Leaching network dissolution control

Waste form leach rates in a geologic repository will be affected by unknown water flow rates and by extensive cracking of the waste form monolith. An understanding of these effects is important in predicting the geochemical behavior of disposed radioactive waste forms over the full range of possible scenarios. The dependence of the waste form source term on the rate of renewal of aqueous solution is first established for the simple but important case of solubility-limited network dissolution control. 

Therefore, if leaching is network dissolution controlled, the higher the flow rate the sooner steady-state is achieved and the higher the leach rate, which accords with the experiments1 evidence. In particular, because 3 = A/V and - F/V,... 

Figure 2 shows the predicted, normalized cumulative mass losses based on the behaviors of silicon and sodium for three different values of the leachant renewal frequency. The physical parameters used refer to the leaching of PNL 76 68 borosilicate glass in deionized water at 90°C, (4) and reference is made to the geometric surface area, SA, of the sample. In particular, the curves corresponding to silicon and sodium tend to have the same, constant slope with increasing flow rate. In particular, the curves corresponding to = 1 day 1 practically coincide, indicative of network dissolution control. 

The reaction of acids with glass may be either a leaching process or a complete dissolution process. Acids such as hydrofluoric acid attack silica glasses by dissolving the silica network. Other acids such as hydrochloric acid or nitric acid may react by dissolving certain glasses. However, the reaction mechanism is by selective extraction of alkali and the substitution of protons in a diffusion-controlled process. 

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