Page:4SIGHT manual- a computer program for modelling degradation of underground low level waste concrete vaults (IA 4sightmanualcomp5612snyd).pdf/87

1. INTRODUCTION 1. INTRODUCTION.

The 4SIGHT computer program has been developed by the Inorganic Group of the Building Materials Division at NIST. This program facilitates estimating the service life of low level nuclear waste storage sites by determining the hydraulic conductivity of the vault roof as a function of time. This objective is acheived by considering prominent deterioration mechanisms to predict the intrinsic permeability, effluent, and working thickness of the vault slab.

The condition of the vault slab is assesssed by modelling the ingress of deleterious ion species by both diffusion and convective flow due to hydraulic pressure gradients. Additionally, changes in physical properties due to the leaching/deposition of salts are incorporated into the program. The propagation of ions are calculated using an advection-diffusion equation. After each discrete time step, each computational cell is brought to chemical equilibrium through precipitation/dissolution of ionic salts.

The program incorporates a one-dimensional model of the vault since the critical deterioration will most likely occur at the ceiling of the vault. The ceiling is the most likely element to experience moisture, stress, and external ions such as C and SO4. Addtionally, the majority of the transport will occur at the center of the roof slab since the edges of the slab will be supported. Therefore, a simple one-dimension model of the flow at the center of the slab will give a conservative estimate of the transport for the entire slab.

The program proceeds in a strainghtforward manner; The program gets input parameters from the user; initializes the ion information and scaling parameters; iterates diffusion and convection calculations, bringing each computational element to chemical equilibrium between iterations; determines whether failure has occured; and prints pertenent information about the performance of the vault.

The output of the program can be printed to either the screen or to an ASCII file. The ASCII file can then be used as input to a spreadsheet program for further analysis.