounds per square inch. Spectroscopy is used to determine the purity of the resulting plutonium oxide. If the expected plutonium content is low (one microgram per gram or less), gross alpha counting is used to calculate the plutonium-238 content.

Analyses of actinide impurities, including uranium-234, americium-241, neptunium-237, and plutonium-236, are performed by radiochemical methods (gross alpha and gamma counting, gamma and alpha spectroscopy, and radionuclide separations). The plutonium isotopic composition is determined by thermoionization mass spectrometry. Direct-current arc and inductively coupled plasma mass spectrometry techniques are used to determine nonactinide cationic and anionic impurities.

Because of material-at-risk issues at the CMR Building, Los Alamos has begun consolidating plutonium-238 operations. The majority of the plutonium-238 chemical analysis capabilities will be moved into the TA-55 Plutonium Facility within the next several years.

Physical measurement capabilities

Spontaneous fission of plutonium-238 produces approximately 2,220 neutrons per second per gram. Energetic alpha particles react with light isotopes, producing even higher neutron emission rates. To reduce the neutron emission rate, researchers treat the purified oxide with oxygen-16 exchange to reduce oxygen-17 and -18 in the products. Researchers then measure the neutron emission rate of the oxide using a thermal neutron counter.

Calorimetry is used to determine the power output of heat-producing materials. Plutonium-238 has a half-life of 87.74 years and a power output of 0.567 watts per gram. Several types of calorimeters are used at TA-55 to measure low-wattage (up to five watts) and high-wattage (up to 200 watts) fuel. A typical GPHS fueled clad contains approximately 150 grams of plutonium-238 oxide and has 61 to 62 watts of power.

An LWRHU contains 2.67 grams of plutonium-238 oxide and has a nominal heat output of one watt. Calorimetric measurements are also used to perform material accountability measurements of plutonium-238.

A plutonium-238 GPHS fuel pellet is hot-pressed and encapsulated in an iridium alloy container with a weld shield. Each iridium clad contains a sintered iridium powder frit vent designed to release the helium generated by the alpha particle decay of the fuel. Ultrasonic immersion testing is performed to examine the weld integrity of the plutonium-fueled clad. If a fueled clad has ultrasonic reflectors in excess of the reject specification level, it is inspected with radiography. This additional engineering data determines whether the heat source is accepted or rejected for flight use.

Heat sources must be designed and constructed to survive impact. NMT-9 can determine the particle-size distribution of fines—small particles that are broken off from the fuel pellet during the impact test— recovered from impact tests that are less than 100 microns in diameter. Impact tests on plutonium-238 and simulant fueled clads are conducted to determine the response to probable launch accident scenarios of bare fueled clads and of GPHS modules with up to four clads each.

Particle-size analysis is also used to verify the particle size of milled oxides before dissolution. NMT-9 has equipment that can measure particles ranging in size from 0.5 to 600 microns.

These images show the microstructure of conforming (top) and nonconforming (bottom) welds of clad material. Metallographic examinations are performed on test components to determine possible failure mechanisms. The black spots in the nonconforming weld are bubbles or voids in the weld itself. These are sometimes formed during the welding process when the metal is molten. Voids adversely affect the integrity of the weld, and here they make the weld nonconforming, or unacceptable. The nonconforming weld was identified by ultrasonic testing, and verified by radiography and metallography. The ultrasonic and radiographic examinations indicated the location of the flaw in the nonconforming weld, allowing researchers to precisely section the sample at that location to reveal the microstructure of the nonconforming area. Researchers at TA-55 use a LECO 300 metallograph with a magnification range up to 500 times actual size.

Researchers use metallographic and ceramographic examinations on components recovered from impact tests. The microstructure of the clad material, girth welds, and samples of fuel pellets are also examined. A metallograph with a magnification range up to 500 times actual size is interfaced to the glove-box line through a unique hood extension.

In-line process monitoring capabilities

In the full-scale plutonium-238 aqueous recovery operation, a PPM system will be used to monitor americium, uranium, and plutonium gamma rays during the ion-exchange process. This PPM system will provide real-time elution profiles of actinide impurities that will help in reducing solution waste volume and will provide a monitoring tool during washing and elution steps.

In addition, a SILAC will be used for monitoring the alpha activity in hydroxide filtrate in the full-scale residue polishing operations. By knowing the approximate alpha concentration, an operator can adjust the operating parameters of the ultrafiltration process to maximize the removal of plutonium and uranium from the residue solutions before discharging them into the Laboratory’s Liquid Waste Treatment Facility.

To help meet its goals, NASA is depending on Los Alamos and the Plutonium Facility's unique capabilities to process and fabricate plutonium-238 into heat sources. Several Mars and deep-space exploration programs that require plutonium-238 heat sources are currently ongoing or under development. For example, the twin Mars Exploration Rovers that are being prepared for launch in the summer of 2003 are expected to carry several plutonium-238 LWRHUs on board. Support for the Laboratory's heat source work is provided by DOE's Office of Space and Defense Power Systems.

This article was contributed by Amy Wong, MaryAnn Reimus, Paul Moniz, and Gary Rinehart of Plutonium-238 Science and Engineering (NMT-9).

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