Title

Compatability Assessment of HfN, ZrN, and Graphite with Molten LiCl-NaCl-KCl-UCl₃

Publication Date

4-2008

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Materials Science and Engineering

Department

Materials Science and Engineering

Major Advisor

Darryl P. Butt

Abstract

The thermodynamics and kinetics of reactions between hafnium nitride (HfN), zirconium nitride (ZrN), and graphite with a quaternary salt of composition LiCl-KCl-NaCl-UCl3 were assessed. Coupons of each of the three refractory materials were exposed to the quaternary salt at 525 to 900°C for 4 to 485 hours. The oxygen partial pressure was maintained at 10-8 atm of O2 in most experiments. A limited number of experiments were also conducted at a higher oxygen partial pressure of 10-4 atm of O2 in order to assess the effects of atmosphere on the reaction kinetics. The reaction kinetics of the salt-refractory interactions was assessed through physical and microstructural characterization including optical microscopy, scanning electron microscopy, and X-ray diffraction. Salts were analyzed using inductively coupled plasma-mass spectroscopy.

It was found that both HtN and ZrN lose weight under all conditions investigated while graphite exhibited relatively small weight increases. In the case of HtN and ZrN, the reaction kinetics was complex. In both materials, simultaneous dissolution and oxidation were observed. While multiple mechanisms were evident, it is proposed that oxygen enrichment at the surface tends to inhibit dissolution in the molten salt. It is speculated that the internal oxygen in the base nitrides diffuses to the surface and provides a more corrosion-resistant interface. A negative activation energy for the overall dissolution process was observed in both HtN and ZrN. This seemingly anomalous activation energy was associated with the competition between surface oxide formation and dissolution.

The Graphite showed little or no evidence of reacting with the molten salt. The small weight gains observed were associated with diffusion of salt into native porosity in the as received material. It was found that increasing the oxygen partial pressure increased the rate of dissolution of the nitrides, but has no statistically significant effect on the reactions with graphite. It is proposed that the atmospheric oxygen reacts with UCl3 in the salt producing UO2 and Cl2 (g), the gaseous product of which may accelerate dissolution.

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