A Review of Geochemical Modeling for the Performance Assessment of Radioactive Waste Disposal in a Subsurface System

Abstract

Radionuclides are inorganic substances, and the solubility of inorganic substances is a major factor affecting the disposal of radioactive waste and the release of concentrations of radionuclides. The degree of solubility determines whether a nuclide source migrates to the far field of a radioactive waste disposal site. Therefore, the most effective method for retarding radionuclide migration is to reduce the radionuclide solubility in the aqueous geochemical environment of subsurface systems. In order to assess the performance of disposal facilities, thermodynamic data regarding nuclides in water–rock systems and minerals in geochemical environments are required; the results obtained from the analysis of these data can provide a strong scientific basis for maintaining safety performance to support nuclear waste management. The pH, Eh and time ranges in the environments of disposal sites cannot be controlled, in contrast to those under experimental conditions in laboratories. Using a hypothetical error mechanism for the safety assessment of disposal sites may engender incorrect assessment results. Studies have focused on radionuclide reactions in waste disposal, and have offered evidence suggesting that these reactions are mainly affected by the geochemical environment. However, studies have not examined the thermodynamics of chemical reactions or interactions between water and minerals, such as the surface complexation and adsorption of various nuclide-ion species. Simple coefficient models have usually been applied in order to obtain empirical formulas for deriving Kd to describe nuclide distributions in the solid or liquid phase in water–rock geochemical systems. Accordingly, this study reviewed previous research on the applications of geochemical models, including studies on the development of geochemical models, sources of thermodynamic databases (TDBs) and their applications in programs, the determination of the adequacy of TDBs in surface complexation models and case studies, and the selection and application of activity coefficient equations in geochemical models. In addition, the study conducted case studies and comparisons of the activity coefficients derived by different geochemical models. Three activity coefficient equations, namely the Davies, modified Debye–Hückel, and Pitzer equations, and four geochemical models, namely PHREEQC, MINEQL+, MINTEQA2, and EQ3/6, were used in the study. The results demonstrated that when the solution’s ionic strength was <0.5 m, the differences in the activity coefficients between the Davies and modified Debye–Hückel equations were <5%. The difference between the Pitzer and Davies equations, or between the Pitzer and modified Debye–Hückel equations in terms of the calculated activity coefficients was <8%. The effect of temperature on the activity coefficient slightly influenced the modeling outputs of the Davies and modified Debye–Hückel equations. In the future, the probability distribution and uncertainty of parameters of Kd and the equilibrium constant can be used in geochemical and reactive transport models to simulate the long-term safety of nuclear waste disposal sites. The findings of this study can provide a strong scientific basis for conducting safety assessments of nuclear waste disposal repositories and developing environmental management or remediation schemes to control sites marred by near-surface contamination.

Reactive Transport: A Review of Basic Concepts with Emphasis on Biochemical Processes

“…), and the N definitions of kinetic reaction rates. The latter can be eliminated by substitution of Equation ( 18) into (17). Still, the large number of coupled non-linear equations makes the problem intractable.…”

“…The basic mass balance for a specific microbe, assumed immobile, simply results from Equation (17), which reads:…”

“…Genome sequence data are used to define a metabolic reaction network within the cells. This may consist of several hundreds of metabolites and reactions and exchange rates between the cell and its environment [217] and is represented, similar to Equation (17), by a stoichiometric matrix and metabolic and exchange rates, but neglecting the transport terms and assuming steady state within the cells, yielding 𝑆 𝑟 = 0. Given concentrations of chemical species of the cell's environment, the rates, 𝑟, are optimized for maximum biomass production, ATP (Adenosine Triphosphate) synthesis, or minimize nutrient uptake, subject to thermodynamic constraints, upper and lower bounds of the rates and mass balances of metabolites (i.e., 𝑆 𝑟 = 0).…”

“…Some are centered on problems related to CO storage [12,13], which generates acidity, so it is somewhat related to acid mine drainage [14], more formally revised by Amos et al [15]. Nuclear waste disposal has motivated reviews on Uranium mobility [16], validity of existing models [17], cement-clay interaction [18] or oxygen transport at depth, which motivated the thorough review of MacQuarrie and Mayer [19] on RT in fractured rock. Global issues motivate the reviews of Dignac et al [20], Li et al [21], Arndt at al.…”

The Effect of Porosity Change in Bentonite Caused by Decay Heat on Radionuclide Transport through Buffer Material

“…These complete chemical thermodynamic databases were incorporated into the coupling model of the geochemical and reactive chemical transport. Even the thermal, pressure, and stress fields were coupled with the reactive chemical transport model [64]. Further, the test data of the underground laboratory from multiple years (such as the status of the field FEBEX test after 18 years: heterogeneous bentonite barrier [65]) or those of other montmorillonite materials as the buffer (such as the alkylammonium-modified montmorillonite [66]) were employed.…”