Journal of Hazardous, Toxic, and Radioactive Waste

The contaminant transport parameters such as hydraulic conductivity and diffusion coefficient are determined and compared for barrier composed of (1) soil, (2) soil-bentonite (SB), and (3) soil-polymer (SP) mixes. Chloride (${\mathrm{Cl}}^{-}$) is taken as the contaminant of interest because it is nonreactive. Sodium bentonite (NaB) and super-absorbent polymer (SAP) which is an anionic polyelectrolyte, is used for the study. Addition of NaB reduced the hydraulic conductivity of the soil and the diffusive transport through the soil barrier. Soil mixed with SAP was found to be more efficient in containing contaminants. The values of hydraulic conductivity and diffusion coefficient for SP mixes were lower than those observed for SB mixes. Low values of hydraulic conductivity and diffusion coefficient are attributed to the decrease of free channels for the contaminant transport and increase in the immobile water in the mix along with the increase in effective length. Microanalyses show that the addition of polymer causes the dispersion of soil particles which is similar to the observations from previous studies.

Radioactive liquid waste generated in the closed nuclear fuel cycle is immobilized by fixing the radionuclides in glass by a process known as vitrification. Vitrification is carried out in a joule-heated ceramic melter (JHCM) by passing alternate current in glass melt using electrodes. Silicon carbide resistance heaters are employed to bring the temperature of the glass to the conducting zone. Experiments were carried out on an industrial scale JHCM. Processing capacities were determined for glass melt having a composition of ${\mathrm{SiO}}_2$ (48%), ${\mathrm{B}}_2{\mathrm{O}}_3$ (26.3%), ${\mathrm{Na}}_2\mathrm{O}$ (11.7%), ${\mathrm{TiO}}_2$ (9.5%), and ${\mathrm{Fe}}_2{\mathrm{O}}_3$ (4.5%) for simulated waste with different salt concentrations. The product formed from these trials is qualified for long-term durability based on the study. The energy distribution in an industrial scale JHCM for different salt concentrations, namely, 28 and $56\mathrm{g}/\mathrm{L}$ of simulated waste is calculated based on the experiments conducted. This study also focuses on mathematical modeling of an all-electric glass furnace used for conditioning of hazardous radioactive waste. In this work, a numerical model of the glass furnace was developed and heat flow patterns in a JHCM were studied using finite element analysis. The numerical model is then used to predict the temperature profiles for different stages of the JHCM operation. The temperature profiles predicted by the model are validated using experimental results. The developed model is used for size reduction of the JHCM by decreasing the plenum heating section, thereby facilitating easier decommissioning. A novel method for estimating the effective thermal conductivity of the glass melt based on the experiments and simulations is discussed.

A system for rapid reduction of radioactive contamination and recycle of contaminated waters is called the Integrated Wash-Aid, Treatment, and Emergency Reuse System (IWATERS). First developed for cesium contaminations, IWATERS prescribes the use of salt and surfactant additives to decontaminate radionuclides from urban surfaces. The water is collected and recycled after passing through reactive filtration beds containing selective sorbents. To adapt the IWATERS for strontium contaminations, potential additives to enhance its decontamination from urban surfaces are identified. One possible additive is calcium (${\mathrm{Ca}}^{2+}$). However, its concentration can have a very strong detrimental effect on the ability of selective sorbents to remove strontium from spent wash water. We recognized that studies on off-the-shelf sorbents that include ${\mathrm{Ca}}^{2+}$ concentrations at relevant levels (greater than millimolar) are absent in the literature. To understand better the effect of ${\mathrm{Ca}}^{2+}$, we completed a literature review, batch tests, and surface complexation modeling to reveal few sorbent options. Only silico-titanate sorbents exhibited high $K_d$ values in the presence of ${\mathrm{Ca}}^{2+}$, but have significant drawbacks in cost and availability. Given the state of the art, it is imperative that alternatives to alkaline earth ions in the IWATERS be identified to permit in situ recycle of the wash waters.

A hybrid methodology was developed for identifying unknown groundwater pollution sources. The source identification problem can be solved by using the inverse optimization approach. Being a mixed-integer problem, genetic algorithms (GAs) can be applied to solve the inverse optimization problem. Even though GA can determine the discrete variable efficiently, it is not efficient in obtaining the continuous variables. For such a situation, the gradient-based local search optimization is considered to be an efficient approach. Therefore, because of the advantages of GA and gradient-based approach, they are combined to form an improved approach. GA has been modified to handle the source variables of locations and fluxes differently. It has been observed that GA often gives near-optimal solution. As such, three local location search algorithms, i.e., longitudinal-transverse search (LTS), mutation search (MS), and ripple-migration search (RMS), have been proposed. The efficiency and the applicability of the proposed model were evaluated by applying it in a hypothetical study area. The results show that the proposed model is capable of obtaining optimal source locations and fluxes. The results obtained are superior to those obtained by using embedded optimization method. However, when computational efficiency was compared in terms of the number of function evaluations, it was found that the number of function evaluation for the LTS algorithm was minimum.

To prevent fly ash from becoming an environmental hazard, it is very often used as a fill material in the construction of embankments. The stability of slopes incorporating the unsaturated behavior is of primary concern in these structures. This necessitates estimating the water retention characteristic curve (WRCC) of fly ash. However, the literature notes various uncertainties associated with the measurement of WRCC, mostly attributed to the inadequacy of instruments to measure wide range of suction. This paper investigates the impact of measurement uncertainties in WRCC on the reliability analysis of fly ash slope. The study shows a significant impact of uncertainties in WRCC on determining slope reliability. To quantify these uncertainties, a probabilistic model for WRCC is developed using the Gaussian copula and is incorporated into an unsaturated slope stability framework using Monte Carlo simulation. The study also shows the impact of the degree of correlation among WRCC parameters on the reliability analysis of slopes.