Solubility of Plutonium in Water Containing Carbonate under Reducing Conditions
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6ROXELOLW\RI3OXWRQLXPLQ:DWHU&RQWDLQLQJ&DUERQDWHXQGHU5HGXFLQJ&RQGLWLRQV Arvid Ödegaard-Jensen , Yngve Albinsson and Hans Nilsson Department of Materials and Surface Chemistry, Nuclear Chemistry Chalmers University of Technology $%675$&7 In order to investigate the dissolution of plutonium(III) under reducing condition and in a carbonate rich water solid phases of plutonium hydroxide and carbonate were made. The still wet solid phases were placed in vessels which contained 1 M NaClO4. The atmosphere in the vessels was either H2 or a mixture of H2 and 0.03% CO2. The preliminary results shows that after some time the plutonium concentration in the solutions reached a value between 10-9 M and 10-7 M. The pH in the solutions is dependent of the atmosphere. In a pure H2 atmosphere the pH is changing over time whereas in the H2+CO2 atmosphere the pH stays between 7 and 7.5. At pH 8 the total carbonate concentration in the water was between 10-4 M and 10-3 M. This is the same order of magnitude as the ground water surrounding the proposed repository. ,1752'8&7,21 The final storage for Sweden’s nuclear fuel is proposed to be an underground repository, located at about 500 m depth in a granitic bedrock formation. The disposal conditions allow ground water to contact the canister, in which the fuel elements are placed, by diffusion through the surrounding layer. The corrosion of the canister will provide actively reducing conditions in the ground water reaching the fuel elements. Plutonium is an important constituents of the spent nuclear fuel, and it is important to predict how plutonium behaves when contacted with ground water under reducing conditions. Swedish ground water contains about 0.2 to 1.0 mM of inorganic carbon, mostly as carbonate, and has a pH of 8, Wikberg et al. [1]. To investigate the effect of carbonate on the solubility of plutonium under reducing condition, and to prevent interference from ions other than hydroxide and carbonate or hydroxy carbonate, a 1M NaClO4 was used instead of ground water. The atmosphere was either pure H2 or a mixture of H2 and 0.03% CO2, which at a pH of 8 will give a carbonate concentration of approximately 0.5mM. All experiments were made at normal pressure, which is not the same as the conditions that will prevail in the underground repository. (;3(5,0(17$/ The preparation and outtake of sample for analysis were done in a nitrogen atmosphere glove box with an oxygen removal system. A plutonium ( 94% 239Pu, 6% 240Pu, and 3X@
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)LJXUH Log( [Pu] ) plotted against time. Felmy et.al.[4] have reported a solubility for Pu(OH)3 which is shown as a dashed line in Figure 2. In addition to this line the lines for crystalline PuO2 and amorphous Pu(OH)4, collected by Haschke and Oversby[5], have also been added. From Figure 2 and Table 1 it is seen that the pH, for the carbonate system, rapidly tends to reach a value of 7.2 to 7.3.This is due to the buffering capacity of the carbonate in this system and is not seen in the
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