Targeted uranium recovery from complex alloys using fluoride volatility

https://doi.org/10.1016/j.jfluchem.2020.109539Get rights and content

Highlights

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    Stocks of enriched uranium (EU) in nuclear fuel exist as a variety of alloy types.

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    Recovery of EU and other fuel constituents is time intensive and fuel-type sensitive.

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    A unique low temperature reactivity of NF3 on U imparts unusual reactivity to U alloys.

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    This novel reactivity potentially provides for rapid recovery of EU.

Abstract

A single step separation of molybdenum from a multicomponent uranium alloy, 10 wt.% Mo in uranium metal, (U-10Mo)–Zr is explored using nitrogen trifluoride. The separation takes advantage of an unusually low temperature behavior of NF3. Exposure of the alloy to flowing NF3 between 100–200 °C caused surface area dependent, NF3 concentration, and temperature dependent thermal runaway that threatened the viability of the separation. Gravimetric methods were used to acquire temperature and NF3 concentration dependent profiles to understand the nature of the self-heating and to establish practical conditions for the separation. Scale-up to multigram quantities of the composite is described. In this effort, the use of larger product masses helped to divulge potential metal/MoF6 redox behavior of during the fluorination and acknowledgement of some unexpected chemistries as the metals evolved from the metallic state to their higher fluorides in the presence of what appears to be a catalytic or otherwise activated NF3-uranium metal surface.

Introduction

Beginning in the 1970s, investigations of new metal fuel compositions and mechanical designs were funded by the US DOE towards the development of a low-enriched (<20 % 235U) fuel candidate that would replace high-enriched (HEU) fuel types that are still used today in high performance research reactors (HPPRs) in the US and other countries. The overarching development effort was to produce a fuel with substantially increased uranium density that would offset the decrease in enrichment. Extensive research concerning design, fabrication, mechanical testing, and irradiation performance of uranium metal thin plate fuels was focused on additives such as Mo [1,2], Zr [3], Si [2,4], and Al [5]. The most successful fuel composition to date, a 10 % Mo metal alloyed with uranium metal (U-10Mo), saw design variants in diverse reactor types in the US [1], Europe [6,7], South Korea [8,9], and Russia [10], spanning nearly 70 years. The historical fabrication and testing campaigns for improved fuel types, consequently, provide for a significant recovery of used and unused LEU and HEU for the repurposing of these valuable materials.

Industrial scale recovery of uranium from used fuel using fluoride volatility has been demonstrated in the United States to be quite rapid and good decontamination of uranium from used plate fuels has been reported using HF and F2 gas [11]. Other reagents that have been shown to have similar efficacy are ClF3 and BrF3, but also present significant handling and health concerns [[12], [13], [14]]. We have been investigating the gas-solid reactions of nitrogen trifluoride (NF3) as fluorinating agent to recover valuable constituents from used nuclear-fuel [15,16]. While NF3 was originally discovered in 1928 [17], it has not been reported on widely for its potential as a metal separations reagent. NF3 has limited chemical reactivity at room temperature, and this considerably reduces risks associated with fluoride volatility-based operations. At higher temperatures, NF3 exhibits a wide variation in its thermal reactivity with metals and non-metals across the periodic table that lead to useful differences in the temperature required to volatilize a given metal or oxide [15,16]. In this article, we have used NF3 to separate molybdenum from a metal alloy comprised of 10 % molybdenum in uranium metal. As fabricated, the fuel is a layered composite; Al–Zr–(U-10 % Mo)–Zr–Al [18]. The Al clad material was impervious to direct fluorination but we demonstrate that (U-10Mo)–Zr can still be separated with a sufficient mechanical disruption of the Al clad. The results here set out the conditions and the physical underpinnings of why the general separation of U-metal alloys of dispersed or monolithic Mo, Si, Zr, Al, and Pu is effective using NF3 and fluoride volatility in general.

Section snippets

Thermal excursion in the separation of (U-10Mo)–Zr

Whereas the reactions of NF3 with metals are exothermic, its thermal excursion behavior is not widely reported in the literature, even for powdered metals [19]. The data in Fig. 1 was acquired using a thermal gravimetric (TG) experiment and is an example of thermal runaway that was initiated using 20 mg of U-10Mo, in 50 % flowing NF3 along a 10 °C min−1 ramp. Near 186 °C, the system heat spiked to 600 °C in less than 2 s; a 400 °C increase overall. The mass loss indicated in Fig. 1 (dotted

Chemistry of the reaction of NF3 with (U-10Mo)–Zr

The chemistry of the reaction of pure U metal with NF3, for which precise product ratios are still under investigation [21], proceeds stepwise as in Eqs. (1), (2), (3).U+NF3→UF3+12N2UF3+13NF3→UF4+16N2UF4+23NF3→UF6+13N2

For stoichiometric reaction, we have observed by use of cryogenic isolation of the products in gas tight, quartz cuvettes that the major product of Eqs. (1) and/or (2) is dinitrogen. After the removal of condensed NF3 from the cuvette, gaseous, red-brown nitrogen dioxide was

Conclusions

Several fluoride and chloride volatility concepts for the recovery of uranium from uranium-based matrices have been demonstrated [11,[33], [34], [35], [36], [37]]. In general, these approaches force all volatile species to congruently depart the reactor, leaving non-volatile ones behind. Alternately, targeted selective separations of U from Mo, Tc, Si, Pu, Zr and other common fuel additives can be achieved by using the appropriate choice of fluorination reagent and conditions. Toward this

Materials

Ultra-high purity (UHP, 99.999 %) Argon was purchased from Oxarc (Spokane, WA) and high purity NF3 (99.999 %) was purchased from Air Liquide (Houston, TX). U-10Mo plates obtained from BWTX (Lynchburg, VA) were cut with shears into small rectangles for thermal sampling. MoF6 (99.5 %) was purchased from Advanced Research Chemicals, Inc. (Catoosa, OK) and was used as received. The U-10Mo fuel was received as enclosed within a Zr metal diffusion barrier (U-10Mo)–Zr. Other samples included

Declaration of Competing Interest

There are no conflicts of interest associated with this work.

Acknowledgments

We thank Dr. Christina Arendt for a large statistical set of analyses that quantified the compositional spread in as-received samples U/Mo. This research was funded by the National Security Division Laboratory Directed Research and Development program at the Pacific Northwest National Laboratory. The laboratory is operated for the US Department of Energy by Battelle under Contract DE-AC05-76RL0-1830.

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