DOE OSTI2015
salt-stone is a cementitious, low-level waste (LLW) material processed and stored at the Savannah River Site (SRS). It consists of fly ash (FA), blast furnace slag (BFS), and Portland cement that are blended with LLW salt solution. The dry feed materials and the salt solution are mixed at the salt-stone Production Facility (SPF), and the fresh grout subsequently pumped into a salt-stone Disposal Unit (SDU) in the salt-stone Disposal Facility (SDF) where it cures to form a rigid, cementitious monolith. In order to demonstrate the long-term performance of salt-stone, non-radioactive simulant samples are processed and cured in a laboratory environment and subsequently tested with respect to key performance properties. Of particular relevance to assessing the long-term stabilization and immobilization of radionuclides in salt-stone is the permeability, or saturated hydraulic conductivity, which determines the rate of infiltration for oxygen-bearing ground waters that can affect redox sensitive contaminants, such as {sup 99}Tc. Environmental conditions in the SDU, in particular temperature and humidity, are believed to affect the cured properties of salt-stone. With respect to humidity it has been determined that the average vapor space humidity in an SDU is at least 95%, and in addition the assumption can be made that underlying grout is subjected to a moisture-saturated environment by the grout poured over it. With respect to temperature, the SDUs contain thermo-well arrays that provide detailed vertical, and to a lesser degree radial, temperature profiles. Temperature data from the SDUs have subsequently been utilized to better simulate curing temperatures for simulant salt-stone in the laboratory. However, no two thermal profiles are the same, varying with respect to the following parameters: initial temperature, rate of temperature increase, maximum temperature attained, time at maximum temperature, and cooling rate. These temperature variables are dependent on the depth of grout (due to insulating effects of underlying and overlying grout), frequency of grout addition to the SDU (with more frequent additions causing grout temperatures to rise due to the exothermic hydration reactions), and, to a lesser degree, the ambient temperature of the external environment. Due to the variability in each thermal profile recorded in an SDU, the question arises as to which profile (or profiles) should be used in the simulated laboratory environment to best mimic the curing of field emplaced salt-stone, such that there is confidence that the properties measured for laboratory-prepared samples are representative of the grout in the SDU. Since the current SDU design contains sixty-six thermo-wells it is obviously not practical to process simulant samples according to each profile. Rather the aim would be to choose a reduced number of profiles that adequately encompass the range of recorded profiles, or to choose a single profile that is considered most conservative with respect to the resulting properties of interest. This is typically thought to be the profile exhibiting the highest peak temperature since for cement-based materials higher curing temperatures (> 60 deg. C), while resulting in enhanced reaction kinetics and strength development for short curing durations (∼28 days), have been shown to adversely affect the physical properties of the material after extended cure times. In choosing representative profiles it is important to consider not only the maximum temperature, but also the influence of rate of temperature rise to the maximum temperature, and time at or close to the maximum temperature on the post-cured properties. The intent of this paper is to consider the varied curing temperature profiles to which the salt-stone is exposed in an SDU, and to analyze the currently available data for elevated temperature curing of salt-stone to determine the potential thermal effects on the saturated hydraulic conductivity of salt-stone emplaced in an SDU. This may subsequently facilitate a more informed approach to simulating the curing of salt-stone in a laboratory environment. (authors)
12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗