Thorium and Uranium Phosphate Syntheses and Lixiviation Tests for their Use as Hosts for Radwastes
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THORIUM AND URANIUM PHOSPHATE SYNTHESES AND LIXIVIATION TESTS FOR THEIR USE AS HOSTS FOR RADWASTES
MICHEL GENET, N. DACHEUX, C. MERIGOU and V. BRANDEL. Universit6 de Paris-Sud, Institut de Physique Nucldaire/IN2P3, Groupe de Radiochimie, BP n'l, 91406 Orsay, Cedex, France. ABSTRACT Thorium and uranium phosphates were synthetized via soft chemistry and then calcined at high temperature, respectively up to 850 and 1170'C. Th3 (PO 4 ) 4 was produced in agreement with published results,while the analog uranium phosphate: U 3 (PO4 ) 4 was not found. Instead, we found we had made a new mixed valence compound: U(U0 2 )(PO 4 ) 2 , for which crystal structure, absorption spectrum in ultraviolet and visible, and X-ray photoelectron spectroscopy were done. All of these experiments confirmed the presence of uranium in both tetravalent and hexavalent states. 22 7 Lixiviation of thorium phosphate labelled with radioactive isotopes : Th and
22 3
Ra
1 allowed us to determine the upper limit of the solubility for this compound = 7.6x10-1 M 3 and, the percentage of 2 23 Ra released in deionized water (about 5x 10- %). 230 U (radiochemically Similar lixiviation tests on U(U0 2)(PO4 ) 2 labelled with
produced) gave an uranium solubility value 10 times greater than uranium measured by laser spectrofluorimetry. This discrepancy is attributed to the radiation chemistry inherent in the use of radioactive isotope at high specific activity. The lixiviated uranium fraction from U(U0
2 )(PO 4 ) 2
is seven to eight orders of magnitude higher than for thorium. Unexpected
behaviour was observed for uranium solubility which increases by two orders of magnitude after 2000 hours of lixiviation. This phenomenon is not yet understood.
I - Introduction The suggestion of storing radioactive waste in phosphate matrices, especially monazite, was extensively discussed several years ago [1]. At that time, monazite structure was recognized as being resistant to natural erosion and it was tempting to replicate the nature by synthetizing materials of the same type as LaPO4. We investigated thorium and uranium phosphates which are not isostructural with monazite, but are also very insoluble and chemically stable. The initial goal of this work was to study thorium phosphate in the frame work of the wastes produced during the chemical treatment of lanthanides ores. Most of them contained 5 to 8% of thorium and 0.5 % of uranium. The radioactive families are in 228
2 26
Ra. More recently we investigated uranium phosphate. Both compounds were studied as material with labelled radioactive isotopes. Their lixiviation properties is the main objective of this report. equilibrium, especially with
Ra and
Mat. Res. Soc. Symp. Proc. Vol. 353 © 1995 Materials Research Society
800
II - Thorium phosphate results
1) Synthesis of thorium phosphate Syntheses of crystallized thorium phosphate well known in literature [2-4] as Th3 (PO4 ) 4 were performed via two main routes : solide state and liquid. First, we duplicated
solid state syntheses. Thorium oxide and a
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