Neptunium(V) Incorporation/Sorption with Uranium(VI) Alteration Products
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Neptunium(V) Incorporation/Sorption with Uranium(VI) Alteration Products J. I. Friese1*, M. Douglas2, E. C. Buck1, S. B. Clark2, B. D. Hanson1 1
Radiological Processing Laboratory, Pacific Northwest National Laboratory, Richland WA, 99352 *Author email: [email protected] 2 Department of Chemistry, Washington State University, Pullman WA, 99164
ABSTRACT An initial uranium phase that has been observed to form during the corrosion of spent nuclear fuel is the uranium oxy-hydroxide metaschoepite. It has been proposed that neptunium(V) solubility can be limited by its association with this uranium phase. Metaschoepite has been synthesized in the presence of neptunium(V) over the pH range modeled in the proposed Yucca Mountain geologic repository. Uranium (VI) phases were synthesized by varying pH and neptunium concentrations. Results of neptunium association with the uranium alteration phases are presented and the relationship to dissolved neptunium concentrations discussed. INTRODUCTION The substitution of radionuclides into uranium(VI) phases during the oxidative dissolution of spent fuel continues to be of interest [1,2]. Burns et al. have suggested that neptunium(V) may substitute into the crystal structure of uranium(VI) solid phases owing to the structural similarities between UO22+ ion and the NpO2+ ion [3]. This incorporation mechanism to form a solid solution was proposed as a potential mechanism for limiting the migration of neptunium(V) into the surrounding environment at the Yucca Mountain Repository [4]. For incorporation to occur, various charge balancing mechanisms must be present to account for the difference in charge between the NpO2+ and UO22+ cations. Two charge balancing mechanisms that may be important for incorporation to occur are the exchange of O with OH in the crystal structure and/or the addition or substitution of other cations in the crystal structure [3]. The dissolved concentration model for radionuclides for the Yucca Mountain Project currently uses Np2O5 as the solubility limiting controlling phase [5]. Under conditions similar to that expected at Yucca Mountain, it can be calculated that the dissolved concentration of Np(V) will be approximately 1×10-5 M using Np2O5 in the model [6]. In contrast, drip tests [7] and batch static water tests [8,9] conducted with spent nuclear fuel (SNF) indicate that the actual dissolved concentration of neptunium is less than 1×10-7 M [10]. With electron energy loss spectroscopy (EELS), Buck et al. provided preliminary evidence for the occurrence of neptunium in schoepite related phases from the corrosion of SNF under vapor conditions [11]. New data on neptunium and uranophane indicates that neptunium can be incorporated into this uranium(VI) phase [12]. X-ray fluorescence spectroscopic analysis with a bent-Laue crystal to reduce the contribution from uranium indicate that Sr was present in a boltwoodite-uranophane mix, but that no neptunium was observed in high-drip rate tests on SNF [13]. Cation exchange between the interlayer of uranopha
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