SNF dissolution

The dissolution rates by the electrochemical and chemical processes are (Ahn, 1996a):

S

Rdis = ±kef (E) [7.2]

and

Rdis = Sk — (Cs — Ct)

[7.3]

Rdis = Vk+ (Ct — Co) + VCt + N ParCt

[7.4]

where S is surface area of the dissolving phase, V is leachate volume, ke is rate constant for electrochemical dissolution, f(E) is dissolution rate as a function of electrochemical potential E, k. is rate constant for SF dissolu­tion, Cs is effective solubility limit of dissolving phase, C. is elemental con­centration under consideration, k+ is rate constant for growth of the reprecipitated phase, F is flow rate of ground water, and Npar is formation or growth rate of colloids per unit leachate concentration.

Equation [7.2] is for electrochemical process, Eq. [7.3] is for chemical process, and Eq. [7.4] is for release rate from the dissolution processes of the first two equations.

The fractional mobilization rate is the dissolution rate multiplied by the specific surface area of the SNF matrix. Conservatively, the fractional mobi­lization rates can be assumed constant within uncertainty ranges at a given temperature. The environmental conditions are important in determining the dissolution rates, including near field water chemistry, temperature, pH, or reducing or oxidizing conditions. Important water chemistry includes carbonates, and cations such as calcium or silica species (Ahn and Mohanty, 2008).

In connecting the dissolution rate to the fractional mobilization rate, the specific surface area is determined by the average fragment size (radius) and density of the waste form. Typically, the fragment size of commercial SNF is 0.1 cm (0.04 inch) (Ahn and Mohanty, 2008).

If the temperature exceeds 100°C (212°F), solid-state oxidation or hydra­tion will occur, depending on the RH. Higher uranium oxides (UO[.4 or U3O8) that form by oxidization of the UO2 matrix dissolve at a rate similar to the unoxidized UO[ matrix. Hydrated UO[-xH2O (x = 0.8, 2) dissolves 10-20 times faster than unhydrated oxides. However, the rate of hydration (i. e., the formation rate of UO3-xH2O) is slower than the aqueous dissolu­tion rate. Ahn and Mohanty (2008) summarized the effects of oxidation and hydration on the dissolution.

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