Nagelschmidtite as a candidate host phase for actinides, rare earth and different waste elements
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Nagelschmidtite as a candidate host phase for actinides, rare earth and different waste elements Sergey V. Stefanovsky,1 Olga I. Stefanovsky,1 Ivan L. Prusakov2 1
Laboratory of Radioecology and Radiation Problems, Frumkin Institute of Physical Chemistry and Electrochemistry RAS, 31-4 Leninskii av., Moscow, 119071 Russia
2 Department of Glass and Glass Ceramics, D. Mendeleev University of Chemical Technology, Miusskaya sq. 1, 125047, Russia
Abstract
Nagelschmidtite, Ca7P2Si2O16, is an end-member of continuous solid solution Ca2SiO4 – Ca3(PO4)22Ca2SiO4 within the pseudo-binary system Ca3(PO4)2 – Ca2SiO4 (whitlockite – larnite). This phase is capable to wide isomorphic exchanges in Ca, P and Si sites: Ca 2+ = Sr2+; Ca2+ = Eu2+; Ca2+ + P5+ = (RE,An)3+ + Si4+, 2Ca2+ = Na+ + (RE,An)3+; 2Ca2+ = An4+ + ; Ca2+ + Si4+ = (RE,An)3+ + (Al,Fe)3+; Ca2+ + Si4+ = Na+ + P5+; 2Ca2+ = Na+ + (Al,Fe)3+; Ca2+ + P5+ = Na+ + S6+. It was found in metallurgical slags and geological formations. We revealed nagelschmidtite-type phase in vitrified phosphorus-bearing radioactive incinerator slags. The materials were glass-crystalline and contained nano-sized nagelschmidtite crystals distributed in vitreous matrix phase. Average chemical composition of the largest (few microns) crystals was recalculated to formula Na1.21K1.05Ca2.22Al2.02Fe0.46Si2.69P1.26U0.08O15.76. Significant oxygen misbalance suggests higher than U(IV) oxidation state for uranium – U(V) or U(VI). Capability of nagelschmidtite to be crystallized from melt makes it promising phase for actinides, rare earths and some other fission and corrosion products at using a melting route to nuclear waste forms including cold crucible induction melting and self-propagating high-temperature synthesis.
Introduction Currently glass is used as the only high-level waste (HLW) form providing for safe immobilization of radionuclides. Liquid HLW is vitrified in a Joule heated ceramic or an induction heated metallic melters. Moreover glass is also considered as a promising intermediate-level waste (ILW) form [1]. Some both HLW and ILW may be inhomogeneous and have complex chemical composition including so-called troublesome components such as high-fusible simple and complex oxides insoluble in silicate-based melts which may play a role of nucleating agents promoting crystallization of glass, sulfates, molybdates, chlorides, etc. As a result the vitrified products can be glass-crystalline [1,2]. Chemical composition of glass-ceramics may be adjusted so that high chemically durable crystalline phase concentrates actinides and other long-lived isotopes while vitreous phase contains primarily shorter-lived radionuclides (Cs, Sr, corrosion Downloaded from https://www.cambridge.org/core. North Carolina State University, on 24 Dec 2017 at 12:37:53, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/adv.2017.614
products). Sphene-based glass ceramics are known to be a typical material of this type [2]. The glass ceramics may be prod
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