A Glass-Ceramic Material for Fixation of Radioactive Waste

In this article, a starting mixture for the preparation of glass-ceramic material for radioactive waste (RW), consisting of 85-95 mass % basanite and 5-15 mass % oxides of elements in І-VIII group of the Periodical table of elements imitating RW, is proposed. The glass-ceramic material is obtained by melting the starting mixture in air at 1450°С for 1 hour and by further crystallization of the melts at 950°С for 30 minutes. It has been noticed that the texture of the glass-ceramic material is microgranular. The main mineral is pyroxene, while a mixture phases are magnetite, hematite and residual glass. It was shown that the RW elements are fixed in the pyroxene and partly in the admixture phases.


Introduction
Basalts castings in the shape of tiles and pipes are industrially produced in the Russian Federation, Ukraine, Czech Republic, Germany, Bulgaria, etc. [1].Their distinctive features are high mechanical strength, high acid and high abrasion resistance.
The method of glazing has been applied for fixation and safe disposal of different waste (slag and ashes after the "burning" of waste, radioactive waste, etc.), after heating at 1200-1400 °С for insertion of heavy metals into the glass [2].
Radioactive depositions which contain cesium 137 are fixed in a cement matrix where vermiculite and bentonite have been added [3].Best results were obtained when 5 mass % vermiculite was added.Bentonite is the most preferred buffer material for depth conservation of radioactive waste in clay and granite rocks [4].The addition of quartz and graphite to bentonite improves the heat transfer to the host rock.
The results of the microstructural studies and the linear thermal expansion of the polymineral material Synroc-B, which is used for burial of nuclear waste, have been provided herein [5].Perovskite, zirconolite and hollandite are present in its composition.The sintering technology of the Synroc-B involves pressing of the starting powder and consequent firing at 1250 °С.No microcracks can be found in the prepared material.
Glasses, vitroceramics and synthetic rocks (Synroc) of titanate and zirconate compositions can be used for fixation of radioactive waste as well [6].The use of glass and Synroc materials is limited due to the high prices of the base raw materials and to the relatively complex technological scheme.
The main mass of the radioactive waste consists of the oxides of uranium and plutonium, which have to be extracted during the radiochemical reprocessing from the radioactive waste for reuse in heat elements [7].The total content of the composition elements' salts and of the radionuclides in the radioactive waste solutions is about 10 mass %.
The aim of the present study is to confirm the possibility of including RW elements in the form of solid solution into pyroxene structure -a mineral possessing a large isomorphous capacity which can prevent their extraction.

Experimental
The starting mixture for glass-ceramic material for fixation of radioactive waste (RW) is composed of 85-95 mass % basanite and 5-15 mass % oxides of the elements from І to VIII group of the Periodical table of elements, imitating the RW (Tab.I).The composition of the basanite from the V. voda locality (Bulgaria) is (in mass %): 44.93 SiO  The glass-ceramic material is obtained following the petrurgical method.Melting of the starting mixture was carried out in corundum crucibles in air at 1450°С for 1 hour.Crystallization of the melts was carried out in a single-stage thermal treatment at 950°С for 30 minutes.The petrurgical method (crystallization of melts) is cheaper than the conventional two-stage method and the modified conventional single-stage method which use crystallization from glass [8].
Fig. 1 shows the crystallization scheme of the cooled melts using the petrurgical method [8], Fig. 2 -diffractograms of the glass-ceramic material, and Fig. 3 -the electron-microscopic photo of this glass-ceramic material.

Results and discussion
The glass-ceramic material prepared following the petrurgical method is comprised of 85-95 mass % basanite and 5-15 mass % oxides of the elements in І-VIII group of the Periodical table of elements imitating RW (Tab.I-II).The melting is carried out at 1450 °С for 1 hour and the crystallization of the melts at 950 °С for 30 minutes.The glass-ceramic based on basanite is composed of pyroxene of diopside structure as a main phase, magnetite which initiates the crystallization by forming nuclei, and residual glass (Fig. 2a).The basanite glass-ceramic material with 10 mass % RW (Fig. 2b) is composed of the same phases and hematite.Pyroxene is identified by its characteristic interplanar spaces (in nm): 0.298-0.323-0.294(9-460); magnetite -0.252-0.148-0.295(4-0755) and hematite -0.269-0.251-0.161 .The residual glass encloses the crystal phases.The diffractograms of the basanite glass-ceramic materials with and without RW additives (Fig. 2,  a, b) are very similar due to their anchimonomineral pyroxene composition.The texture of the glass-ceramic materials with and without RW additives is microgranular with crystal size from 20-30 µm (Fig. 3, a) to 5-10 µm (Fig. 3, b).The twobarrier character of the glass-ceramic structure with respect to the RW elements was formed by their introduction in pyroxene (I barrier) and by the residual glass, enclosing the crystal phases (II barrier) [9].

Conclusion
The glass-ceramic material for fixation of RW is prepared following the petrurgical method using basanite (85-95 mass %) and oxides of the elements from І to VIII group of the Periodical table of elements (5-15 mass %) imitating RW.It is composed of pyroxene forming the main phase, magnetite, hematite and residual glass.The RW elements are fixed isomorphously mainly in the structure of pyroxene and partly in the admixture phases.The basanite glass-ceramic material with pyroxene composition is a potentially new material for burial of RW, completely compatible (in a geologic aspect) with the volcanic-sedimentary rocks in the Earth's crust.

Fig. 1 .
Fig. 1.Temperature dependency of the rate of the secondary nuclei formation and rate of the crystal growth (a) at a single stage thermal treatment of the cooling melts (b) T NG -optimal temperature of crystallization
The elements from І to VIII group of the Periodical table of elements (I group -Na, K Cu; II group -Sr,Ba,Zn; III group -Ga,Y, Ce, Pr,Nd,Gd; IV group -Sn, Pb,Ti,Zr; V group -Bi,Nb; VI group -Cr,W; VII group -Mn; VIII group -Fe, Co, Ni) are shown in Tab.II as oxides.