High Level Waste System Research Articles

Fractal approach in expansive clay-based materials with special focus on compacted GMZ bentonite in nuclear waste disposal: a systematic review.

Knowledge of the behavior of highly compacted expansive clays, as an engineered barrier, in disposal of high-level nuclear waste (HLW) systems to prevent the pollution due to migration of radionuclide is extremely essential. The prominent properties of globally and widely used bentonites have been extensively studied during past two decades. In China, GaoMiaoZi (GMZ) bentonite is the first choice as a buffer or backfill material for deep geological repositories. This review article presents the recent progresses of knowledge on water retention properties, hydromechanical behavior, and fractal characteristics of GMZ bentonite-based materials, by reviewing 217 internationally published research articles. Firstly, the current literature regarding hydrogeochemical and mechanical characteristics of GMZ bentonite influenced by various saline solutions are critically summarized and reviewed. Then, the role of osmotic suction π alongside the application of surface fractal dimension Ds is presented from the standpoint of fractal theory. Finally, the strength characteristics of GMZ bentonites using fractal approach have been discussed. Furthermore, this study sheds light on gaps, opportunities, and further research for understanding and analyzing the long-term hydromechanical characteristics of the designed backfill material, from the standpoint of surface fractality of bentonites, and implications of sustainable buffer materials in the field of geoenvironmental engineering.

Mercury in aqueous tank waste at the Savannah River Site: Facts, forms, and impacts

ABSTRACTOver the past two years, there has been an intense effort to understand the chemistry of mercury across the Savannah River Site’s high-level liquid waste system to determine the impacts of various mercury species. This effort began after high concentrations of mercury were measured in the leachates from a toxicity characteristic leaching procedure (TCLP) test on the low-level cementitious waste form produced in the Savannah River Saltstone facility. Speciation showed the dominant form of leached mercury to be the methylmercury cation. Neither the source of the methylmercury nor its concentration in the Saltstone feed was well established at the time of the testing. This assessment of mercury was necessary to inform points in the process operations that may be subject to new separation technologies for the removal of mercury.

Studies of Potential Inhibitors of Sodium Aluminosilicate Scales in High-Level Waste Evaporation

For a decade, the evaporation of highly alkaline High-Level nuclear wastes as practiced in some DOE nuclear waste facilities has produced sodium aluminosilicate scales causing both operation and criticality hazards. Segregation of aluminum-rich wastes from silicate-rich wastes minimizes the amount of scale produced and reduces cleaning expenses but does not eliminate scaling nor increase operational flexibility in waste processing. Similar issues have affected the aluminum refining industry for many decades. Over the past several years, successful commercial products have been identified to eliminate aluminosilicate fouling in the aluminum industry but have not been utilized in a nuclear environment. Laboratory quantities of three proprietary aluminosilicate scale inhibitors produced by Cytec industries have been shown to prevent formation of aluminosilicate scales. The Savannah River National Laboratory (SRNL) has been actively testing these potential scale inhibitors to examine their radiation stability, radiolytic degradation behaviors, and downstream impacts to determine their viability within the High-Level nuclear waste system at the Savannah River Site (SRS). One of the tested polymers has been found to successfully meet the established criteria for application in the nuclear waste environment.

Investigation of Plutonium and Uranium Precipitation Behavior with Gadolinium as a Neutron Poison

:The caustic precipitation of plutonium (Pu)-containing solutions has been investigated to determine whether the presence of 1 :3 Pu : uranium (U) in solutions stored in the H-Canyon Facility at the U.S. Department of Energy’s Savannah River Site (SRS) would adversely impact the use of gadolinium nitrate [GcHNO3)3] as a neutron poison. In the past, this disposition strategy has been successfully used to discard solutions containing approximately 100 kg of Pu to the SRS high-level waste (HLW) system. In the current experiments, gadolinium [as Gd(NO3)3] was added to samples of a 1 : 3 Pu : U solution, a surrogate 3 g/L U solution and a surrogate 3 g/L U with 1 g/L Pu solution. A series of experiments was then performed to observe and characterize the precipitate at selected pH values. Solids formed at pH 4.5 and were found to contain at least 50% of the U and 94% of the Pu but only 6% of the Gd. As the pH of the solution increased [e.g., pH >14 with 1.2 or 3.6M sodium hydroxide (NaOH) excess], the precipitate contained greater than 99% of the Pu, U, and Gd. After the pH >14 systems were undisturbed for one week, no significant changes were found in the composition of the solid or supernale for each sample. The solids were characterized by X-ray diffraction (XRD) which found sodium diuranate (Na2U2O7) and gadolinium hydroxide [Gd(OH)3] at pH > 14. Thermal gravimetric analysis (TGA) indicated sufficient water molecules were present in the solids to thermalize the neutrons, a requirement for the use of Gd as a neutron poison. Scanning electron microscopy (SEM) was also performed, and the accompanying back-scattering electron analysis (BSE) found Pu, U, and Gd compounds in all PH >14 precipitate samples. The rheological properties o! the slurries at pH >14 were also investigated by performing precipitate settling rate studies and measuring the viscosity and density of the materials. Based on the results of these experiments, poisoning the Pu-U solutions with Gil and subsequent neutralization is a viable process for discarding the Pu to the SRS HLW system.