Page:4SIGHT manual- a computer program for modelling degradation of underground low level waste concrete vaults (IA 4sightmanualcomp5612snyd).pdf/39

 the uncracked portion, resulting in virtually no pressure drop across the crack. This increases the pressure gradient across the remaining untracked concrete.

Although each LLW facility may be unique, most underground facilities can be represented schematically as in Figure 1. Below the ground surface there will be an engineered barrier to deflect surface water away form the concrete vault below. The entire facility is sited on a geologically suitable location.

From the schematic in Figure 1 it appears as though the roof is the most critical element because it is most likely to have a moist environment, especially upon the failure of the engineered barrier. Because of this, the analysis of the entire concrete vault can concentrate on the roof. If the roof is treated as a simple slab, the analysis simplifies further. Since the flow through the roof slab will be approximately uniform over the surface of the slab, a one-dimensional analysis of transport vertically through the slab should serve as a sufficiently accurate model for transport through the roof of the vault.

A single transport equation is developed to propagate ions through the slab. This equation can be converted into a finite difference equation so that it can be implemented in a computer program. A result of this approach is that time will advance in discrete intervals. After every time interval each computational element is put in chemical equilibrium using solubility products and a charge balance. This step is achieved through dissolution/precipitation of available salts. Any change in the quantity of solid salts in the pore space will change the