Kinetic Studies of Natural Uranium Minerals for the Long-Term Evolution of Spent Nuclear Fuel under Oxidizing Conditions
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KINETIC STUDIES OF NATURAL URANIUM MINERALS FOR THE LONG-TERM EVOLUTION OF SPENT NUCLEAR FUEL UNDER OXIDIZING CONDITIONS IGNASI CASAS, E. CERA AND J. BRUNO MBT TA, Parc Tecnol6gic del Vall6s (CENT), Cerdanyola, 08290 Spain.
ABSTRACT The time scale of spent fuel dissolution studies is of the order of magnitude of 2 to 10 years, while the performance of a spent fuel repository should be assessed for much longer times (105-106 years). These time scales can be bridged using appropriate natural analogues. Among other important information, the study of natural systems can give insight of which can be the oxidative alteration of spent fuel in granitic environments. However, in studying such systems, thermodynamic and kinetic data of relevant natural solid phases are needed. In this work we present preliminary results of dissolution experiments carried out under oxidizing conditions with selected and well characterized natural samples of the alteration chain of uraninite (i.e., uraninite, schoepite, uranophane). The experiments have been performed using a synthetic granitic groundwater as a leachant, in contact with air and at 25 oC.
INTRODUCTION A vast amount of information has been collected in the recent years concerning the stability and dissolution behavior of spent nuclear fuel and its chemical analogues, U0 2 and SIMFUEL. However, the long term for which the performance of a spent fuel repository must be assessed (105-106 years) is clearly not covered by experimental studies, the time scale of which is of 2 to 10 years in the best case. This time span can be bridged using appropriate natural analogues. Among those, the Shinkolobwe site constitutes a very nice example of complete alteration of uraninite, while the Cigar Lake site is an example of preservation of the original U0 2 . A schematic pathway for the oxidative alteration of uraninite can be depicted as follows: 1.- Initial radiolytic surface oxidation of the U0 2: U0 2 + (x/2 )02
-
UO2+x
2.- Full oxidation to U(VI)-hydroxides (i.e., schoepite), sometimes including divalent cations (i.e., becquerelite)[1]: UO2+x + ((1"x)/ 2 )0 2 + (1+y)H 20 -- U0 2(OH) 2.yH 20 6UO 2+x+ (3-3x)0
2
+ Ca 2+ + 12H 20 -
[CaU6019.11H 20](cr)+ 2H+
3.- Final alteration to either silicates or phosphates, depending on the Si0 2/PO 4 ratio in the groundwaters (i.e., in the presence of Ca, reaction of becquerelite to give uranophane)[1]:
Mat. Res. Soc. Symp. Proc. Vol. 294. ©1993 Materials Research Society
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[CaU 6 o',-llH 20](cr) + 2Ca 2÷ + 6SiO2(,q) + 9H 2 0 - 3[CaU 2Si2 Ol1"6H2 O](Ce) + 4H+ Recently, Rod Ewing and coworkers at the University of New Mexico (UNM) have devoted much attention to the mineralogical and crystallographic studies of the pathways of uraninite alteration [2] and the consequences for the long-term stability of spent fuel. However, in order to include this mineralogical information in the perspective of the performance assessment of a spent fuel repository, the thermodynamics and kinetics of the critical pathways of the oxidative alteration of urani
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