Progress in alkaline peroxide dissolution of low-enriched uranium metal and silicide targets Page: 4 of 28
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Dissolution Kinetics of Uranium Silicide
We developed a U3Si2 dissolution rate model in the past year. The dissolution of uranium
silicide in alkaline peroxide is proposed as:
U3Si2 + 25H202 + 100H -+ 2 SiO + 3 U02(02H)4 + 24 H20 (1)
The dissolution kinetics of the U3Si2 particles was determined by use of the initial rate method.
Experiments were carried out in an open, batch-reactor. Grap samples were taken at predetermined
time intervals during dissolution and then analyzed by inductively coupled plasma-mass spectrometry
(ICP-MS). The concentration of uranium was plotted against time. The initial reaction rate is the
slope of a linear-regression fit for uranium concentration vs. time at the start (0.5-5 min) of each
experiment.
Initial reaction rates were obtained for both comminuted and atomized U3Si2 particles [3,4].
The same mass of spherical atomized particles dissolved more slowly than the comminuted particles.
This is due mainly to the surface area. The large surface area in the atomized particles is attributed
to pore structure. It is believed that the internal surface of atomized particles was not effectively
exposed to reactants during dissolution because the gas bubbles produced by the vigorous reaction
inhibited the diffusion of reactants into the pores. However, the comminuted particles have a fine
surface and not a porous structure. Thus, a reasonable effective surface area for the comminuted
particles (200 cm2/g) is about twice that of the atomized particles (100 cm2/g). When the effective
surface area is considered, the atomized and comminuted U3Si2 particles have similar uranium
dissolution rates at 50*C.
The dissolution rate was found to vary with the initial base and peroxide concentration. A
thermodynamic model for dissolution with an alkaline peroxide solution, developed by means of a
software package called the Environmental Simulation Program (ESP) [5], predicts that hydrogen
peroxide (H202) and hydroxyl ion (OH-) are equilibrated with the peroxyl ion (02H) (Fig. 1). When
the dissolution rate is plotted against the equilibrium 02H" concentration (Fig. 2), it is clearly shown
that uranium dissolution rates depend most strongly on the equilibrium concentration of the peroxyl
ion (02H). This effect can be explained by 02H ~ being required for uranium solubility and being a
controlling factor in the dissolution.
We believe that the function of the O2H- as a rate-controlling factor is surface related: 02H-
is required to produce an activated complex on the uranium surface and to allow the dissolution to
proceed. Thus, a kinetic model for uranium dissolution can be developed by assuming that the total
uranium on the surface of the U3Si2 particles ([U,]) exists in three distinct states: the unreacted
surface ([A]); the reactive complex of A with 02H~ ([B] = K1 [A][O2H ]eq); and a second,
unreactive complex of A with OH" ([C]= K2 [A][OH ]eq). Then, the expressions for the reactive
complex [B] can be solved from the relation [U,]=[A]+[B]+[C], as shown in Eq. 2:
KI [02H Jeq
[B] = [Us] (2)
1 + K1 [02H ]eq + K2 [OH ]eq
A uranium dissolution rate model developed from Eq. 2 is shown in Eq. 3:
1 dU Ki [02H Jeg
Ru - -= ka[B] = ka _ (3)
S dt a + K1 [02H ]eq + K2[OH ]eq
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Chen, L.; Dong, D.; Buchholz, B. A.; Vandegrift, G. F. & Wu, D. Progress in alkaline peroxide dissolution of low-enriched uranium metal and silicide targets, article, December 31, 1996; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc680457/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.