Investigation of Diffusion of UO 2 Cl 2 in Saturated NaCl Solutions at Various Temperatures
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INVESTIGATION OF DIFFUSION OF U02CI 2 IN SATURATED NaCI SOLUTIONS AT VARIOUS TEMPERATURES HARRY MURSO
BODO PLEWINSKY
,
DIETER LEOPOLD AND GONTER MARX
Free University of Berlin Institute of Inorganic and Analytical Chemistry Fabeckstra~e 34-36, 1000 Berlin 33 (Dahlem), Germany
INTRODUCTION
Even in the early 1960's it had already been decided to place high-level radioactive waste in the salt formations of the Federal Republic of Germany. However a major concern is the formation of highly corrosive brines in the event of water filling the repositories. In order to estimate the risks of waste storage in salt repositories it is necessary to investigate the physicochemical properties of the waste materials and of those radionuclides extracted into brine. The extraction of radionuclides from borosilicate glasses or spent fuel elements has been discussed2 , taking into consideration the two transport mechanisms: diffusion and convection. Diffusion measurements of actinides in saturated NaCl solutions have been made in order to determine which mechanism is rate determining. The diffusion coefficients of U02 C12 in saturated NaCl solutions were measured by use of an ultracentrifuge at 25, 40 and 50 0C. The results are compared with those determined for the binary system U02 CI2 - H20. Measurements of viscosity and molar mass were made in order to elucidate the difference in diffusion velocities in the systems studied. EXPERIMENTAL DETAILS
Diffusion coefficient and molar mass determinations were carried out with a Beckman Instruments, Model E analytical ultracentrifuge, fitted with a standard schlieren optical system and an absorption optical system connected to a photoelectric scanner. The apparatus, methods of operation and data handling systems are described fully in references 3, 4, 5 *
Part of theses
**
to whom correspondence should be addressed, new address: Bundesanstalt fUr MaterialprUfung (BAM), Unter den Eichen 87, 1000 Berlin 45, Germany
492
Viscosity measurements were carried out with the aid of an automatic capillary viscosimeter (Ubbelohde type). DIFFUSION IN TERNARY SYSTEMS
A ternary system - e. g. U02 CI2 dissolved in saturated NaCl solutions obeys 6 Fick's law as follows: H20 (1) -
U02 C12 (2)
-
NaCl (3),
-J2
= D2 2 gradc 2
-J3
= DD 32 gradc 2 + D3 3gradc 3.
+ D23 gradc 3 9
Ji is the current density of component i and equals the amount of substance i being transported perpendicularly across a plain of reference of unit area in unit time. In such a system, four diffusion coefficients are required in order to quantitatively describe the diffusion behaviour. There are two main-term diffusion coefficients, D22 and D33. and two cross-term diffusion coefficients, D23 and D32 In this case, the main-term diffusion coefficient D22 describes the molecular transport of U02C12 caused by its own concentration gradient. In addition, the NaCl concentration gradient can also cause the diffusion of U02 CI2. The extent of this coupling effect is indicated by D23. Of the three optical systems available wi
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