Impact of the Synthesis Process on Structure Properties for AFCI Fuel Candidates

Fuels Campaign (TRP) Transmutation Research Program Projects

2007

Kenneth Czerwinski University of Nevada, Las Vegas, [email protected]

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Repository Citation Czerwinski, K. (2007). Impact of the Synthesis Process on Structure Properties for AFCI Fuel Candidates. 60-61. Available at: https://digitalscholarship.unlv.edu/hrc_trp_fuels/72

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This Annual Report has been accepted for inclusion in Fuels Campaign (TRP) by an authorized administrator of Digital Scholarship@UNLV. For more information, please contact [email protected]. Task 28 Impact of the Synthesis Process on Structure Properties for AFCI Fuel Candidates K. Czerwinski

BACKGROUND actinide nitrides. x To characterize actinide nitrides structurally and thermally. Synthesis of actinium mononitrides using carbothermic reduction x To use high resolution TEM techniques to explore the micro- of the corresponding oxides has a few outstanding issues, includ- structure of the radioactive samples. ing the formation of secondary phases such as oxides and carbides and low densities of the final product. Furthermore the require- RESEARCH ACCOMPLISHMENTS ment of a high process temperature at 1700°C, for more than 12 hours is also a drawback particularly for Americium-bearing sam- Uranium based nitride synthesis and characterization ples. Therefore, it is important to explore the use of other possible routes to synthesize actinide mononitrides. The fluoride route was successfully used to synthesize three ura- A low temperature process is used in this research to produce nium nitride samples with different stoichiometry (UN2, U2N3, actinide mononitrides using a fluoride route in which the first step and UN). Experimental conditions were optimized to synthesize is to mix the actinide oxide with NH4HF2. The second step in- high phase purity UN (97 wt.%). Thermal decomposition of UN2 volves the heat-treatment of the resulting ammonium actinide under different atmospheric conditions was also studied, and ultra fluoride salts in ammonia atmosphere. Using different analytical high purity argon could successfully be used to reduce the sample techniques available, the experimental conditions can be studied completely to UN at 1100°C, see graph on the opposite page. UN2 and optimized to synthesize the required materials with high and U2N3 decomposition kinetics into UN were studied under phase purity. Such available techniques are X-ray Powder Dif- argon at three different temperatures (1000, 1050, and 1100°C). fraction (XRD), Thermogravimetry and Differential Scanning Calorimetry (TG/DSC), and microscopic techniques such as Optical microscopy and SEM were used to explore the morphol- Scanning Electron Microscopy (SEM) and Transmission Electron ogy of uranium nitride samples. Bright Field Transmission Elec- Microscopy (TEM). Once the experimental conditions are stud- tron Microscopy was also used to confirm the morphological ob- ied and optimized, a number of actinide nitride systems (uranium, servations. Microstructural studies of the samples were carried thorium, and neptunium) will be synthesized and characterized to out using high resolution (HR) TEM with the help of selected provide knowledge on the chemistry of the systems. Characteriza- area diffraction (SAD) patterns. X-ray energy dispersive spec- tion of these nitride systems will include chemical phase identifi- trometry of TEM was utilized to characterize the elemental distri- cation, lattice parameter refinements, morphological studies, mi- bution and to verify the phase purity of the samples. Powder XRD crostructural verifications, thermal behavior, reaction mechanism, patterns of the as-synthesized uranium nitrides, UN2, U2N3, and and reaction kinetics. UN were collected and analyzed. Optical microscopic studies showed that the particle sizes of these uranium nitride samples RESEARCH OBJECTIVES AND METHODS range from 100 to 5000 nm. The microstructure of the UN sample shows the presence of UO2 as a secondary phase on the sur- The research objectives are: face of the sample. In this region, the lattice fringes correspond to x To explore a low-temperature fluoride route to synthesize the (222) interplanar d-spacing of UO2. X-ray Energy Dispersive

Scanning Electron Microscopic images of the (a)7NH4F.6UF4 and (b) (NH4)4ThF8 samples. (a) 7NH4F.6UF4 particles are well- crystallized (hexagonal unit cell with a rhomb-centered, a (b) = 15.40 Å and c = 10.49 Å and UN2 is cubic (fcc) with a = 5.310 Å) (b) Well-crystallized (NH4)4ThF8 acicular-shaped particles (triclinic unit cell with lattice parameters a = 8.477, b = 8.364, and c = 7.308 Å).

60 UN wt.% ACADEMIC YEAR HIGHLIGHTS 100 U2N3 wt.% UO2 wt.% i G.W.C. Silva, T. Hartmann, and K. Czerwinski, “Nitridization of Zr-U-Er-Cxides,” AFCI Semi-Annual Meeting, Sept., 2006. 80 i G.W.C. Silva, C.B. Yeamans, G.S. Cerefice, and K. Czerwin- ski, “Reaction Mechanism of UN2 conversion to UN,” ACS -1 k = 0.076172 (18355) min 233rd National Meeting & Exposition, Chicago, IL, March 25- 60 29, 2007. i C.B. Yeamans, G.W.C. Silva, G.S. Cerefice, K.R. Czerwinski, wt.% x T. Hartmann, A.K. Burrell, and A.P. Sattelberger, “Oxidative 40 Ammonolysis of Uranium(IV) Fluorides to Uranium(VI) Ni- UN tride,” Journal of Nuclear Materials (in press).

20 FUTURE WORK

0 The next phase of the project involves accomplishing the follow- ing tasks: 0 5 10 15 20 25 30 35 x Explore other experimental conditions and chemicals to syn- Time/ min thesize thorium nitrides. x Study any reaction mechanism and kinetics involved in tho- Pseudo-first-order kinetics of UN2 denitriding at 1100°C. rium nitride formations. x Characterize thorium-based nitrides using the above men- Spectrometry (XEDS) demonstrated that U was prominent, but it tioned techniques. is difficult to identify the presence of N due to overlaps with x Determine the ammonium bifluoride reaction with neptunium peaks from O and C. However, the magnified XEDS spectra veri- oxide. fies the presence of N in samples, and this figure also displays the x Explore the reaction route to synthesize Np-based nitrides. presence of O only in the UN sample. Thus, the XEDS verifies the phase purity of the synthesized sample.

Thorium based nitride synthesis and characterization Use of the fluoride route was successful only up to the formation of ThNF. The removal of fluorine, which should have lead to the formation of thorium nitrides was unsuccessful at different experimental conditions. However, the characterization of ammonium thorium fluoride and ThNF was done using the above mentioned techniques. High resolution TEM images of (a) UN and (b) ThNF samples. (a) Crystallography of UN was confirmed using the lattice fringes of HRTEM image, and the secondary oxide phase was only identified at the surface of the particle edge. (b) ThNF crystal structure which is determined using XRD was confirmed by the HRTEM and SAD pattern.

Research Staff Ken Czerwinski, Principal Investigator, Associate Professor, Department of Chemistry Gary Cerefice, Assistant Research Professor, Harry Reid Center for Environmental Studies

Students Chinthaka Silva, Graduate Student, Department of Chemistry Charles Yeamans, Graduate Student, University of California, Berkeley

Collaborators Al Sattelberger, Argonne National Laboratory