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Yongliang Xiong1

1, Sandia National Laboratories, Carlsbad, New Mexico, United States

Actinide oxalates are a family of materials important for nuclear industry. In the field of nuclear waste management, actinides as well as oxalate are present in nuclear waste in geological repositories. Therefore, the interactions between actinides and oxalate are important for performance assessment (PA) for geological repositories for nuclear wastes. As an example, in the Waste Isolation Pilot Plant (WIPP), a U.S. DOE geological repository for defense-related transuranic (TRU) waste in the bedded salt formations in New Mexico, USA, the inventory of oxalate in waste was 1.99 × 104 kg for the WIPP Compliance Application Re-Certification in 2014 (CRA-2014). The oxalate concentrations were 1.18 × 10–2 M. As actinide oxalates have low solubilities, actinide oxalates could form in geological repositories to become solubility-controlling phases for actinides.
In the nuclear power fuel cycle, long-lived and alpha-emitting actinides with high radiotoxicity are usually separated from other radioactive elements, as oxalates for immobilization for disposal or for transmutation, or purification for reuse. For instance, 241Am can be separated, and be used as radioisotope thermoelectric generators (RTGs) for space programs. RTGs are a key technology for data acquisition and communications in space missions. Pu in used nuclear fuel can also be co-separated with U as oxalates, which then can serve as a feeder for manufacturing mixed oxide (MOX) fuel. In addition, the uranyl oxalate system has been used as a chemical actinometer.
As actinide oxalates are important to numerous fields, the knowledge of their solubility constants is a key to the aqueous processes in which they are precipitated. In this work, we present our evaluation of the solubility constants of actinide oxalates including americium, curium, plutonium and uranium oxalates. In our evaluation, we use the Pitzer model to describe activity coefficients of aqueous species. The computer code, EQ3/6 Version 8.0a, is used as a modeling platform. The solubility data that are selected for evaluation are in oxalic and nitric acids, and in the mixtures of these two acids. The media are characterized with high ionic strengths, up to ~ 8 m.

The solubility constants of actinide oxalates evaluated by this study are expected to find applications in many fields, including the description of the stability of actinide oxalates in the near field of geological repositories.

A This research is funded by the WIPP programs administered by the Office of Environmental Management (EM) of the U.S. Department of Energy.
B Sandia National Laboratories is a multimission laboratory operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. SAND2017-11366 A

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