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Title: Np Incorporation into Uranyl Alteration Phases: A Quantum Mechanical Approach

Abstract

Neptunium is a major contributor to the long-term radioactivity in a geologic repository for spent nuclear fuel (SNF) due to its long half-life (2.1 million years). The mobility of Np may be decreased by incorporation into the U{sup 6+} phases that form during the corrosion of SNF. The ionic radii of Np (0.089nm) and U (0.087nm) are similar, as is their chemistry. Experimental studies have shown Np can be incorporated into uranyl phases at concentrations of {approx} 100 ppm. The low concentration of Np in the uranyl phases complicates experimental detection and presents a significant challenge for determining the incorporation mechanism. Therefore, we have used quantum mechanical calculations to investigate incorporation mechanisms and evaluate the energetics of Np substituting for U. CASTEP, a density functional theory based code that uses plane waves and pseudo-potentials, was used to calculate optimal H positions, relaxed geometry, and energy of different uranyl phases. The incorporation energy for Np in uranyl alteration phases was calculated for studtite, [(UO{sub 2})O{sub 2}(H{sub 2}O){sub 2}](H{sub 2}){sub 2}, and boltwoodite, HK(UO{sub 2})(SiO{sub 4})* 1.5(H{sub 2}O). Studtite is the rare case of a stable uranyl hydroxyl-peroxide mineral that forms in the presence of H{sub 2}O{sub 2} from the radiolysis ofmore » H{sub 2}O. For studtite, two incorporation mechanisms were evaluated: (1) charge-balanced substitution of Np{sup 5+} and H{sup +} for one U{sup 6+}, and (2) direct substitution of Np{sup 6+} for U{sup 6+}. For boltwoodite, the H atomic positions prior to Np incorporation were determined, as well as the Np incorporation mechanisms and the corresponding substitution energies. The preferential incorporation of Np into different structure types of U{sup 6+} minerals was also investigated. Quantum mechanical substitution energies have to be derived at Np concentrations higher than the ones found in experiments or expected in a repository. However, the quantum mechanical results are crucial for subsequent empirical force-field and Monte-Carlo simulations to determine the thermodynamically stable limit of Np incorporation into these uranyl phases.« less

Authors:
; ;
Publication Date:
Research Org.:
Yucca Mountain Project, Las Vegas, NV (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
894739
Report Number(s):
NA
MOL.20061026.0147, DC# 48764; TRN: US0700410
DOE Contract Number:  
NA
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS; CHEMISTRY; CORROSION; DETECTION; FUNCTIONALS; GEOMETRY; HALF-LIFE; NEPTUNIUM; NUCLEAR FUELS; RADIOACTIVITY; RADIOLYSIS; SILICATE MINERALS; URANIUM MINERALS

Citation Formats

Huller, L C, Win, R C, and Ecker, U. Np Incorporation into Uranyl Alteration Phases: A Quantum Mechanical Approach. United States: N. p., 2006. Web. doi:10.2172/894739.
Huller, L C, Win, R C, & Ecker, U. Np Incorporation into Uranyl Alteration Phases: A Quantum Mechanical Approach. United States. https://doi.org/10.2172/894739
Huller, L C, Win, R C, and Ecker, U. 2006. "Np Incorporation into Uranyl Alteration Phases: A Quantum Mechanical Approach". United States. https://doi.org/10.2172/894739. https://www.osti.gov/servlets/purl/894739.
@article{osti_894739,
title = {Np Incorporation into Uranyl Alteration Phases: A Quantum Mechanical Approach},
author = {Huller, L C and Win, R C and Ecker, U},
abstractNote = {Neptunium is a major contributor to the long-term radioactivity in a geologic repository for spent nuclear fuel (SNF) due to its long half-life (2.1 million years). The mobility of Np may be decreased by incorporation into the U{sup 6+} phases that form during the corrosion of SNF. The ionic radii of Np (0.089nm) and U (0.087nm) are similar, as is their chemistry. Experimental studies have shown Np can be incorporated into uranyl phases at concentrations of {approx} 100 ppm. The low concentration of Np in the uranyl phases complicates experimental detection and presents a significant challenge for determining the incorporation mechanism. Therefore, we have used quantum mechanical calculations to investigate incorporation mechanisms and evaluate the energetics of Np substituting for U. CASTEP, a density functional theory based code that uses plane waves and pseudo-potentials, was used to calculate optimal H positions, relaxed geometry, and energy of different uranyl phases. The incorporation energy for Np in uranyl alteration phases was calculated for studtite, [(UO{sub 2})O{sub 2}(H{sub 2}O){sub 2}](H{sub 2}){sub 2}, and boltwoodite, HK(UO{sub 2})(SiO{sub 4})* 1.5(H{sub 2}O). Studtite is the rare case of a stable uranyl hydroxyl-peroxide mineral that forms in the presence of H{sub 2}O{sub 2} from the radiolysis of H{sub 2}O. For studtite, two incorporation mechanisms were evaluated: (1) charge-balanced substitution of Np{sup 5+} and H{sup +} for one U{sup 6+}, and (2) direct substitution of Np{sup 6+} for U{sup 6+}. For boltwoodite, the H atomic positions prior to Np incorporation were determined, as well as the Np incorporation mechanisms and the corresponding substitution energies. The preferential incorporation of Np into different structure types of U{sup 6+} minerals was also investigated. Quantum mechanical substitution energies have to be derived at Np concentrations higher than the ones found in experiments or expected in a repository. However, the quantum mechanical results are crucial for subsequent empirical force-field and Monte-Carlo simulations to determine the thermodynamically stable limit of Np incorporation into these uranyl phases.},
doi = {10.2172/894739},
url = {https://www.osti.gov/biblio/894739}, journal = {},
number = ,
volume = ,
place = {United States},
year = {Tue Sep 05 00:00:00 EDT 2006},
month = {Tue Sep 05 00:00:00 EDT 2006}
}