4SIGHT Manual: A Computer Program for Modelling Degradation of Underground Low Level Waste Concrete Vaults/Introduction

by Kenneth A. Snyder and James R. Clifton

1Introduction

The computer program 4SIGHT has been written to facilitate Low Level Waste (LLW) disposal facility performance assessment. 4SIGHT can be used to predict the service life and hydraulic conductivity of a buried concrete vault. This estimate is based upon an analysis of the concrete vault roof given the material properties of the concrete and the external ion concentrations.

4SIGHT incorporates multiple degradation mechanisms by using a single transport equation for ions. Individual degradation mechanisms are typically controlled by the concentration of a single ion species. However, as various ion species diffuse into the concrete, the transport properties change, which changes the rate of ion diffusion. As the pore solution pH changes, available salts either precipitate or go into solution. The precipitation and dissolution processes changes the porosity, which in turn changes the transport properties. It is the act of maintaining chemical equilibrium that affects the synergism of the different degradation mechanisms.

Previous efforts [1][2] have yielded mathematical models for various degradation mechanisms. From these equations, the service life could be calculated for any one degradation mechanism. However, combining the effects of multiple mechanisms was not feasible analytically. The advantage of the method used by 4SIGHT is the ability to incorporate the synergism of multiple degradation processes. Rather than calculate rates of attack, 4SIGHT uses a single transport equation to propagate ions through the concrete. Keeping the system in chemical equilibrium allows interaction of degradation mechanisms.

4SIGHT is a command-driven program that awaits input parameters from the user, using default values when necessary. Once all known parameters are entered, 4SIGHT reiterates the values of all the input parameters used in the calculation with a note as to whether the value was specified by the user or whether a default value was used. The computations begin by 4SIGHT propagating ions in discrete time increments. At regular intervals 4SIGHT prints to the screen the bulk hydraulic conductivity and diffusivity, and the depth of sulfate and chloride attack. The calculation continues until one of the following occurs: corrosion of the steel reinforcement, sufficient sulfate attack that the roof fails structurally, failure of a joint, or a user specified time limit has been exceeded. Upon termination, 4SIGHT prints the state of the system, composed of the concentration of user selected ions and moles of user selected salts, as a function of depth into the roof.

What follows in this Report is a complete user's guide to 4SIGHT, including installation information and source code. The goal is an all-inclusive document, acting as a single body of work, that facilitates review of any aspect of the program. To facilitate source code review, 4SIGHT was written using a literate programming tool which incorporates both the programming code and typeset comments. The typeset output does not conform to the structure of a NISTIR. Therefore, the typeset output, as it normally appears, is attached to the end of this report.