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NMT Division Recycles, Purifies Plutonium-238 Oxide Fuel for Future Space Missions

Plutonium-238 (238Pu) has proven to be an excellent radioisotope for space power applications because of its availability, power density, useful lifetime, minimal shielding requirements, and oxide stability. The Laboratory's experience with the 238Pu isotope goes back many years to pioneering efforts associated with developing pacemakers and designing, fabricating, and testing heat sources for space. More recent LANL work has been associated with the design and fabrication of general purpose heat source (GPHS) units and radioisotope heater units (also known as RHUs) to support the Cassini mission to Saturn (Reported in Actinide Research Quarterly, Fall 1996).


Figure 1: High-energy ball mill used in preparing plutonium-238 dioxide feed for dissolution. Because 238Pu, used for space power applications, is no longer available from DOE nuclear reactors, NMT proposes to recycle old sources of 238Pu in the DOE complex to make the necessary fuel.

Plutonium-238 is made in a nuclear reactor by neutron irradiation of neptunium-237 to form neptunium-238. Short-lived neptunium-238 (half-life of 2.35 days) then decays by beta emission to 238Pu (half-life of 87.74 years). The shutdown of most DOE nuclear reactors has raised doubts about the production of new 238Pu for future space missions. The NMT Division proposes to recycle old sources of 238Pu that exist in the DOE complex to make highly purified fuel. Recycled fuel could also be mixed with higher-isotopic fuel obtained from the Former Soviet Union, England, France, or Canada.

The purity specifications for 238Pu to be used in heat sources are rigorous. Small amounts of impurities in 238Pu fuel could interfere with the function of heat sources in several ways, including grain growth in the iridium used in the inner clad layer to encapsulate the fuel; plugging of the fueled clad vent, leading to potential pressurization and rupture; and interference with instrumentation onboard the spacecraft from emitted radiation, primarily from neutrons produced by (a,n) reactions with light elements.

Our initial effort has focused on purification of 238PuO2 fuel that fails to meet GPHS specifications because of impurities. The most notable nonactinide impurity was silicon, but aluminum, chromium, iron, and nickel were also near to or in excess of limits specified by GPHS fuel powder specifications. The 234U was by far the largest actinide impurity observed in the feed material because it is the daughter product of 238Pu by alpha decay. Older heat sources to be reclaimed would have relatively large amounts of 234U ingrowth (half-life of 24,500 years), a valuable isotope that could be recovered for use in tracer studies of U.

Chemical processing of 238Pu fuel offers several unique challenges. One step in making 238PuO2 fuel extremely stable for heat sources is high-temperature firing of up to 1600°C. Unfortunately, this makes the material very hard to dissolve by usual methods should additional purification be required. We have observed a marked improvement in dissolution efficiency when the 238PuO2 feed is first milled in a high-energy ball mill. The very fine powder obtained dissolves at an acceptable rate in a mixture of refluxing nitric and hydrofluoric acids.

Plutonium (III) oxalate precipitation was selected for this portion of the demonstration because of its simplicity, speed, and adequacy of purification. In particular, plutonium (III) oxalate precipitation requires no temperature control, shows little detrimental effect from excess oxalic acid, and has a rapid reaction, precipitation, and filtration time. Reagent-grade chemicals were used to minimize introduction of contaminants. Teflon or polypropylene apparatus was used for 238Pu solutions in this demonstration to avoid leaching silicates from glassware.

Plutonium exhibits some of the most complex and interesting chemistry of any element, with several oxidation states possible in solution. Controlling the oxidation state of plutonium is challenging under any circumstances, but it is particularly so in the presence of so much alpha radiation. As 238Pu has an alpha activity of 17.1 Ci/g, even a moderate batch size (80 g) of 238Pu can have over 1000 Ci of alpha in a single small solution. To mitigate effects of radiolysis caused by this level of radioactivity, combinations of reducing agents were used in excess, and all steps were performed as rapidly as possible. Our best results to date combined hydroxylamine nitrate with either sulfamic acid or urea.

Decontamination factors for uranium, silicon, chromium, iron and nickel were very good using the plutonium (III) oxalate precipitation method employed. Up to 96%­99% reduction in impurities was noted. The purity of the 238PuO2 recovered from this demonstration was significantly better than GPHS specifications, and in fact better than that of any fuel material received at LANL for use in the Cassini mission to Saturn.

NMT Division has expertise and facilities that make larger-scale recovery and purification of 238Pu for oxide fuel a practical option. Additional supplies of 238Pu oxide fuel from many existing sources can be recovered in a glove box facility like PF-4. Plutonium-238 materials targeted for recovery include impure oxide and scrap items. For many 238PuO2 feeds, including those with significant 234U ingrowth, the plutonium (III) oxalate precipitation procedure is adequate to meet the GPHS fuel standard. For scrap items that are lean in 238Pu values, anion exchange separation offers a method to concentrate and purify additional amounts of the isotope.

Efforts continue to develop the capability for efficient, safe, cost-effective, and environmentally acceptable methods to recover and purify 238PuO2 fuel in a glove box environment. Los Alamos is the Department of Energy's "Lead Lab" for plutonium, and NMT Division has the resident plutonium experience and required infrastructure to complete this project successfully. One of the most valuable contributions is the waste minimization efforts underway in NMT to reduce the activity and volume of liquid and solid wastes associated with plutonium processing. Technology transfer of these capabilities to processing of 238PuO2 will greatly support our ability to provide high-purity fuel for future space power applications.


Figure 2: After ball milling, the plutonium-238 dioxide feed is dissolved in a mixture of nitric and hydrofluoric acids in this apparatus consisting of a Teflon round-bottomed flask, Teflon reflux condenser, and Teflon-compatible heating mantle. The dissolution is one step in producing a 238Pu fuel that meets the rigorous specifications for high purity to ensure proper performance for space applications.

Jacob Espinoza, Elizabeth Foltyn, and Gary Rinehart (NMT-9) and Louis Schulte, Gary Silver, Larry Avens, and Gordon Jarvinen (NMT-6) are the principal developers of this project. They acknowledge many contributors to this work: Kevin Ramsey and Jim Jones (NMT-9) for help with hardware and glove box development; Charles V. Puglisi, Carlos D. Dozhier, Christina M. Lynch, Paul F. Moniz, Robert W. Mathews, Richard T. Romero, and Mary Severinghaus (NMT-9) for the help with hot jobs and sample removal; Tim George (NMT-9), Keith Fife, Steve Yarbro, and Steve Schreiber (NMT-2), Mark Dinehart, Jerry Lugo, and Randy Vaughn (NMT-6) for helpful conversations; Johnny N. Quintana (CST-4), and Margaret T. Trujillo and Nelson D. Stalnaker (CST-8) for radiochemical analysis of solutions; and numerous others in LANL groups CST-8 and CST-9 for elemental and radiochemical analysis of the plutonium-238 oxide products.


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