NMT-9 Recycles Heat Source Fuel

Los Alamos has been involved in the fabrication of heat sources to power experiments for space exploration since 1979. With the shutdown of the Savannah River reactors, the United States has purchased ²³⁸PuO₂ from Russia to supplement US heat source material for NASAšs future space missions. These missions include the Pluto/Kuiper Express, Europa/Orbiter, Solar Probe, and a number of Mars surface science and sample return missions. A significant fraction of the heat source fuel in the Los Alamos inventory, including the Russian material and fuel from disassembled milliwatt heat sources, requires purification before it can be fabricated into new heat sources.

Heat source fuel was previously recycled and purified in facilities at Savannah River. For future missions, members of the Power Source Technologies Group (NMT-9) are currently qualifying a ²³⁸Pu aqueous recovery process to provide feed for the heat source fabrication process. A new glove box line is currently being installed for this work, and the full-scale aqueous recovery process is expected to become operational in FY01.

Figure 1. Welding a capsule of ²³⁸PuO₂ fuel to power experiments in space exploration. The Power Source Technologies Group is currently quantifying a ²³⁸Pu aqueous recovery process to provide feed for the heat source fabrication process.

The impure oxide is first dissolved in a nitric/hydrofluoric acid solution and then follows one of two routes for purification. If the feed contains only impurities that can be successfully removed by oxalate precipitation, the solution is reduced to Pu(III) using hydroxylamine nitrate as the reductant and sulfamic acid or urea as a holding agent. This solution is precipitated as plutonium oxalate by adding oxalic acid to the solution.

If the feed contains impurities that cannot be removed by oxalate precipitation, the solution is oxidized to Pu(IV) with sodium nitrite and loaded onto a nitrate anion exchange resin (Reillex-HPQ). The eluate from this process is reduced to Pu(III) and precipitated as plutonium oxalate. Plutonium oxalate is converted to purified plutonium oxide by calcining the oxalate in the presence of oxygen enriched in 16O at 750°C. The use of 16O is very important in the processing of ²³⁸PuO₂. The 17O and 18O that are present in atmospheric oxygen have large (a,n) cross sections. The neutron emission rate of ²³⁸PuO₂ fabricated from atmospheric oxygen has a neutron emission rate of about 16,400 n/s/g ²³⁸Pu (13,700 n/s from atmospheric oxygen and 2,700 n/s from spontaneous fission of ²³⁸Pu). During the calcining process the 17O and 18O atoms are exchanged with 16O atoms leading to a neutron emission rate as low as 4000 n/s/g ²³⁸Pu. This lower neutron emission rate reduces personnel radiation exposure and lowers the risk of radiation damage to sensitive electronic equipment on the spacecraft.

Effluents and filtrates from the aqueous processing steps are precipitated as hydroxide cakes, and the hydroxide filtrates undergo ultrafiltration/polymer filtration before they are sent to TA-50 for disposal. The pure oxide feed from the aqueous processing is formed into sintered granules for most terrestrial heat sources and into hot-pressed pellets for space heat sources. Most terrestrial heat sources are designed to contain the helium that builds up as a result of alpha decay. The terrestrial heat sources use a granular fuel that has a relatively low power density. They have significant free space within the capsule to accommodate the helium inventory and are heavy-walled and usually triply encapsulated to withstand the helium pressure. Space heat sources need to have high power-to-mass ratios because of the high cost of launching items into space. These heat sources are normally singly encapsulated and contain vents to let helium escape to the atmosphere.

Fabrication steps involved in forming the oxide feed into a completed 62-Wth general-purpose heat source (GPHS) or a light-weight radioisotope heater unit (LWRHU) are nearly identical. The most important step in obtaining the desired microstructure in the hot-pressed pellet is the sintering temperature of the less than 210-mm diameter granules. A total of 60% of the granules are sintered at 1100°C and the remainder at 1600°C. During the hot pressing at 1500°C, the high-fired, non-reactive granules form a microstructural skeleton, around which the more active low-fired granules sinter during pressing and post-press sintering. The graphite die that is used to contain the fuel during the hot pressing reduces the fuel to a stoichiometry of about PuO_1.9. The post-press sintering of the pellet at 1527°C oxidizes the fuel to PuO_2.00 and increases the grain size within the pellet to about 20-30 mm. This larger grain size helps prevent the formation of respirable fines during a launch accident. The sintered pellet has a density that is 85% of theoretical density and has connected porosity to assist the release of helium from its interior.

Figure 2. The LWRHU assembly consists of the fueled capsule, three layers of pyrolytic graphite thermal insulation, and the outer, impact-resistant graphite fiber aeroshell. Three of these heat sources were recently used to keep electronics at normal operating temperature on the Mars Pathfinder rover, and 117 are on their way to Saturn on the Cassini spacecraft.

Assembly and welding of both the GPHS and LWRHU capsules are performed in helium atmosphere glove boxes. The LWRHU welding fixture is shown in Figure 2. An empty Pt-30% Rh capsule is loaded into the fixture, and a fuel pellet is placed in the capsule. Then a weld shim is placed over the fuel pellet and an end cap is placed on the capsule. The welding fixture rotates the capsule around a vertical axis in front of a welding torch held at an angle of 45° to the axis of rotation. The weld current and rotation speed are computer controlled. The entire welding cycle requires 1.9 s of arc time.

Welded capsules are then decontaminated in a nitric/hydrofluoric acid solution and subjected to numerous nondestructive tests to assure the heat sources meet specifications. Tests include helium leak testing, neutron emission rate and calorimetry measurements, radiography and ultrasonic examination of the weld, and dimensional measurements. The GPHS capsules are normally shipped to the Mound Facility for assembly into radioisotope thermoelectric generators (RTGs), but the LWRHU capsules are assembled into their final configuration for use on the spacecraft before they are shipped to the Kennedy Space Center. Figure 2 is a photo of the LWRHU assembly, which consists of the fueled capsule, three layers of pyrolytic graphite thermal insulation, and the outer, impact-resistant graphite fiber aeroshell. The complete assembly weighs 40 g. Three of these heat sources were recently used to keep electronics at normal operating temperature on the Mars Pathfinder rover, and 117 are on their way to Saturn on the Cassini spacecraft. Figure 3 is a photo of the GPHS iridium-clad capsule showing the equatorial tungsten arc weld and the iridium frit-vent at the pole, which releases helium from the alpha decay of plutonium. The Cassini spacecraft employs 216 of these heat sources to provide thermal power for the three RTGs on the orbiter.

Figure 3. Photo of the GPHS iridium-clad capsule showing the equatorial tungsten arc weld and the iridium frit-vent at the pole, which releases helium from the alpha decay of plutonium. GPHS capsules are normally shipped to the Mound Facility for assembly into RTGs. The Cassini spacecraft employs 216 of these heat sources to provide thermal power for the three RTGs on the orbiter.

This article was contributed by Gary H. Rinehart (NMT-9).

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