Suitability of Bioapatite as Backfill Material for Nuclear Waste Isolation
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Suitability of Bioapatite as Backfill Material for Nuclear Waste Isolation
A.J. Finlay1, A.E. Drewiczl, D.O. Terry, Jr.1, D.E. Grandstaff1, and Richard D. Ash2 1
Earth and Environmental Science, Temple University, Philadelphia, PA 19122 USA
2
Department of Geology, University of Maryland, College Park, MD 20742 USA
ABSTRACT Bioapatite, found in vertebrate bones and teeth, is highly reactive and may incorporate high concentrations of some radionuclides, including U, Pu, and Sr. Therefore, bioapatite may be useful in backfill or overpack materials in nuclear waste repositories. The dissolution rate for bioapatite is constant at pH > 4 and is about 5 times faster than fluorapatite. In terrestrial environments, bioapatite recrystallizes over periods of up to ca. 40 ka. INTRODUCTION Monazite (CePO4), apatite, and other phosphate minerals can contain high concentrations of actinides, lanthanides, and other elements with anthropogenic short-lived isotopes (e.g., 90Sr) found in nuclear waste. Because of their high capacity and durability, phosphate minerals or phosphate-silicate solid solutions have been proposed as waste-forms for nuclear waste disposal [1]. These characteristics have also led to proposals for phosphate minerals to be used as backfill or overpack materials in high-level nuclear waste repositories [2,3,4]. Bioapatite (dahllite), found in vertebrate bones and teeth, has also been investigated as a reactive barrier material to remove U, Pb, Cd, and other heavy metals from solution at contaminated sites [5,6,7]. We suggest that bioapatite may also be used in backfill or overpack materials in nuclear waste repositories. Common apatite types include hydroxy- (HAP)[Ca5(PO4)3(OH)], chlor- [Ca5(PO4)3(Cl)], and fluorapatite (FAP)[Ca5(PO4)3F], found in igneous and metamorphic rocks and carbonate fluorapatite (CFA), in sedimentary phosphorites. In CFA and bioapatite, CO32- substitutes for PO43-. The charge deficiency is usually compensated by omission of Ca, hydroxyl, or substitution of monovalent cations, such as Na, producing a defect structure [8]. Carbonate apatite crystals in bone are poorly crystalline, plate- or tablet-shaped and extremely small, with average dimensions of 50 x 25 x 2 to 4 nm and very large specific surface areas of ca. 240 m2/g [8,9]. The carbonate substitution, poor crystallinity, and defect structure make bioapatite more soluble and reactive than many other apatites [5,9]. In apatite-containing backfill, concentrations of nuclear waste species may be controlled either by solubility of their phosphate minerals or by sorption on apatite [7,10]. Therefore, the higher dissolved phosphate concentrations resulting from greater solubility of bioapatite would produce lower concentrations of radionuclides. In near-neutral pH solutions, measured sorption constants [3,4,6] between apatite and U, Pu, and
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rare earth elements (REE) range from ca. 5 x 105 to 1 x 107. Therefore, sorption could also significantly decrease dissolved waste concentrations. During diagenesis, adsorption and incorpora
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